Forces — Print · Physical Science: Matter, Forces & Energy, Grade 8

Balanced & Unbalanced Forces

Learning goals

• I can find the net force on an object from two or more forces.
• I can predict whether an object's motion will change based on whether forces are balanced or unbalanced.

Readings for this unit

• A moving cart and a magnet across a gap

In the real world, objects almost never feel just a single force. Right now, gravity is pulling this book down, the table is pushing it up, and if you slid it, friction would drag against it too. What actually determines how an object moves isn't any one of those forces alone, but the net force — the single overall force you get after combining all of them, carefully accounting for direction. Forces pointing the same way add together; forces pointing in opposite directions partly or fully cancel.

When all the forces on an object cancel out to a net force of zero, we say the forces are balanced. A balanced object's motion doesn't change one bit. If it was sitting still, it stays still; if it was already gliding along, it keeps gliding at the same steady velocity. That's Newton's first law in action — no net force means no change in motion. A book resting on a table is the classic case: gravity pulls it down, the table pushes up with an equal force, and the two perfectly balance.

When the forces don't cancel — when there's some net force left over — the forces are unbalanced, and now the object's motion has to change. It accelerates in the direction of that leftover net force, speeding up, slowing down, or changing direction. Push a shopping cart harder than friction pushes back, and it rolls forward faster and faster. The bigger the unbalanced net force, the greater the acceleration it produces.

Working out the net force is mostly careful arithmetic with direction. Two forces in the same direction simply add: a 40 N push plus another 40 N push the same way is 80 N. Two forces in opposite directions subtract, and the net force points the way the larger one was pointing: 60 N to the right against 20 N to the left leaves 40 N to the right. Watch out for a common misconception here — an object moving at a steady speed does NOT need a net force to keep going. Constant velocity actually means the forces are balanced, exactly like an object standing still.

This single question — is the net force zero or not? — lets you predict what will happen to almost anything. A tug-of-war rope stays put while both teams pull equally hard, then suddenly lurches the instant one side gains an edge and the forces become unbalanced. From a car cruising at a constant speed (balanced) to a rocket blasting upward (wildly unbalanced), reading the net force tells you whether an object's motion will hold steady or change.

Choose one object and draw only the forces acting on it. Opposite equal arrows give zero net force. The object can then remain still or move in a straight line at constant velocity; balanced forces do not require zero speed.

Unbalanced forces change velocity, which can mean changing speed, direction or both. A turn at constant speed still involves acceleration. To predict the change, add forces as directional quantities rather than adding every magnitude as positive.

If a book rests on a table, the table's upward force balances the Earth's downward force on the book. Those two forces act on the same book and are not a third-law pair. Third-law forces act on different objects and therefore cannot cancel in one object's force diagram.

Build your understanding

Where this fits

This unit helps answer the chapter question: How can forces explain stability and changes in motion? Keep a notebook table with the question, evidence, explanation and one unresolved issue. Revise a previous model only when you can name the new evidence or idea that changes it.

Use three teaching sessions of about 40–45 minutes: first observe the phenomenon, draft a model and study Parts 1–2; next explain Parts 3–4 with the figures and evidence; then work the application, examples, practice, model revision and exit check. Divide any difficult part into smaller steps and revisit after feedback. Investigations, review and quizzes have separate sessions.

Part 1 · List forces acting on one object

Choose one object and draw arrows for external forces acting on it. Label the source of each force and its direction. Include pushes, pulls, friction, gravity, and support forces where relevant. A force diagram represents interactions, not a trail showing where the object has already traveled.

Read the visual model

Why does a force diagram need a named object?

Visual reasoning — after your attempt

Forces acting on different objects cannot be combined to find the net force on one object. Naming the object keeps the interactions and arrows consistent.

For a book resting on a table, Earth pulls down and the table pushes up. Draw both arrows on the book. Do not put the book’s downward push on the table in the book’s own force diagram; that force acts on the table.

Pause and explain

Why does a force diagram need a named object?

Check your reasoning after answering

Forces acting on different objects cannot be combined to find the net force on one object. Naming the object keeps the interactions and arrows consistent.

Part 2 · Find the net force along a line

Net force is the combined external force on the chosen object. In one dimension, forces in opposite directions subtract. Equal opposite forces give zero net force along that direction. Zero net force means zero acceleration, not necessarily zero velocity. Use direction words with the result.

Read the visual model

Choose the receiver before adding arrows. A sign records direction along the chosen line. Net force is the combined result of the real forces, not an extra push that exists in addition to them.

If the left force becomes 12 N while the right stays 10 N, what is the new net force and direction?

Visual reasoning — after your attempt

10−12 = −2 N with right positive: 2 N left. That predicts leftward acceleration, not necessarily leftward current velocity.

A cart experiences 8 N right and 3 N left, giving 5 N right. If the forces were 8 N each, the net force would be zero; a cart already moving could continue at constant velocity in the ideal model.

Pause and explain

Can balanced forces act on an object that is moving?

Check your reasoning after answering

Yes. With zero net force its velocity does not change. It can remain at rest or continue moving at constant speed in a straight line.

Part 3 · Separate velocity direction from acceleration direction

An unbalanced force changes velocity. If force acts opposite current motion, speed can decrease; if force acts in the same direction, speed can increase. If force changes direction relative to motion, the path can bend. The net-force arrow alone does not specify the object’s current velocity.

Read the visual model

Does a leftward net force prove the cart is already moving left?

Visual reasoning — after your attempt

No. It specifies leftward acceleration. A right-moving cart could be slowing, and a resting cart could begin moving left.

A cart moving right while experiencing a leftward net force initially slows. It does not instantly move left. A motion arrow and a force arrow answer different questions and should have different labels.

Pause and explain

Does a leftward net force prove the cart is already moving left?

Check your reasoning after answering

No. It specifies leftward acceleration. A right-moving cart could be slowing, and a resting cart could begin moving left.

Part 4 · Use evidence to revise an incomplete diagram

If measured velocity changes, a zero-net-force diagram is incomplete or the chosen model has failed. Look for omitted friction, air resistance, or a push that changes over time. A slow or stationary appearance is not sufficient evidence of exact balance; use observations over a measured interval.

Read the visual model

A mismatch can expose a missing force or a method problem. Revise the diagram with a named, testable interaction rather than inventing an unexplained arrow just to make the arithmetic match.

Why is a diagram containing only the 6 N push incomplete for these records? What further measurement could test the revision?

Visual reasoning — after your attempt

It predicts 3 m/s² rather than the supplied 2 m/s². Measure or investigate opposing resistance, while also checking mass and acceleration measurement uncertainty.

A toy car slows after the hand releases it. The hand no longer supplies a forward push, while rolling resistance and air resistance can act against motion. Include these interactions before deciding the first law was contradicted.

Pause and explain

What does slowing motion suggest about a zero-net-force claim?

Check your reasoning after answering

Slowing is a change in velocity, so the zero-net-force claim needs revision. Identify interactions that can provide the nonzero net force.

Apply the whole unit · Explain a braking cart

Illustrative horizontal forces on a cart moving right: pull 6 N right and resistance 10 N left. Vertical support and weight balance. In a second trial, pull and resistance are both 6 N.

Your task

Draw both force diagrams, calculate horizontal net forces, and predict how the rightward velocity initially changes. Explain why neither diagram alone establishes the starting speed.

What your response should show

• Use the supplied observations and measurements accurately; include units where needed.
• Explain the science behind your answer, rather than giving only a number or label.
• Show a labeled model, calculation, or evidence comparison as requested.
• State an assumption or limitation and suggest a useful next measurement.

Response guidance — after your attempt

First trial: net force 4 N left, giving leftward acceleration and initially slowing rightward motion. Second trial: zero horizontal net force, so rightward velocity stays constant in the model. Equal vertical forces give no vertical acceleration. Starting speed requires motion information; it is not determined by force balance alone.

Scientific references and adapted teaching resources

• OpenSciEd 8.1 Contact Forces — public unit overview
• OpenSciEd 8.3 Forces at a Distance — public unit overview
• OpenSciEd instructional model — storyline routines
• Creative Commons Attribution 4.0 — middle-school material license

Parent / teacher teaching notes

Use distinct arrow colors for forces and motion. Ask acting on which object for every arrow. Revisit constant velocity versus rest before adding numerical acceleration. Adaptation credit: OpenSciEd middle-school teaching routines and public task structures by OpenSciEd, CC BY 4.0. This independent version adds original explanatory text, data and figures, terminology support and a home-study schedule. It does not reproduce the complete teacher editions or their classroom validation.

Net Force

F net = F₁ + F₂ + …

add forces pointing in the same direction; subtract forces pointing in opposite directions, to find the overall net force.

Worked examples

Avoid these mistakes

• Zero net force means zero acceleration, not necessarily zero velocity.

Words to know

net force

The overall force on an object after combining all individual forces, accounting for direction.

balanced forces

Forces that combine to a net force of zero, so motion doesn't change.

unbalanced forces

Forces that combine to a net force that isn't zero, causing acceleration.

model

A representation that explains selected features and makes predictions; it leaves other details out.

evidence

Relevant observations or records used to support or challenge a claim.

system boundary

The chosen limit separating the objects being studied from their surroundings.

assumption

A condition taken as true for a model or calculation and needing to be checked against the case.

When forces don't line up: combining forces at an angle

Adding forces is easy when they point along the same line — just add or subtract. But when two forces act at a right angle to each other, like a boat's engine pushing it forward while a current pushes it sideways, you combine them using the same Pythagorean theorem (a² + b² = c²) you use in geometry. A 3 N push east and a 4 N push north combine into a single net force of √(3² + 4²) = √25 = 5 N, angled diagonally between the two — physics and math turning out to be exactly the same tool.

Terminal velocity: when a falling object's forces finally balance

A skydiver jumping from a plane doesn't just keep speeding up forever — as speed increases, air resistance (drag) pushing upward grows stronger too, until it exactly equals the pull of gravity. At that point, net force becomes zero, and the skydiver falls at a constant, unchanging speed called terminal velocity — for a human in a typical belly-down position, that's about 195 km/h (54 m/s). It's a perfect real-world example of balanced forces: even though the skydiver is very much still moving, their forces are balanced and their velocity stops changing.

Cruising at 35,000 feet: an airplane in perfectly balanced flight

It's tempting to think that if an airplane's engines are running, it must be accelerating — but a plane cruising at a constant altitude and constant speed has perfectly balanced forces acting on it: lift balances weight (up vs. down), and engine thrust balances air drag (forward vs. backward). The engines work constantly not to speed the plane up, but simply to keep matching the drag trying to slow it down. This is the same principle as a car maintaining a steady highway speed — the engine isn't 'winning' against friction, just fighting it to a draw.

Free-body diagrams: the engineer's trick for tracking every force

When several forces act on an object at once, physicists and engineers draw a free-body diagram: a simplified sketch of just the object, with an arrow for every force acting on it, sized to show relative strength and pointed in the right direction. Bridge engineers use exactly this technique to make sure the combined forces of traffic, wind, and the bridge's own weight stay balanced by the strength of its cables and supports — an unbalanced design isn't just a physics homework mistake, it's a structure that could actually fail.

In the real world

Tug-of-war is a great example of unbalanced forces — the moment one side pulls harder, the rope and both teams accelerate that way.

Exit check

Could the top cart be moving right? Explain the relevant mechanism and one model limitation.

Interaction Pairs & Force Diagrams

Learning goals

• I can label each force’s source and receiver and keep interaction partners on separate diagrams.
• I can find a one-dimensional net force and distinguish it from velocity.

Readings for this unit

• A moving cart and a magnet across a gap

When a hand pushes a cart, the cart also pushes the hand. An interaction involves two objects affecting each other.

A force is a push or pull, and direction matters. A force diagram represents the forces acting on one chosen object; it is not a picture of every force in the room.

The length of an arrow can represent relative force size if a scale is stated. Begin with named objects and observations before adding arrows.

This prevents the common error of putting an equal-and-opposite interaction pair on the same object and concluding that all motion must be impossible.

An observation is what a person or instrument records. An inference is a proposed interpretation of those records. Keep the two separate: a graph can show a pattern while more than one mechanism might explain it. State the chosen system, or part of the world being studied, and its boundary before accounting for material or energy transfers.

Use each figure as a reasoning tool. Read its caption and labels, describe the pattern, then explain a cause. If the picture is schematic, use the stated numbers rather than measuring its printed size. Check whether its simplified conditions match the problem before applying a formula.

Build your understanding

Where this fits

This unit helps answer the chapter question: How can forces explain stability and changes in motion? Keep a notebook table with the question, evidence, explanation and one unresolved issue. Revise a previous model only when you can name the new evidence or idea that changes it.

Use three teaching sessions of about 40–45 minutes: first observe the phenomenon, draft a model and study Parts 1–2; next explain Parts 3–4 with the figures and evidence; then work the application, examples, practice, model revision and exit check. Divide any difficult part into smaller steps and revisit after feedback. Investigations, review and quizzes have separate sessions.

Part 1 · Name the object receiving each force

Write forces as “force of A on B.” B is the receiver and belongs in the diagram being constructed. Gravity from Earth, support from a table and a push from a hand come from different interactions. The support force is often called the normal force; normal here means perpendicular to the surface.

Read the visual model

Which receiver belongs in a cart diagram?

Visual reasoning — after your attempt

The cart. Forces exerted on the table or on Earth belong in separate diagrams, even though they are connected interactions.

For a cart resting on a horizontal table, draw Earth-on-cart downward and table-on-cart upward. The table is not doing nothing merely because the cart is still; it deforms slightly and supplies support.

Pause and explain

Which receiver belongs in a cart diagram?

Check your reasoning after answering

The cart. Forces exerted on the table or on Earth belong in separate diagrams, even though they are connected interactions.

Part 2 · Keep an interaction pair on separate objects

Newton’s third law describes a pair: A-on-B and B-on-A are equal in size and opposite in direction during the same interaction. They act on different objects. Equal interaction forces do not imply equal changes in motion when the object masses differ.

Read the visual model

Read a force name as source-on-receiver. That naming convention makes the third-law pair explicit. Balanced forces on one receiver are a different comparison from this equal-and-opposite pair between two receivers.

Which of these arrows belongs on the cart’s free-body diagram? Where does the other belong?

Visual reasoning — after your attempt

Only the hand-on-cart arrow belongs on the cart’s diagram. The equal cart-on-hand partner belongs on the hand’s diagram.

A person pushes a cart rightward. The cart pushes the person leftward at the same instant. The cart’s diagram contains the person-on-cart force; the person’s diagram contains the cart-on-person force.

Pause and explain

Why do the two forces not cancel in the cart diagram?

Check your reasoning after answering

Only one acts on the cart. Adding a force on the person to the forces on the cart mixes receivers and gives an invalid net force.

Part 3 · Find the net force on one receiver

Net force means the directional combination of forces on the chosen object. Along one line, choose a positive direction and subtract opposing forces. Balanced means the net force is zero, which allows rest or constant velocity; it does not require absence of forces.

Read the visual model

A free-body diagram includes forces on the chosen object and the sources of those forces. Then compare directions to determine net force. To predict speeding up or slowing down, also identify the current velocity.

If the cart is moving left when these forces act, what can happen to its speed initially?

Visual reasoning — after your attempt

It initially slows because rightward acceleration opposes its leftward velocity. It can later stop and reverse if the same net force persists.

A cart has a 6 N rightward push and a 2 N leftward resistive force, so its horizontal net force is 4 N rightward. Newton, written N, is the force unit. Its vertical support and weight can balance separately.

Pause and explain

Does a 4 N horizontal net force require an unbalanced vertical force?

Check your reasoning after answering

No. Horizontal and vertical comparisons are separate in this simple model. The vertical forces may balance while horizontal motion changes.

Part 4 · Test what a diagram predicts

A model should connect arrows with observed changes in motion and state missing information. A nonzero net force predicts acceleration, a change in velocity, rather than a particular speed without time and mass information. A force arrow is not automatically a velocity arrow.

Read the visual model

A prediction connects one representation to another. Translate arrows into net force, net force into acceleration, and acceleration into the slope of a velocity–time record. Check the assumptions about mass and the reference frame.

Would a claimed zero-net-force diagram agree with this graph? Explain the evidence rather than just saying no.

Visual reasoning — after your attempt

No. The velocity changes by 6 m/s in 3 s, so the supplied mass requires a nonzero net force of 4 N right. Zero net force would predict constant velocity in this model.

A cart moving rightward can have a leftward net force while slowing. Draw the motion arrow and force arrows with different labels. Use before-and-after position or speed records to check the interpretation.

Pause and explain

If a cart is moving right, must its net force point right?

Check your reasoning after answering

No. It can be slowing under a leftward net force or moving steadily with zero net force. Motion direction and net-force direction are different quantities.

Apply the whole unit · Two diagrams for one push

Illustrative case for analysis, not measurements collected by the learner. In an illustrative interaction a hand pushes a cart rightward with 8 N. Floor resistance on the cart is 3 N leftward.

Your task

State the cart’s horizontal net force and the cart-on-hand force; explain why they differ. Use the supplied conditions, show intermediate reasoning and distinguish the supported conclusion from a broader claim.

What your response should show

• Use the specified quantities and units.
• Explain an intermediate step or comparison.
• Connect the pattern to the physical mechanism.
• State an assumption or limitation.

Response guidance — after your attempt

1. Choose the cart as receiver for the net-force sum.

2. Subtract 3 N left resistance from 8 N right push: net force is 5 N right.

3. For the hand–cart interaction, the cart-on-hand partner is 8 N left; its receiver is the hand.

4. Do not subtract the partner on the hand when adding forces on the cart.

The cart’s net force is 8−3=5 N rightward. The cart pushes the hand with 8 N leftward as the partner of the hand-on-cart interaction. The 3 N floor force belongs to another interaction. Net force on the cart and the pair force on the hand answer different questions.

Scientific references and adapted teaching resources

• OpenSciEd 8.1 Contact Forces — public unit overview
• OpenSciEd 8.3 Forces at a Distance — public unit overview
• OpenSciEd instructional model — storyline routines
• Creative Commons Attribution 4.0 — middle-school material license

Parent / teacher teaching notes

Ask the learner to describe the phenomenon before naming a formula. Discuss each checkpoint after an attempt. Use the supplied records when equipment is unavailable and label them illustrative rather than actual measurements. The final explanation should identify a mechanism and a limit, with different defensible wording accepted. Adaptation credit: OpenSciEd middle-school teaching routines and public task structures by OpenSciEd, CC BY 4.0. This independent version adds original explanatory text, data and figures, terminology support and a home-study schedule. It does not reproduce the complete teacher editions or their classroom validation.

Worked examples

Avoid these mistakes

• Interaction partners act on different objects; acceleration depends on the net force on each object separately.
• Do not treat a schematic drawing as a scale measurement or a universal guarantee.
• Keep the stated comparison conditions and measurement boundary explicit.
• A correct numerical result needs units and an explanation of its meaning.

Words to know

interaction

Mutual influence between named objects.

force diagram

Representation of the forces acting on one chosen object.

receiver

Object on which a named force acts.

normal force

Contact support force perpendicular to a surface.

net force

Directional sum of forces on one object.

model

A representation that explains selected features and makes predictions; it leaves other details out.

evidence

Relevant observations or records used to support or challenge a claim.

system boundary

The chosen limit separating the objects being studied from their surroundings.

assumption

A condition taken as true for a model or calculation and needing to be checked against the case.

In the real world

In an illustrative interaction a hand pushes a cart rightward with 8 N. Floor resistance on the cart is 3 N leftward.

Exit check

With 3 N leftward friction, what is the cart’s horizontal net force? Explain the relevant mechanism and one model limitation.

Gravity & Friction

Learning goals

• I can describe how gravity depends on mass, and calculate weight using W = mg.
• I can describe how friction opposes motion and identify factors that change its strength.
• I can argue from mass/distance evidence and distinguish free fall from significant drag.

Readings for this unit

• A moving cart and a magnet across a gap

Gravity is an attractive interaction between objects with mass. Its strength depends on the masses and separation. Near Earth, the ideal free-fall acceleration is approximately 9.8 m/s² when air resistance is negligible. A heavier object has greater weight but also greater inertia, giving the same ideal acceleration. Different shapes can fall differently in air because drag affects their motion. State the model assumptions rather than claiming that every real falling object has identical acceleration.

That pull gives every object a weight, calculated as weight = mass × g, or W = mg, and measured in newtons (N) like any other force. It's tempting to treat weight and mass as the same thing, but they're genuinely different. Mass is the amount of matter packed into an object (measured in kilograms), and it never changes no matter where the object goes. Weight is the force of gravity acting on that mass, so it changes whenever gravity changes.

The Moon makes the difference vivid. An astronaut has the exact same mass on the Moon as on Earth — she's made of the same amount of matter either way. But the Moon is much smaller, so its gravity is only about a sixth as strong, and she therefore weighs only about a sixth as much. She could bound across the surface in giant leaps not because she lost any mass, but because the force pulling her down got weaker.

Friction resists relative sliding, or the tendency to slide, between contacting surfaces. It can depend on surface materials, microscopic contact interactions, normal force, and whether surfaces are already sliding. Surface roughness alone is not a universal predictor. Friction may oppose a sliding box’s motion, but static friction can provide the forward force that helps a person walk. Describe the contact and relative motion before assigning a friction arrow.

Air resistance is a specific kind of friction — the push of air against anything moving through it. It's the reason a feather drifts slowly to the floor while a rock plummets, even though gravity pulls on both. The feather's large surface area and tiny mass let air resistance hold it back far more, relative to its weight. This leads to a famous surprise: in a vacuum with no air at all, a feather and a rock dropped together actually hit the ground at the exact same instant — a result astronauts have demonstrated on the airless Moon.

Mass measures inertia and is measured in kilograms. Weight is gravitational force and is measured in newtons. Near Earth's surface weight is approximately mass×9.8 N/kg, although using 10 N/kg is suitable when a question says to approximate.

Friction depends on contact materials, surface conditions and the forces pressing surfaces together. It can slow sliding, but it also lets shoes grip the ground and tires push a road. Static friction can act without visible sliding.

Gravity acts without contact and depends on the masses and their separation. In everyday falling objects, air resistance can hide the simple gravitational pattern. Compare shapes and release conditions before assuming a heavier object always has a larger downward acceleration.

A thrown ball also keeps accelerating downward while it is rising. After it leaves the hand, gravity changes its vertical velocity; an initial horizontal velocity continues in an ideal model without drag. The trajectory is the ball’s curved path through space. At its highest point the vertical component of velocity is zero for an instant, while gravity and any horizontal component of velocity are still present. Study the labeled throw below before using the graphs.

Weight

W = mg

m is mass in kg; g is the acceleration due to gravity (9.8 m/s² on Earth); W (weight) comes out in newtons (N).

Worked examples

Avoid these mistakes

• Friction is not always harmful, and mass is not measured in newtons.

Words to know

gravity

An attractive force between any two objects with mass; bigger masses and closer distances create a stronger pull.

weight

The force of gravity on an object's mass, calculated as weight = mass × g (W = mg).

mass

The amount of matter in an object, measured in kilograms; it doesn't change no matter where the object is.

friction

A force that opposes motion between two surfaces in contact.

air resistance

A type of friction caused by air pushing against a moving object.

model

A representation that explains selected features and makes predictions; it leaves other details out.

evidence

Relevant observations or records used to support or challenge a claim.

system boundary

The chosen limit separating the objects being studied from their surroundings.

assumption

A condition taken as true for a model or calculation and needing to be checked against the case.

Newton's universal law of gravitation: an inverse-square rule

Newton didn't just say gravity exists — he wrote down exactly how it works: F = G(m₁m₂)/r², where m₁ and m₂ are the two objects' masses, r is the distance between their centers, and G is a very small fixed number called the gravitational constant. The 'r²' in the denominator makes this an inverse-square law: double the distance between two objects, and the gravitational force between them drops to one-quarter as strong, not one-half. This is also why gravity between you and a nearby friend is technically real but far too weak to notice — it takes a planet-sized mass for the pull to become obvious.

Henry Cavendish 'weighs the Earth'

Newton's equation includes G, the gravitational constant, but Newton himself never measured its actual value. In 1798 — over a century later — British scientist Henry Cavendish used an extremely sensitive torsion balance (two small lead balls on a thin wire, twisted by the gravitational pull of larger lead spheres) to measure G directly for the first time. Once G was known, scientists could finally calculate Earth's actual mass using nothing but the strength of its gravity at the surface — earning Cavendish's experiment the nickname 'weighing the Earth.'

Static vs. kinetic friction: why the first push is the hardest

Try shoving a heavy couch across a floor and you'll notice something odd: it takes a big effort just to get it moving, but once it's sliding, keeping it moving takes noticeably less force. That's because static friction (between surfaces that aren't yet sliding) is almost always stronger than kinetic friction (between surfaces already in motion) — microscopic surface bumps have to be 'unstuck' from each other before sliding can even begin. This is exactly why cars equipped with anti-lock brakes (ABS) stop faster than cars with locked, skidding tires: rolling tires stay in the stronger static-friction regime, while skidding tires fall into weaker kinetic friction.

The hammer, the feather, and the Moon

In 1971, Apollo 15 commander David Scott performed a now-famous live television demonstration on the airless Moon: he dropped a geologist's hammer and a falcon feather at the same moment, from the same height. With no atmosphere to create air resistance, both hit the lunar surface at exactly the same instant — visual proof of something Galileo had predicted centuries earlier using thought experiments alone. It's a perfect real-world bookend to the feather-and-rock idea from this unit's explanation: gravity accelerates every mass equally, and it's air resistance alone (not gravity) that normally makes light, spread-out objects fall more slowly on Earth.

In the real world

Engineers design tire treads to increase friction with the road, helping cars brake safely, especially in the rain.

Exit check

Why does the object retain its mass on the Moon? Explain the relevant mechanism and one model limitation.

Electric & Magnetic Forces

Learning goals

• I can use a field model to describe forces between objects that are not touching.
• I can design a fair test of how magnet separation affects its interaction with another object.
• I can interpret coil-strength records and model stored energy in changed field arrangements.

Readings for this unit

• A moving cart and a magnet across a gap

Some forces act without contact. A charged comb can attract small paper pieces, and a magnet can attract certain metals. A field model describes how an object can exert a force on another object at a distance. Field lines are a drawing tool; they are not physical strings.

Electric charges can be positive or negative. Like charges repel and unlike charges attract. An electrically neutral object can sometimes be attracted when its charges shift slightly in response to a nearby charged object. Magnetic poles also come in two kinds: like poles repel and unlike poles attract. An ordinary magnet attracts iron but does not attract every metal.

The strength of an interaction depends on factors such as separation and the objects involved. To investigate distance, keep the same magnet, test object, surface, and orientation while varying separation. Repeat measurements. A stronger pull at a smaller separation supports a relationship; a single trial does not establish a universal numerical law.

A field represents how an object could experience a force at a location without touching the source. Field diagrams use arrows to show direction and density of lines to suggest relative strength. The drawn lines are a model, not material strings filling space.

Electric forces can attract or repel. Neutral objects can sometimes be attracted because their charges shift slightly within the material. Magnets have north and south poles, and ordinary cutting of a magnet makes two smaller magnets, each with both poles.

A fair comparison of field strength changes distance while keeping the sources and measurement method the same. Observe from multiple positions and repeat. Electric and magnetic forces are related but are not interchangeable: a stationary electric charge and a permanent magnet require different introductory models.

Build your understanding

Where this fits

This unit helps answer the chapter question: How can forces explain stability and changes in motion? Keep a notebook table with the question, evidence, explanation and one unresolved issue. Revise a previous model only when you can name the new evidence or idea that changes it.

Use three teaching sessions of about 40–45 minutes: first observe the phenomenon, draft a model and study Parts 1–2; next explain Parts 3–4 with the figures and evidence; then work the application, examples, practice, model revision and exit check. Divide any difficult part into smaller steps and revisit after feedback. Investigations, review and quizzes have separate sessions.

Part 1 · Describe an interaction without contact

Electric and magnetic forces can act across a gap. A field model describes how a source can influence other objects nearby. Arrows or lines represent field direction and patterns, not material threads connecting objects. The strength of an interaction can depend on separation and the properties of both objects.

Read the visual model

What does a field drawing help explain?

Visual reasoning — after your attempt

It represents where and in what direction a source can influence suitable test objects. It does not show physical strings or prove identical force at every location.

A magnet can attract a suitable object before touching it. Compare the same objects at different distances. Do not assume every shiny metal is strongly attracted, because magnetic response depends on the material.

An electric field tells us the direction in which a small positive test charge would be pushed. Around a positive source the arrows point outward; around a negative source they point inward. A negative test charge feels electric force in the opposite direction. A field arrow is therefore not automatically the force arrow for every object. The test charge is assumed small enough not to significantly change the source arrangement.

Read a field drawing in three steps: identify the source, identify the test object, then translate the field direction into the force on that test object. Arrows describe influence at a location; they do not mean that a charge must follow a drawn line or move at constant speed. We use a separate field model for magnets below.

Pause and explain

What does a field drawing help explain?

Check your reasoning after answering

It represents where and in what direction a source can influence suitable test objects. It does not show physical strings or prove identical force at every location.

Part 2 · Track charges rather than creating them

Objects can acquire net electric charge when electrons transfer. Like net charges repel and unlike net charges attract in simple comparisons. A charged object can also attract a neutral object by rearranging its charges, so attraction alone does not prove the second object has opposite net charge.

Read the visual model

Does attraction prove both objects have opposite net charges?

Visual reasoning — after your attempt

No. A charged object can attract a neutral object through polarization. Repulsion in a suitable simple test can be stronger evidence of like net charges.

A charged plastic object can attract small neutral paper pieces through polarization. Draw negative and positive regions shifting slightly within the neutral piece, while its total charge remains balanced. This differs from transferring electrons to create a net charge.

For a simple two-charge comparison, force acts along the line joining the charges. When both charges are positive, each force points away from the other charge; the same happens when both are negative. Opposite signs give forces toward one another. The equal and opposite forces act on different objects, consistent with Newton’s third law. Do not add them as two forces on one receiver.

Polarization means a small separation or shift of positive and negative charge within material. In paper, bound electron clouds can shift slightly relative to positive atomic regions. The near-side attraction can be stronger than the far-side repulsion while the paper still has equal total positive and negative charge. The diagram exaggerates the shift to make it visible; it does not show free protons crossing the paper.

Electric charge is a property of matter responsible for electric interactions. Electrons carry negative charge and protons positive charge. An object with balanced total amounts is neutral. Transferring electrons can give one object an excess (net negative charge) and leave another with a deficit (net positive charge). In ordinary rubbing, it is electrons that transfer; rubbing does not manufacture new charge.

Pause and explain

Does attraction prove both objects have opposite net charges?

Check your reasoning after answering

No. A charged object can attract a neutral object through polarization. Repulsion in a suitable simple test can be stronger evidence of like net charges.

Part 3 · Recognize magnetic poles and material response

Magnets have north and south poles. Like poles repel and unlike poles attract in the classroom model. Cutting a bar magnet produces smaller magnets with both poles, rather than isolated poles. Electric charge and magnetic pole are different concepts and should not be mixed in one labeling system.

Read the visual model

Can attraction to a steel piece identify a magnet’s north pole?

Visual reasoning — after your attempt

No. Either pole can attract a suitable steel piece. Compare with a known marked magnet or another appropriate reference to identify the pole.

Compare two bar magnets using their marked ends. A steel piece may be attracted by either end because it responds to the field. Attraction to steel does not identify which pole faces it. Keep magnets away from items they could damage and avoid small powerful magnets.

Now read the green magnetic-field diagram. Outside a bar magnet the conventional direction is north to south; through the magnet it returns from south to north, forming closed loops. A compass has its own magnetized needle: the needle’s north-seeking end points along the local field. This direction convention is different from the electric positive-test-charge convention. North and south are magnetic pole names, not electric plus and minus signs.

The direction changes with position around the magnet, so a compass on one side need not point the same way as a compass elsewhere. The chosen field lines never cross in a simple field drawing because the field has one direction at a location. The iron-filings photograph shows a physical response and approximate pattern; the labeled diagram supplies directions and the closed-loop model that the photograph alone cannot show.

Cutting a magnet does not produce a lone north pole and a lone south pole. Each piece remains a magnet with both poles. This is an explanation model, not a suggested cutting activity. Attraction to ordinary steel is also different from comparing two marked magnetic poles: either end of a bar magnet can attract suitable steel.

Pause and explain

Can attraction to a steel piece identify a magnet’s north pole?

Check your reasoning after answering

No. Either pole can attract a suitable steel piece. Compare with a known marked magnet or another appropriate reference to identify the pole.

Part 4 · Make a fair distance investigation

Keep the objects, orientation, and measurement method comparable while changing separation. Record a measurable outcome such as force from supplied sensor data. Counting attracted items can be a useful rough comparison but also depends on arrangement and cannot give a universal force law.

Read the visual model

What must stay comparable when testing the effect of separation?

Visual reasoning — after your attempt

Use the same source, test object, orientation, and measuring method. Change separation deliberately and repeat observations to check variation.

A supplied data set may show weaker force at greater gaps. Conclude only for the tested pair and range. If orientation changes with distance, the investigation has changed two factors and cannot isolate separation as clearly.

In the supplied graph the independent variable is gap, measured in centimetres (cm). The vertical axis is force in millinewtons (mN): 1 mN is one thousandth of a newton. The three points are 12, 6 and 3 mN at 1, 2 and 3 cm. Read the trend and units before making a claim. The line segments help the eye connect records; they are not measurements between the points. These mN values are a separately labeled illustrative dataset; the initial evidence case used arbitrary response units rather than a calibrated force scale.

From 1 to 2 cm this particular supplied force halves, but that does not prove a universal distance rule for magnets. Magnet shape, orientation and the test object affect the relationship. The inverse-square rule for ideal isolated electric point charges is a different model; it must not be silently applied to bar magnets. Repeat each gap and record variation before drawing a stronger conclusion.

Pause and explain

What must stay comparable when testing the effect of separation?

Check your reasoning after answering

Use the same source, test object, orientation, and measuring method. Change separation deliberately and repeat observations to check variation.

Field strength and stored energy

The gap between interacting objects can affect force, while rearranging them can also change stored energy in the two-object system.

Current means the rate of electric-charge flow through a circuit, measured in amperes (A). A coil is wire wound into repeated turns. Together they make a useful cause-and-effect model: current through the turns produces a magnetic field, so the coil can act as an electromagnet. An iron core may strengthen its response. Increasing turns and increasing current are different changes and must be tested separately. The labeled schematic shows the idea; no homemade energized coil is needed.

A coil is wire arranged in turns. Current through a coil can produce a magnetic influence, making an electromagnet. Supplied coil records can test how turns, current and separation affect a response. A clip-lift count is a proxy, an indirect indicator, for performance and is not force in newtons. Do not infer a universal proportional rule from a short table or change coil turns and current together and call that an isolated comparison.

Potential energy belongs to the interacting arrangement, not a mysterious substance held by one object alone. Pushing repelling magnets closer can require work on the two-magnet system and increase its stored energy; releasing can transfer energy into motion and other paths. For an attractive pair, separating it can similarly require work and increase the store. Keep orientation and reference condition explicit before comparing arrangements.

Electric, magnetic and gravitational examples can all be modeled as forces acting across gaps with arrangement-dependent potential energy. A charged balloon and nearby hair illustrate electric interactions without creating new charge; charges are transferred or redistributed. A field model uses responses of a suitable test object to represent influence at different locations. It is not a line of material connecting the objects. A claim that effects require air can be tested against magnetic interaction across a vacuum, distinct from ordinary sound.

Follow the speaker cutaway: a fixed magnet provides a field, and current in a movable coil makes an interaction that can push the coil. The cone is attached to the coil. Changing current changes the force and lets the cone vibrate; the cone then makes the surrounding air vibrate. This is the chain that connects electric input, magnetic interaction, motion and sound. A diagram should explain the parts and connections, not merely label the word magnet.

For the additional illustrative turns investigation, current is held fixed and the trial clip counts are 3, 3, 4 for 10 turns; 5, 5, 6 for 20 turns; and 6, 7, 6 for 30 turns. Means are about 3.33, 5.33 and 6.33. Counts vary, so increased mean lifting performance does not prove exact proportionality or give force in newtons. These records test turns; the supplied evidence table separately tests current using matched turns.

Evidence to interpret

Coil turns	Current (A)	Clip-lift count (illustrative)
10	0.2	3
20	0.2	5
20	0.3	7

Use this method safely

Use supplied coil data; do not wind or energize homemade coils, use mains electricity or handle small powerful magnets. Any optional magnet observations use large classroom magnets with adult guidance.

Explain and apply

Use the coil records to ask an investigable question about each changed factor. Then draw a repelling-magnet energy model and contrast the work needed for separating an attractive pair.

• Isolate turns/current using the proper records.
• Distinguish proxy measurement from force.
• Model potential energy, orientation and work.

Reasoning and marking guidance — after your attempt

At fixed 0.2 A, changing turns from 10 to 20 raises the illustrative lift count 3→5; at fixed 20 turns, current 0.2→0.3 A changes it 5→7. These proxy outcomes support questions and a bounded trend, not a direct calibrated force law. Moving repelling magnets closer requires input work; separating attracting magnets can require input work instead. Energy is associated with the pair’s arrangement and can transfer into later motion.

Apply the whole unit · Interpret an interaction experiment

Illustrative measurements for the same magnet and test piece with fixed orientation: gaps 1, 2, and 3 cm give forces 0.8, 0.3, and 0.1 N. In a separate case, a charged rod attracts uncharged paper.

Your task

Graph the magnetic results, describe the supported trend, and explain why the data cannot establish a universal formula. Draw a charge-redistribution model for the paper without giving it an unsupported net charge.

What your response should show

• Use the supplied observations and measurements accurately; include units where needed.
• Explain the science behind your answer, rather than giving only a number or label.
• Show a labeled model, calculation, or evidence comparison as requested.
• State an assumption or limitation and suggest a useful next measurement.

Response guidance — after your attempt

The force decreases across the tested gaps, but three measurements for one setup do not establish a universal inverse-distance formula. Label axes and units and keep orientation fixed. In the paper, charges redistribute so the nearer region interacts more strongly; the total can remain neutral. Attraction alone is insufficient to claim opposite net charge.

Scientific references and adapted teaching resources

• OpenSciEd 8.1 Contact Forces — public unit overview
• OpenSciEd 8.3 Forces at a Distance — public unit overview
• OpenSciEd instructional model — storyline routines
• Creative Commons Attribution 4.0 — middle-school material license
• OpenSciEd 8.3 Lesson 3 — Framing Hypotheses (public handout)
• OpenStax: electric field direction and drawing conventions
• OpenStax: magnetic fields, closed loops and compass direction

Parent / teacher teaching notes

Keep electric charge signs and magnetic pole labels separate. Use the supplied measurements for quantitative work. Avoid small powerful magnets and any experiment involving wall electricity. Adaptation credit: OpenSciEd middle-school teaching routines and public task structures by OpenSciEd, CC BY 4.0. This independent version adds original explanatory text, data and figures, terminology support and a home-study schedule. It does not reproduce the complete teacher editions or their classroom validation. The illustrated study “Field strength and stored energy” is required curriculum content. Teach it before the associated application or distribute it over the assigned unit sessions; it is not an optional enrichment. Its practice/review/quiz response provides evidence of understanding. Model/data work must not be recorded as an actual physical experiment.

Worked examples

Avoid these mistakes

• A neutral object's attraction does not mean it had a net charge beforehand.

Words to know

field

A model describing the influence an object has in the space around it.

repel

Push apart.

controlled variable

A factor kept the same during a comparison.

model

A representation that explains selected features and makes predictions; it leaves other details out.

evidence

Relevant observations or records used to support or challenge a claim.

system boundary

The chosen limit separating the objects being studied from their surroundings.

assumption

A condition taken as true for a model or calculation and needing to be checked against the case.

electric charge

A property responsible for electric interactions. In this introductory model, excess electrons give an object net negative charge and an electron deficit can give it net positive charge.

polarization

A slight shift or separation of positive and negative charge within material; the whole object can remain electrically neutral.

current

The rate of electric-charge flow through a chosen part of a circuit, measured in amperes (A).

coil

Wire wound into repeated turns. Current through the turns produces a magnetic field.

electromagnet

A magnet produced by electric current, often using a wire coil and an iron core.

compass

An instrument with a magnetized needle; its north-seeking end points along the local magnetic field.

In the real world

Exit check

Why not change the test magnet size between positions? Explain the relevant mechanism and one model limitation.

Pressure & Buoyant Forces

Learning goals

• I can describe fluid pressure and how contact area affects pressure.
• I can explain floating using upward buoyant force and downward weight.

Readings for this unit

• A moving cart and a magnet across a gap

Liquids and gases are fluids: they flow and push on surfaces. Pressure is force per unit area. If the same force acts on a smaller area, pressure is greater. That is why snowshoes can reduce how far a person sinks into snow: they spread weight over more area.

Within a stationary liquid, pressure increases with depth because more fluid lies above. Fluid pushes in all directions. The greater pressure on the lower parts of a submerged object produces a net upward force called buoyancy. This is different from the object’s weight, which acts downward.

An object floating steadily has equal upward buoyant force and downward weight. Zero net force does not mean that no forces act. When an object sinks, the forces may initially be unbalanced downward; fluid resistance also matters once it moves. Changing the shape of a sealed boat hull can increase the volume of water displaced without greatly increasing mass.

Pressure is force spread over an area. For the same force, a smaller area produces greater pressure. Within a resting liquid, pressure generally increases with depth because more liquid above contributes weight.

A fluid pushes on an immersed object from all sides. The greater pressure at the bottom than at the top creates a net upward buoyant force. In an introductory model, that force equals the weight of fluid displaced by the submerged part.

A floating object reaches a balance between its weight and buoyant force. A heavier load makes a boat sit deeper, displacing more water. If water enters the boat, its average density may rise and the available buoyancy may no longer support it. Keep depth, volume and density separate in the explanation.

Worked examples

Avoid these mistakes

• Buoyant force exists on sinking objects as well as floating objects.

Words to know

pressure

Force acting per unit area.

fluid

A liquid or gas that flows.

buoyant force

The net upward force a fluid exerts on an immersed object.

model

A representation that explains selected features and makes predictions; it leaves other details out.

evidence

Relevant observations or records used to support or challenge a claim.

system boundary

The chosen limit separating the objects being studied from their surroundings.

assumption

A condition taken as true for a model or calculation and needing to be checked against the case.

In the real world

Exit check

Which displacement must be smaller in the ideal lift? Explain the relevant mechanism and one model limitation.