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Forces and Their Effects

Forces on an object in equilibrium

  • 3 things to know
  • Paper 2

Overview

Resultant force and balanced forces, calculating a resultant from a push and friction, and objects in equilibrium, including a lift moving steadily, a skydiver at constant velocity, a book on a table and a rope pulled from both ends.

What you need to know

Answer each one before you reveal it.

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Key term

Explain the motion of a skydiver whose weight equals air resistance, a book on a table, and a rope pulled equally both ways.

Key term

Explain what is meant by balanced forces acting on an object.

Key term

Explain why the resultant force on a lift moving upwards at a steady speed is zero.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    State what is meant by the resultant force on an object.

    Model answer

    It is the single force that has the same effect as all of the forces acting on that object.

  2. 2

    Explain what happens to the motion of an object when the resultant force acting on it is zero.

    Model answer

    It stays at rest, or carries on moving at a constant velocity.

  3. 3

    Explain what is meant by balanced forces acting on an object.

    Model answer

    The forces cancel each other out, giving a resultant force of zero.

  4. 4

    A crate is pushed with a force of 80 N while friction of 30 N opposes the motion. Calculate the resultant force and state its direction.

    Model answer

    The resultant is 80 − 30 = 50 N in the direction of the push.

  5. 5

    Explain why the resultant force on a lift moving upwards at a steady speed is zero.

    Model answer

    The tension in the cable balances the weight of the lift exactly.

  6. 6

    A skydiver of weight 700 N experiences an air resistance of 700 N. Explain the effect of these forces on their motion.

    Model answer

    The resultant force is zero, so the skydiver falls at a constant velocity.

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