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Homework Help: What happens to the lost energy in these situations?

  1. Dec 14, 2009 #1
    What happens to the "lost" energy in these situations?

    (a) A box sliding across the floor stops due to friction. How did friction take away that KE and what happened to that energy?

    (b) A car stops when you apply the brakes. What happened to its kinetic energy?

    (c) air resistance uses up some of the gravitational potential energy of a falling object. What type of energy did the lost PE become?

    (d) When a returning space shuttle touches down on the runway, it has lost almost all its KE and gravitational PE. Where did all that energy go?

    ==========================

    (a) I know the energy goes into the earth but I'm not sure how friction takes away the KE

    (b) The KE goes into the earth ?

    (c) does some of the PE get transferred as KE to the surrounding air?

    (d) The earth?

    correct me if I'm wrong
     
  2. jcsd
  3. Dec 14, 2009 #2
    Re: What happens to the "lost" energy in these situations?

    Well, in many of your examples the energy is translated into heat(thermal energy).

    The energy doesn't "go" anywhere or into anything. It is energy, not matter.

    Let us talk about the car question though.(b)
    When you apply the breaks, friction is created. This friction generates heat.
    Now, what if you stopped your car by driving up a hill until the car stopped?
    The energy would have changed from kinetic energy to potential energy.

    The basic lesson is that energy can be changed from one state to another.
    The secondary lesson is that this process is always inefficient, and you will always have less energy when it is over.

    Hope that helps!
     
  4. Dec 14, 2009 #3

    AEM

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    Re: What happens to the "lost" energy in these situations?

    In all of these situations, if you were to take very careful temperature measurements of

    (a) the box and the ground,

    (b) the brake rotor and brake pads,

    (c) the air through which the object fell and the object,

    (d) the space shuttle and the air along its path,

    you would find a rise in temperature. In each case kinetic energy has been turned into the type of molecular motion that gives rise to what we call heat.
     
  5. Dec 14, 2009 #4

    cepheid

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    Staff Emeritus
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    Gold Member

    Re: What happens to the "lost" energy in these situations?

    This is just plain wrong. Energy is conserved. In many situations, in order to see that, you have to consider all possible forms into which it might have been changed. That is the point of doing this type of problem.
     
  6. Dec 14, 2009 #5
    Re: What happens to the "lost" energy in these situations?

    Yea that does help...thanks...but what about question c? Was I right about that? When an object falls some of the PE gets translated into KE which slows the object down right?
     
  7. Dec 14, 2009 #6

    AEM

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    Re: What happens to the "lost" energy in these situations?

    Nope. The translation of PE into KE speeds the object up to maintain the conservation of energy. Some of the PE is lost to heat, so the correct equation is

    PE goes to KE + energy dissipated as heat
     
  8. Dec 14, 2009 #7
    Re: What happens to the "lost" energy in these situations?

    Oh okay thanks
     
  9. Dec 15, 2009 #8
    Re: What happens to the "lost" energy in these situations?

    Absolutely correct...I dont really know what I was trying to say there....
    If you account for all energy, it should be a perfectly balanced equation
     
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