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Mechanics

Free-body diagrams and Newton's first and second laws

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Overview

Recall questions on what belongs on a free-body diagram, the statements of the first and second laws, the definition of the newton, and applying F = ma.

What you need to know

Answer each one before you reveal it.

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Key term

What should the length of each arrow on a free-body force diagram indicate?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What is shown on a free-body force diagram?

    Model answer

    Only the forces acting on one chosen body, shown as labelled arrows pointing in the directions in which the forces act.

  2. 2

    Why are the forces that the chosen body exerts on other objects left out of its free-body diagram?

    Model answer

    Those forces act on other bodies, so they cannot contribute to the resultant force on the body being considered.

  3. 3

    What should the length of each arrow on a free-body force diagram indicate?

    Model answer

    The relative magnitude of that force.

  4. 4

    Which forces act on a book resting on a horizontal table?

    Model answer

    Its weight acting vertically downwards and the normal contact force from the table acting vertically upwards, which are equal in magnitude.

  5. 5

    Which forces act on a car moving at constant velocity along a level road?

    Model answer

    Weight downwards, normal contact force upwards, the driving force forwards and the resistive forces backwards, with each pair equal in magnitude.

  6. 6

    What does Newton's first law of motion state?

    Model answer

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

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