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[Gravitational Potential Energy] In a movie stunt, a 65 kg skier..

  1. Nov 22, 2016 #1
    1. The problem statement, all variables and given/known data

    In a movie stunt, a 65 kg skier starts from a rest position at the top of a hill 30 m high. She slides down the hill to the bottom, where she collides with a 45 kg stationary skier. The collision is completely inelastic. Find the final velocity of the skiers.

    2. Relevant equations
    I got the answer, but I feel I did it wrong.

    3. The attempt at a solution
    Please look at Image.
    ETFETF.jpg
     
    Last edited by a moderator: Nov 22, 2016
  2. jcsd
  3. Nov 22, 2016 #2
    Please ignore the multiple uploads of pictures. I'm new here and I thought it didn't upload the picture. Anyone of the pictures will do.

    In the beginning you can see I did

    ETF= ETF

    Can I do that? My reasoning for doing ETF=ETF is because it's total mech. energy AFTER the collision and Final velocity
     
  4. Nov 22, 2016 #3

    gneill

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    Staff: Mentor

    Moderator note: I've inserted one copy of your image in-line in your post and deleted the extra copies of the attachment.

    I'm not sure what you mean to imply by saying ETF = ETF. Of course a given quantity is equal to itself. If you mean that the total mechanical energy is conserved from start to finish, then I would disagree (what type of collision is involved? Is energy conserved in that type of collision?).

    Only the 65 kg skier starts at the top of the hill, so I don't see why you've included both masses in the change of gravitational potential (your second line in your image).

    I'd expect to see an examination of the inelastic collision that occurs when the two collide at the bottom of the hill.
     
  5. Nov 22, 2016 #4
    Thank you for doing that.


    Yeah, TME is conserved from start to finish. The collision is completely inelastic.

    I included both masses since it's the combined energy of both masses after the collision.
     
  6. Nov 22, 2016 #5

    gneill

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    Energy is NOT conserved in an inelastic collision. What is conserved?
    But both masses do not go down the hill. Only one gains energy by going down the hill.
     
  7. Nov 22, 2016 #6

    Hmm I don't know how to answer the question. I mean the question

    " In a movie stunt, a 65 kg skier starts from a rest position at the top of a hill 30 m high. She slides down the hill to the bottom, where she collides with a 45 kg stationary skier. The collision is completely inelastic. Find the final velocity of the skiers. "

    Is all I have to work with, and we were given the answer but they don't show us how the actual work is done. How would you answer this question?
     
  8. Nov 22, 2016 #7

    gneill

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    There are really two separate parts to this problem. Break the problem into its sequential parts:

    i) 65 kg skier slides down hill (30 m height change). What's her speed at the bottom?

    ii) Two skiers collide inelastically. The 65 kg skier has the speed from (i) and collides with the 45 kg skier who is initally at rest. What's their speed after the collision?
     
  9. Nov 23, 2016 #8
    So I took your advice and redid the question. This time I feel more confident in my answer than my first attempt. Please check the picture and let me know what you think.
     

    Attached Files:

  10. Nov 23, 2016 #9

    gneill

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    Yes, that's much better. The final answer looks good.

    You should make it clear that when you say:
    upload_2016-11-23_6-41-1.png
    that you mean the initial velocity just before the collision.
     
  11. Nov 25, 2016 #10

    Ahh true! Thank you so much for your help!
    I went back to double check my work (And to make that change that you suggested.) And I feel like an idiot for not getting it in the first place. Again thank you so much!
     
  12. Nov 25, 2016 #11

    gneill

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    Staff: Mentor

    You're welcome!
     
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