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Energy Problem

  1. Apr 1, 2007 #1
    A84 kg skier jumper starts fro rest at the top of a 220 m hill. What is the velocity of the skier jumper, if the jump is 105 m from the bottom of the hill.

    My Proof:

    m=84 KG
    h=105 m
    Eg=84kg*9.8N/kg*105m
    Eg=86436J
    so Eg=Ek
    86436J=1/2*85*V^2
    V=45..36m/s


    Is that right or Wrong?
     
  2. jcsd
  3. Apr 1, 2007 #2

    hage567

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    It's wrong.

    You must consider the energy he has at the top of the hill. Apply conservation of energy between that height and the height of the jump.
     
  4. Apr 1, 2007 #3
    i didn't understand u
    can u explain me?
     
  5. Apr 1, 2007 #4

    hage567

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    Well for one thing, the kinetic energy the skier has at the time he reaches the jump must come from somewhere. The change in potential energy between the top of the hill and the jump is what will determine the kinetic energy at the jump. It will determine his velocity.

    Why don't you think the potential energy the skier has at the top of the hill is important in finding out how much kinetic energy he has at the jump?
     
  6. Apr 1, 2007 #5
    can u give me proof?
     
  7. Apr 1, 2007 #6

    hage567

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    Tell me why you think it's wrong.

    You have not shown any reasoning for your answer.
     
  8. Apr 1, 2007 #7
    because i am not sure about my answer
     
  9. Apr 1, 2007 #8

    hage567

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    Well you must have done your answer that way for some reason.

    What kind of proof are you looking for? I'm not going to give you the answer if that's what you mean.

    Have you tried to use my idea to solve this problem? What about it is giving you trouble?
    Can you find the total energy of the skier when he is at rest at the top of the hill?
     
  10. Apr 1, 2007 #9
    Eg=mghA
    Eg=84kg*9.8N/kg*220m
    Eg=181104J


    Eg=mghB
    Eg=84kg*9.8N/kg*105m
    Eg=86436J


    Eg=181104J-86436J
    Eg=94668J

    So Eg=Ek
    94668JJ=1/2*85*V^2
    V=47.47m/s


    Is that right or Wrong?
     
  11. Apr 1, 2007 #10

    hage567

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    I would say that is right.
     
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