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Potential Energy Help Needed!

  1. Dec 17, 2005 #1
    How would you find the gravitational potenetial energy change for a box being slide down a slope? The weight of the box is 490N, the kinetic coefficient of friction is 0.12N, the slant of the slope is 45 degrees. I knwo how to find the forces acting ont he boxes and have calculated all of them but I have no clue how to solve for the potential energy. Please help!
     
  2. jcsd
  3. Dec 17, 2005 #2

    mukundpa

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    change in gravitational potential energy of a body of mass m is mgh, where h is the vertical height between the two positions.
     
  4. Dec 17, 2005 #3
    but given just an angle how would I know the height?
     
  5. Dec 17, 2005 #4
    are you sure that's all the information you've been given?
     
  6. Dec 17, 2005 #5

    mukundpa

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    1. The vertical height should be given.
    2. They may give the slant height and with that and the angle known we can calculate the vertical height.

    Just check again the question otherwise it is not possible to caclculat the change in KE with given data.
     
  7. Dec 17, 2005 #6
    THis was thwe actual complete question but I am just trying to solve the last part:

    A 50.0 kg crate slides down a slope of 45 degrees for a distance of 200.0m. how much work will be done by gravity? How much work will be done by friction, if the kinetic coefficent of friction is 0.12? What speed will it attain at this point if it was originally at rest? By how much will its gravitaional potential energy change during the slide?

    Again I have solve all of the problem except for the last question.
     
  8. Dec 17, 2005 #7

    mukundpa

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    here
    the distance covered is given 200m
    hence the vertical height fallen is 200 sin 45deg. = 2000*707 = 141.4 m.
    now you can calculate mgh
    remember the height is lost means the potential energy decreases(may be the answer is having a negative sign)
     
  9. Dec 17, 2005 #8
    How did you solve the question1?
    Please show your method. I am asking avb - others don't reply.
     
  10. Dec 17, 2005 #9

    mukundpa

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    the distance covered down the slope is given = 200m hence the vertical component of this distance is 200 sin45deg. = 200*0.707
    mgh is the loss in potential energy.(_ve)
     
  11. Dec 17, 2005 #10
    I solved for the weight which was 490N, then I multiplied the weight times the 200m distance times cos(45degrees). y?
     
  12. Dec 17, 2005 #11
    Ok thank you mukundpa!
     
  13. Dec 17, 2005 #12
    Just think o the coincidence of the first question with the last one. I think you should know the relation between energy and work. What is the definition of energy? So you anyway solved it before posting?
     
  14. Dec 17, 2005 #13

    mukundpa

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    ya you may solve it in that way too if you have calculated the speed at the end
    loss in potential energy = gain in KE + work done against friction
     
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