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Homework Help: Inelastic Collision

  1. Mar 31, 2004 #1
    Two identical balls made of putty, each of mass 1.50 kg, are suspended from strings of length 28.0 cm so that they touch when in their equilibrium position. We pull one of the balls back until its string makes an angle theta = 73.0 degrees with the vertical and let it go. It collides totally inelastically with the other ball, which means the two balls remain stuck together after the collision. How high will the stuck-together balls rise above the bottom most point?

    So this is a Completely Inelastic Collision in Two deminsions, right?

    Do i have all the information I need or do i have to find velocities, I don't know any formulas with theta involved.
  2. jcsd
  3. Mar 31, 2004 #2


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    While it is in 2-dimensions, the problem only has one degree of freedom (θ). Try completing the following steps to solve the problem:

    - Copmpute the velocity of the first ball immediately prior to its collision with the second ball. Do this by considering the conversion of the potential energy the ball has at its original height into purely kinetic energy (ignore friction/air resistance).
    - Compute the velocity of the two balls together immediately after the collision. This is simply a 1D momentum conservation equation (this involves the assumption that the motion of the balls just before and just after the collision are roughly horizontal; this assumption is supported by the description of the equilibrium position of each ball).
    - Compute the height of the balls based on their initial velocity (the velocity right after the collision). (Use the simple geometry of the situation to find the correspondence between the angle of the string and the height of the ball). This is found using the same calculation as step 1; i.e., use the conservation of energy.
  4. Mar 31, 2004 #3


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    1. calculate the potential energy of the raised ball
    2. calculate the kinetic energy it will have at the bottom
    3. calculate the velocity of the 2 balls stuck together (momentum conserved, not energy)
    4. calculate the kinetic energy of the 2 balls
    5. calculate the height where the potential energy equals the kinetic energy you found in part 4
  5. Mar 31, 2004 #4
    missed deadline by 2 mins hehe ohwell, thanks for the tips.
  6. Apr 1, 2004 #5
    Next time post your questions a bit earlier so they may be answered in time.
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