Gravity & Friction — Practice · Grade 8 Science

Chapter: Forces

What this practice covers

By the end of Gravity & Friction you should be able to:

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.

15 questions, numeric, multiple choice and written answer, each with a worked explanation.

Vocabulary used: gravity, weight, mass, friction, air resistance, model, evidence, system boundary, assumption.

Problem 1

Find the weight of a 50 kg student on Earth. Use g = 9.8 m/s² and W = mg.

Answer and explanation

490 N

W = mg = 50 × 9.8 = 490 N.

Problem 2

A rock has a mass of 12 kg. What is its weight on Earth? (g = 9.8 m/s²)

Answer and explanation

117.6 N

W = mg = 12 × 9.8 = 117.6 N.

Problem 3

Which stays the same no matter where in the universe an object is?

its weight
its mass
the pull of gravity on it
none of these

Answer and explanation

its mass

Mass is the amount of matter in an object and doesn't change with location, unlike weight.

Problem 4

Which surface pairing would create the MOST friction?

ice on ice
rubber on dry pavement
oiled metal on metal
polished wood on polished wood

Answer and explanation

rubber on dry pavement

Rough, grippy surfaces like rubber on dry pavement create much more friction than smooth or lubricated ones.

Problem 5

Explain why a feather falls slower than a rock through air, even though gravity pulls on both.

Answer and explanation

Air resistance pushes up on the feather much more relative to its weight, because of its large surface area and very low mass, which slows its fall a lot. In a vacuum with no air resistance, a feather and a rock would actually fall at the same rate.

Problem 6

An object weighs 98 N on Earth. Using W = mg with g = 9.8 m/s², find its mass.

Answer and explanation

10 kg

m = W ÷ g = 98 ÷ 9.8 = 10 kg.

Problem 7

Using g≈10 N/kg, find the weight of a 3 kg object and explain why its mass stays the same on the Moon. Explain the reasoning in your own words.

Answer and explanation

30 N on Earth; the same 3 kg mass has a smaller weight on the Moon.

Weight near Earth is 3×10=30 N.

A different gravitational field changes force, but not the object's amount of matter or inertia.

Response criteria

Use the stated conditions.
Show the evidence or calculation.
Explain the conclusion and any stated limitation.

Problem 8

A box slides right across a rough floor. State the direction of kinetic friction from the floor. Show the reasoning, not just the conclusion.

Hints

The book's weight is still present even though it is not falling.

Answer and explanation

Left, opposing the box's sliding relative to the floor.

Problem 9

An object of mass 3 kg has weight 30 N in a simplified g=10 N/kg model. Would its mass change on the Moon?

Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Mass remains 3 kg; weight changes with the local gravitational field.

Problem 10

Use the table to construct a gravitational claim about mass and separation. Then explain why a feather and stone can fall differently in air without proving different ideal free-fall acceleration.

Hints

Use the illustrated study titled Gravity: masses, distance and free fall. Keep the observations, model and conditions separate.

Answer and explanation

The force rises from 8 to 16 relative units when one mass doubles at fixed separation, and falls to 2 when separation doubles at the reference masses. These model comparisons support attractive, mass- and distance-dependent interaction under the stated conditions. Feather/stone differences in air also involve drag and inertia; an ideal negligible-drag comparison does not depend on mass in the same way. Separate receivers for the interaction pair and name the model limit.

Response criteria

Compare one changed factor at a time.
Explain attraction and appropriate force receivers.
Distinguish gravity-only motion from significant drag.

Problem 11

In the no-drag throw with initial horizontal velocity 6 m/s, what are horizontal velocity, vertical velocity and vertical acceleration at the top? Does gravity stop acting?

Answer and explanation

Horizontal velocity 6 m/s; vertical velocity 0; vertical acceleration −10 m/s². Gravity continues acting downward.

Separate the two velocity components.

Distinguish zero vertical velocity from zero acceleration.

Response criteria

State all three values with units.
Keep gravity downward.
Avoid claiming zero total velocity.

Problem 12

Keep upward launch speed 10 m/s and g = 10 m/s². Change only horizontal speed to 2 m/s. Predict flight time, maximum height and range back to release height.

Answer and explanation

2 s; 5 m; 4 m respectively, in the same no-drag model.

Vertical motion is unchanged.

Range = 2 m/s × 2 s = 4 m.

Response criteria

Keep flight time and height unchanged.
Calculate range with units.
State no drag and same return height.

Problem 13

Two balls start 5 m above the ground with zero vertical velocity; one is dropped and the other launched horizontally. In a no-drag model, which lands first? Would adding an upward launch component keep this comparison the same?

Answer and explanation

They land together (after 1 s using g = 10 m/s²). An upward component changes initial vertical velocity, so the original comparison no longer applies.

Horizontal motion does not change the shared vertical acceleration.

Distinguish horizontal from upward launch.

Response criteria

Use equal-height and vertical-velocity conditions.
State the no-drag limit.
Explain why an upward throw is different.

Problem 14

Two quantities with different units that everyday speech runs together. The kilogram figure is unchanged by location; the newton figure depends on the gravitational field strength where you are.

For this unit’s illustrated case, what remains unchanged when the same object moves to a weaker gravitational field?

Answer and explanation

Its mass remains the same if no matter is added or lost. Its gravitational weight decreases because the local field is weaker.

A 2 kg object has weight about 19.6 N near Earth using 9.8 N/kg. On a location with weaker gravitational field, the same object retains 2 kg mass but has lower weight. State which quantity a scale reports.

Its mass remains the same if no matter is added or lost. Its gravitational weight decreases because the local field is weaker.

Response criteria

Use the named labels, directions or supplied quantities accurately.
Connect the visual evidence to the scientific mechanism or calculation.
State the relevant model conditions or limitation.

Problem 15

For a no-drag ball launched upward at 8 m/s and horizontally at 3 m/s, use g = 10 m/s². At 0.8 s state both velocity components and acceleration, then predict horizontal range when it returns to release height.

Answer and explanation

At the top after 0.8 s: horizontal velocity 3 m/s, vertical velocity 0, vertical acceleration −10 m/s². Total flight is 1.6 s, so range is 4.8 m under the same-height ideal model.

Vertical velocity loses 10 m/s each second, reaching zero after 0.8 s.

Symmetric ideal same-height return takes twice that time; multiply by horizontal speed.

At the top after 0.8 s: horizontal velocity 3 m/s, vertical velocity 0, vertical acceleration −10 m/s². Total flight is 1.6 s, so range is 4.8 m under the same-height ideal model.

Response criteria

Use the named labels, directions or supplied quantities accurately.
Connect the visual evidence to the scientific mechanism or calculation.
State the relevant model conditions or limitation.

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