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AP Physics 1: Work, Energy and Power Review

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Key Questions

  • Work is done when a force moves an object through a distance.

    There are three keys requirements in order to say that work has been done. They are force, displacement (distance*) and cause. The qualify for work to be done a force must cause a displacement of an object. There must be a force and it must cause the displacement.

    If the displacement and the force are in the same direction, the equation for work is:

    work = force x displacement

    • displacement is a vector quantity, distance is a scalar quantity
  • The SI unit for work is joule (J) or newton-meter.

    When one newton of force is used to move an object one meter you have exerted one joule of force.

    A newton is one #kg\cdotm/s^2#. Then you multiply this by one meter, your unit becomes #kg \cdot m^2#/#s^2#. As you can see, it's much easier to use a joule (J) instead.

  • If you are ignoring friction and are moving the crate horizontally, you do NO WORK because you neither fought resistance nor changed the potential energy of the crate.

    If, on the other hand, you lift the crate up 20 meters then you did work according to Work = Force x Distance or W = 567n x 20m = 11340 nm = 11340 Joules. This is because you increased the crate's potential energy from 0 Joules on the floor to PE= mgh = 11340 joules on the shelf.

    Note that weight = mass x accel of grav, or w = mg. Since PE=W=mgh, it is also true that W = wh, where height (h) is the distance you lifted it.


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Work and Energy