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Mechanics

Work done by a force

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Overview

Recall questions on the definition of work and the joule, the cosine term when a force acts at an angle, and what negative work means.

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How is the work done by a force calculated, including when the force acts at an angle to the motion?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    How is the work done by a force defined, and what is its equation?

    Model answer

    Work done is the force multiplied by the distance moved in the direction of the force, ΔW = FΔs.

  2. 2

    How is the joule defined?

    Model answer

    One joule is the work done when a force of 1 N moves its point of application 1 m in the direction of the force.

  3. 3

    How is the work done calculated when the force acts at an angle θ to the direction of motion?

    Model answer

    The work done is ΔW = FΔs cos θ, using the component of the force along the direction of motion.

  4. 4

    Why is no work done by a force that acts at right angles to the motion?

    Model answer

    Because cos 90° = 0, the force has no component along the displacement and so transfers no energy.

  5. 5

    What is an example of a force that does no work on a moving object?

    Model answer

    The normal contact force on a box sliding along a horizontal floor does no work, because it acts vertically while the motion is horizontal.

  6. 6

    A suitcase is pulled 15 m along level ground by a force of 60 N acting at 25° above the horizontal. How much work does that force do?

    Model answer

    The work done is 60 × 15 × cos 25° = 816 J.

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