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Homework Help: Kinematics/collision problem!

  1. Apr 22, 2008 #1
    An object of mass 3m is launched in vertical direction with velocity Vo. At the same time from the maximum height the 3m object can reach another object of mass m is released without initial velocity. Two objects experience perfectly inelastic collision at some height. Given that Vo = 12m/s and m = 150g. Calculate the time when the two objects collide. What is the velocity of each just before the collision? What is the height where the collision takes place? How much kinetic energy is lost in the collision?

    I've calculated the max height H to be 7.3m but I'm stuck on calculating the time and velocity and all the rest. Can anyone help? Thanks!
  2. jcsd
  3. Apr 22, 2008 #2
    Recall that you are working with a constant acceleration, gravity.

    You've probably seen equations relating velocity, height, and time. Hint these equations are very common and are even posted in one of the stickies.

    Remember that there is a conservation of energy in the system. So the energy of the two individual balls will be equal to the energy of balls when they are stuck together.
  4. Apr 22, 2008 #3

    I know the formulas involved. The problem is the time to collide. With a height of 7.3m, the object drops to the ground in 1.22s. The object fired from the ground comes to a stop in 1.22s also. The collision *should* be when the displacement for both objects is the same - and that should be at the midpoint since the object from the top is accelerating at the same rate the object from the bottom is decelerating. However, I keep getting 1.22 when I expect I should get .61.
  5. Apr 22, 2008 #4
    Could you show some of your work. If you've got the correct formulas then it sounds like it might just be a small error.
  6. Apr 23, 2008 #5
    The 'y' coordinate becomes equal for both at the time for collision.
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