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Energy conservation question help?

  1. Jun 29, 2012 #1
    The sphere with the mass 1 kg is released from point A without initial speed and moves without friction just like in the figure.
    a) Find the mechanic energy in point A
    Solution : E=mgh because v=0
    b) Find the kinetic energy in B
    Using the law of the conservation of mechanic energy, mghA=mghB+Ekb
    C) WILL THE SPHERE REACH THE POINT C?

    To reach the C point, Emb=Emc
    so mghB+ Ekb=mghC+ Ekc
    here i find the V of C.BY proving that the ball has a speed in C ,do I prove it reaches the point?

    http://s1171.photobucket.com/albums/r549/Elaia06/?action=view&current=untitled.jpg
     
  2. jcsd
  3. Jun 29, 2012 #2

    Doc Al

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    Re: nergy conservation question help?

    OK.
    Try it and see. Even easier is to compare the energy at A with the energy at C.
     
  4. Jun 29, 2012 #3
    Re: nergy conservation question help?

    is it correct the c question?
     
  5. Jun 29, 2012 #4

    Doc Al

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    Re: nergy conservation question help?

    The basic approach is correct. Compare the energy at A with the energy at C. What would the velocity of the sphere be at that point? Is it possible?
     
  6. Jun 29, 2012 #5
    Re: nergy conservation question help?

    The energy at A is 25 J.The energy at C is 35.Velocity at a is zero.Velocity at C is sqrt10.Seems possible.
     
  7. Jun 29, 2012 #6

    Doc Al

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    Re: nergy conservation question help?

    Better check those numbers, especially the second. I presume you mean that to be the gravitational PE at C.
    Really? Even though you found that just the PE alone is greater at C than the entire energy at A? How did you solve for the velocity at C. Show the equation.
     
  8. Jun 29, 2012 #7
    Re: nergy conservation question help?

    The law of the conservation of the mechanical energy For points B and C

    mghB+ Ekb=mghC+ Ekc
    1*10*1.5+10=1*10*3+1*V^2/2
    so here V=sqrt10
    what does the energy at A have to do with the energy at C? I mean, I found that it has a speed at C,isnt that enough?
     
  9. Jun 29, 2012 #8

    Doc Al

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    Re: nergy conservation question help?

    Let's do the math:
    15 + 10 = 30 + KE
    25 = 30 + KE
    so... KE = -5 Oops! Can KE be negative?
    Energy is conserved, so it's the same at A, B, and C.
     
  10. Jun 29, 2012 #9
    Re: nergy conservation question help?

    Sorry, u are right :) So there is no kinetic energy at point C,which means that there is no velocity,so the ball cant reach that point.right now?
    What if the velocity would be lets say 5 m/s,would the ball reach the C then?
     
    Last edited: Jun 29, 2012
  11. Jun 29, 2012 #10

    Doc Al

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    Re: nergy conservation question help?

    Right. The sphere cannot roll to a point higher than where it started.
     
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