Newton's Three Laws of Motion — Drills · Grade 12 Science

Chapter: Forces, Rotation & Equilibrium

What these drills covers

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

6 questions, numeric, multiple choice 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).

Problem 1

A net force of 24 N acts on an 8 kg object. Find its acceleration.
Answer and explanation

3 m/s²

a = ΣF ÷ m = 24 ÷ 8 = 3 m/s².

Problem 2

What net force is needed to accelerate a 3 kg ball at 5 m/s²?
Answer and explanation

15 N

ΣF = ma = 3 × 5 = 15 N.

Problem 3

Which property of an object is the direct measure of its inertia?
  1. Its mass
  2. Its weight
  3. Its acceleration
  4. Its speed
Answer and explanation

Its mass

Mass measures inertia — the more mass an object has, the more it resists a change in its motion.

Problem 4

A rifle fires a bullet forward and recoils backward against the shooter's shoulder. This is an example of Newton's…
  1. second law
  2. third law
  3. law of gravitation
  4. first law
Answer and explanation

third law

The rifle pushes the bullet forward; by Newton's third law the bullet pushes the rifle backward with equal force — the recoil.

Problem 5

State Newton's first law of motion and give one everyday example of it.
Answer and explanation

Newton's first law says an object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless a nonzero net force acts on it. Example: a coffee cup slides forward on the dashboard when the car brakes, because the cup tends to keep moving even though the car slows down.

Problem 6

Two horizontal forces act on a 4 kg cart on frictionless ice: 30 N east and 6 N west. Find its acceleration.
Answer and explanation

6 m/s²

Net force = 30 − 6 = 24 N east. a = ΣF ÷ m = 24 ÷ 4 = 6 m/s².

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