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Conservation of Energy w/ Frictional Forces

  1. Oct 26, 2014 #1
    1. The problem statement, all variables and given/known data
    During a rockslide, a 710 kg rock slides from rest down a hillside that is 500 m long and 300 m high. The coefficient of kinetic friction between the rock and the hill surface is 0.23.
    https://physicsforums-bernhardtmediall.netdna-ssl.com/data/attachments/57/57890-5a3321dd6b8a58eb42038e3fc5bbe5e1.jpg [Broken]
    (a) If the gravitational potential energy U of the rock-Earth system is set to zero at the bottom of the hill, what is the value of U just before the slide?
    (b) How much energy is transferred to thermal energy during the slide?
    (c) What is the kinetic energy of the rock as it reaches the bottom of the hill?

    2. Relevant equations
    U
    =mgh
    N=mgcosθ
    Fƒ=μN
    Wƒ=Fƒdcosθ

    3. The attempt at a solution
    a)
    Easy. Uο=(710)(9.8)(300)=2087400 J (Accepted as correct answer)

    b) θ=tan-1(300/500)=31°
    N=(710)(9.8)cos(31)=5964.2 N
    Fƒ=(.23)(5964.2)=1371.8 N
    Wƒ=(1371.8)(583.1)cos(180°)=-799896.6 J (This was not accepted as correct answer)

    (c)
    Uο+Kο=Uf+Kf+Wƒ
    2087400+0=0+Kf+799896.6
    Kf=1287503.4 J (Also not accepted as correct answer)


    Please let me know where I went wrong.
     

    Attached Files:

    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Oct 26, 2014 #2

    haruspex

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    Where did the 583.1 come from?
    Should the answer be positive or negative?
     
  4. Oct 26, 2014 #3
    583.1 is the distance the rock slides found by the Pythagorean Theorem √3002+5002. I tried both positive and negative with those answers. I assume the work done by friction is always negative since it is opposite the motion. and the kinetic energy should be positive because the velocity would be in the positive x-direction.
     
  5. Oct 26, 2014 #4
    You misinterpreted the problem. The 500 is the hypotenuse.

    Chet
     
  6. Oct 26, 2014 #5

    haruspex

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    ... which means this is also wrong:
    θ=tan-1(300/500)​
     
  7. Oct 26, 2014 #6
    Ahh knew I missed something basic. Thanks a lot.
     
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