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Homework Help: Easy Work Problem

  1. Feb 1, 2004 #1
    Hi, i havent been at school this week so i only got the homework and no help.

    I know this is an easy question so just tell me which way of doing it is right.

    A person carries a carton of milk that is a weight of 10 N for a distance of 3.5m along a level surface. How much work is being done?

    is it just 10N * 3.5m * cos(90)

    or is it just 10N * 3.5m

    I would think it would be the first one because it is an angle of 90 degrees between the displacement and the weight, but it doesnt make sense because it says no work is being done.

    Also, one more problem a hor. force of 805 N is needed to drag a crate across a hor. floor with a constant speed. You drag the crate using a rope held at an angle of 32 degrees.
    a. What force do you exert on the rope?
    b. How much work do you do on the crate when moving it 22m
    c. If you complete the job in 8.0s, what power is developed.

    I really just need help on part of that problem.
    THanks alot :)
    Last edited: Feb 1, 2004
  2. jcsd
  3. Feb 1, 2004 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    In the first problem, you are correct. No work is being done at all (against gravity). If the object does not change altitude, then gravity is quite simply not relevant.

    In the real world, of course, you have to contend with friction. In this problem, however, I believe you are to neglect friction.

    The reason no work is done can be easily understood. Imagine sliding a jug of milk along a perfectly smooth, enormous iced-over lake. In the absence of friction, the only effort you'd have to spend is applying a force to get the jug moving. Once moving, sending it a few feet away and sending it clear to the other side of the lake would be (energetically speaking) exactly the same.

    No work is done against (or by) gravity unless something is changing altitude.

    - Warren
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