Newton's Three Laws of Motion — Drills · Grade 8 Science

Chapter: Motion

What these drills covers

By the end of Newton's Three Laws of Motion you should be able to:

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

Vocabulary used: inertia, Newton's first law, Newton's second law, Newton's third law, newton (N), model, evidence, system boundary, assumption.

Problem 1

Newton's first law says an object keeps its motion unless acted on by
  1. gravity only
  2. an unbalanced force
  3. friction only
  4. its own mass
Answer and explanation

an unbalanced force

Without a net force, velocity does not change — this is inertia.

Problem 2

A net force of 24 N acts on a 6 kg mass. What is the acceleration in m/s²?
Answer and explanation

4 m/s²

a = F/m = 24/6 = 4 m/s².

Problem 3

What net force in newtons gives a 3 kg object an acceleration of 5 m/s²?
Answer and explanation

15 N

F = ma = 3 × 5 = 15 N.

Problem 4

A 12 N net force accelerates an object at 4 m/s². What is its mass in kilograms?
Answer and explanation

3 kg

m = F/a = 12/4 = 3 kg.

Problem 5

A swimmer pushes water backward and moves forward. This illustrates
  1. the first law
  2. the second law
  3. the third law
  4. conservation of mass
Answer and explanation

the third law

The third law: the push on the water is matched by an equal and opposite push on the swimmer.

Problem 6

Two objects receive the same net force. The one with greater mass
  1. accelerates more
  2. accelerates less
  3. does not accelerate
  4. gains more force
Answer and explanation

accelerates less

Acceleration is inversely proportional to mass for a given force.

Problem 7

Why do the equal and opposite forces of the third law not simply cancel and leave nothing moving?
Answer and explanation

Because the two forces act on different objects. When a swimmer pushes the water backward, that force acts on the water, and the water's push forward acts on the swimmer. Forces only cancel when they act on the same object, so each object accelerates according to the single force acting on it and its own mass.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 8

Explain why passengers lurch forward when a bus brakes sharply.
Answer and explanation

The passengers were moving with the bus and, by the first law, continue at that velocity unless a force acts on them. The brakes act on the bus, slowing it, but the passengers keep moving forward until a seatbelt, seat or handrail supplies the force that slows them too. It is the absence of a force on the passengers, not a force throwing them forward, that produces the lurch.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

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