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Space, Time and Motion

Newton's laws of motion

  • 3 things to know
  • Components 01, 02 and 03

Overview

Recall questions on Newton's first, second and third laws, F = ma from the momentum form, resultant force, car and football calculations, why third law pairs never cancel, the partner of a book's weight, a passenger thrown forwards, the newton, mass as inertia, rocket propulsion, and the floor force in an accelerating lift.

What you need to know

Answer each one before you reveal it.

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Key term

State Newton's second law of motion in terms of momentum.

Key term

Show how F = ma follows from the momentum form of Newton's second law when the mass is constant.

Numbers

A resultant force of 3.0 kN acts on a car of mass 1500 kg. What is its acceleration?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    State Newton's first law of motion.

    Model answer

    A body remains at rest or continues to move at constant velocity in a straight line unless a resultant force acts on it.

  2. 2

    State Newton's second law of motion in terms of momentum.

    Model answer

    The resultant force on a body equals the rate of change of its momentum, F = Δ(mv)/Δt.

  3. 3

    Show how F = ma follows from the momentum form of Newton's second law when the mass is constant.

    Model answer

    With m constant, Δ(mv)/Δt becomes m Δv/Δt, and Δv/Δt is the acceleration, so F = ma.

  4. 4

    State Newton's third law of motion.

    Model answer

    If body A exerts a force on body B, then B exerts a force on A that is equal in size, opposite in direction and of the same kind.

  5. 5

    What is meant by the resultant force acting on a body?

    Model answer

    It is the single force equal to the vector sum of all the individual forces acting on that body.

  6. 6

    A resultant force of 3.0 kN acts on a car of mass 1500 kg. What is its acceleration?

    Model answer

    a = F/m = 3000/1500 = 2.0 m s⁻².

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