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Question about Energy/Work

  1. Aug 31, 2012 #1
    I'm a little confused about what exactly work/energy is. I understand that if I exert 10 N o a box over a distance of 10 m, 100 J of energy are expended. However, say I exert 10 N on a 100 kg box, and it does not move, why is no energy expended? Such an action certainly requires energy?
     
  2. jcsd
  3. Aug 31, 2012 #2
    Energy is transferred in the form of chemical reactions in your body that fuel your exerting of force. However, no work is done on the 100-kg box due to the fact that there is no movement. The force of static friction counteracts the force you exert.
     
  4. Aug 31, 2012 #3
    First, realize that physicists have a very specific mathematical meaning of "work" and "energy" and they are not the same as "effort".

    You should meditate on the case of a table. A table can hold things up indefinitely without expending any energy at all. No energy is required, because while the table is pushing up on the object, the object isn't moving, so no work is being done.

    It may be tiring to hold the same object stationary in your outstretched palm, but that doesn't mean that your palm is doing work *on the object,* any more than the table is doing work on the object when it supports the object. You get tired because the biology of your muscles is such that they need to burn fuel just to create a force in the first place.

    If the object doesn't move, your muscles burn chemical fuel, releasing energy, but that energy just ends up heating up your muscles instead of doing work on the object.
     
  5. Sep 1, 2012 #4
    Energy is the ability, the potential do do work, rather than actually doing work. So an electromagnmetic field, for example, contains energy, but it might just sit there without doing any work.

    Work is defined as a force times a distance. So in general energy has the ability to exert such a force, but unless something is moved it does not fit the definition. You can push, for example, as hard as you like against a wall but unless it moves the definition says no work has been done. But as noted above, you HAVE expended energy trying to push it.

    A closely related concept is power....work expended per unit time....So if you perform some movement,some work, like accelerating a car, the faster you do it the more POWER is expended. These definitions take some time to get used to, sort of like the distinction between 'affect' and 'effect'....
     
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