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Energy conservation of ball

  1. Nov 29, 2009 #1
    I need help!!

    1. The problem statement, all variables and given/known data
    A 44.0 g ball is fired horizontally with initial speed vi toward a 110 g ball that is hanging motionless from a 1.10 m-long string. The balls undergo a head-on, perfectly elastic collision, after which the 110 ball swings out to a maximum angle = 52.0.

    What was vi ?
    2. Relevant equations

    Conservation of momentum:
    m1vi1 + m2vi2 = m1vf1 + m2vf2

    Conservation of energy:
    1/2m1(vi1^2) = m2gy


    3. The attempt at a solution

    I tried to solve these problem with those 2 equations, but it still doesn't work. I compeletly massed up with these concepts. Can anyone help me with this problem with exact answer?
    Thank you
     
  2. jcsd
  3. Nov 29, 2009 #2

    mgb_phys

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    Re: I need help!!

    You almost have the energy conservation right.

    ke of first ball -> ke of second ball -> potential energy of second ball.
    you know m1, m2, and h so just find v1
     
  4. Nov 29, 2009 #3
    Re: I need help!!

    I do not know how to combine those two equations. Can you through specific steps by using numbers?
     
  5. Nov 29, 2009 #4
    Re: I need help!!

    I do not know how to combine those two equations. Can you through specific steps by using numbers?
     
  6. Nov 29, 2009 #5

    mgb_phys

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    Re: I need help!!

    ke of first ball = 1/2 m1 v^2
    pe of 2n ball = m2 g h

    Just set them equal
    1/2 * m1 * v1^2 = m2 * g * h
    1/2 * 0.044 * v1^2 = 0.110 * 9.8 * h

    You need to do a bit of trig to get h, then it's simple
     
  7. Nov 29, 2009 #6
    Re: I need help!!

    okay, thank your so far. I got the h as 0.110cos53, but is it right? or h is (0.110 - 0.110cos53 )?
    which one is right?
     
  8. Nov 30, 2009 #7

    kuruman

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    Re: I need help!!

    There is no wording in the problem that says that the 44 g is at rest after the collision. It seems to me that the energy conservation equation is missing the kinetic energy of the 44 g ball after the collision. One needs to solve the momentum conservation equation for the velocity of the 44 g mass after the collision and substitute in the modified energy conservation equation.
     
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