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Motion and Forces

Newton's second and third laws

  • 3 things to know
  • Paper 5

Overview

Force = mass × acceleration and calculations with it, how acceleration depends on force and mass, Newton's third law and its equal and opposite force pairs, conservation of momentum in collisions between trolleys and balls, and the proportionality symbol ∝.

What you need to know

Answer each one before you reveal it.

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Key term

Using a person pushing a wall with a force of 50 N, a swimmer and two colliding trolleys, explain Newton's third law of motion.

Key term

State in words the equation that links resultant force, mass and acceleration.

Numbers

A resultant force of 60 N acts on a mass of 12 kg. Calculate the acceleration.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    State in words the equation that links resultant force, mass and acceleration.

    Model answer

    Force = mass × acceleration.

  2. 2

    A resultant force of 60 N acts on a mass of 12 kg. Calculate the acceleration.

    Model answer

    60 ÷ 12 = 5 m/s².

  3. 3

    A car of mass 1200 kg accelerates at 2 m/s². Calculate the resultant force on it.

    Model answer

    1200 × 2 = 2400 N.

  4. 4

    A resultant force of 90 N gives an object an acceleration of 3 m/s². Calculate the mass of the object.

    Model answer

    90 ÷ 3 = 30 kg.

  5. 5

    A trolley of mass 0.8 kg is pulled by a resultant force of 2 N. Calculate its acceleration.

    Model answer

    2 ÷ 0.8 = 2.5 m/s².

  6. 6

    Explain what happens to the acceleration of an object if the resultant force on it doubles while its mass stays the same.

    Model answer

    The acceleration doubles, because acceleration is directly proportional to resultant force.

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Key Concepts of Physics
Motion and Forces
Conservation of Energy
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ALength is measured in metres, with the symbol m.
BMass is measured in kilograms, symbol kg.
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