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Energy of Conservation Lab Report Question

  1. Mar 8, 2014 #1
    1. The problem statement, all variables and given/known data

    In this experiment we tied a cart to a hanging mass and measured the velocity as it passed through a gate, and recorded the mass of the cart. I calculated the potential and kinetic energy of the hanging mass and when I subtracted the potential from the kinetic, I got a negative number. Does this mean that energy was lost? Do these results make sense? Please let me know if I wasn't descriptive enough of the scenario!


    2. Relevant equations
    K=(1/2)mv2
    U=mgh
    K-U


    3. The attempt at a solution
    Here is what I did for trial 1 (all of the units are what they need to be for the formula to work):

    Calculating Kinetic Energy:
    K=(1/2)(0.075)(1.23)2=0.0567 J

    Calculating Potential Energy:
    U=(0.075)(9.8)(0.86) =0.6321

    The difference:
    0.0567-0.6321= -0.575
     
  2. jcsd
  3. Mar 8, 2014 #2

    PhanthomJay

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    Yes energy was lost but check your numbers again because a lot of energy was lost, more than one would expect. Where did the lost energy go?
     
  4. Mar 8, 2014 #3
    I'm not really sure what I'm doing wrong! All the numbers are what I recorded during the lab, the units are right, the equations are right... I don't know, I thought the purpose of this experiment was to show how potential energy could be converted into kinetic energy. So should I have not lost any energy?
     
  5. Mar 8, 2014 #4

    Simon Bridge

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    Follow the energy transformations - you are converting gravitational potential energy in the hanging mass into kinetic energy in what?
     
  6. Mar 9, 2014 #5

    PhanthomJay

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    some energy is lost ...depends on the smoothness of the spinning pulley and cart wheels... But not all that much...what did you measure for the cart mass and what was the value of the hanging mass ?
     
  7. Mar 9, 2014 #6

    Simon Bridge

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    jdawg reports:
    ... taken as written, the change in PE of the hanging mass was subtracted from the change in KE of the hanging mass. This should be a negative number since the PE of the hanging mass went to do more than just accelerate the hanging mass.

    There is another very important place the energy went - and a bunch of other places that probably have a smaller, but also important, effect.

    I'd like to see jdawg work through the places the PE could have gone before providing further help.
     
  8. Mar 9, 2014 #7
    The mass of the cart was 0.5246 kg and the mass of the hanging weight for trial 1 was 0.075 kg

    Ooops... Was I supposed to use the mass of the cart?
     
  9. Mar 9, 2014 #8

    Simon Bridge

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    That's what I figured happened :) It is total energy that is conserved.

    The energy transformation goes like this:
    The PE lost by the weight goes to KE in everything that moves as a result.
    So the weight moves, but it also moves the cart. (re. post #6)

    Add the KE of the cart to the KE of the weight, compare with the PE.
     
  10. Mar 9, 2014 #9
    Haha ohh! Thanks so much! :)
     
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