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Homework Help: Total energy; confusion with teacher's statement

  1. Jan 8, 2005 #1
    This post relates into the idea of post: https://www.physicsforums.com/showthread.php?t=58918

    I was talking to my physics teach and he said some thing confusing. I have a video of the ball drop and the separation; however, I do not have the clip RIGHT before the collision (with the earth). The clip jumps from: the two balls a bit above the table --> to the objects separated. My teacher said something along these lines:

    “You cant prove the change in momentum because you do not have the velocity right before the collision. You need to find the height of where the basketball would have bounced and find the loss of energy. To do so, calculate the loss of Potential energy with each bounce of the basketball after the collision. Find an equation with that data that will show the loss of potential energy. When you do, plug in the original height of the basketball before you released it and see where the basketball would have bounced. This loss of potential energy plus the initial kinetic energy of the tennis ball (after collision) plus the initial kinetic energy of the basketball (after collision) should equal the original potential energy of the system before you let go of the balls. This will allow you find the initial velocity right before the collision to find the change in momentum.”

    I’m not sure what he is trying to tell me. I might have got some ideas wrong but that’s the best I can remember from class. How does finding that all the energies add up to the initial potential energy give me the velocity of the two balls before they hit?
  2. jcsd
  3. Jan 8, 2005 #2
    ayone have any clues as to what my teacher means?
  4. Jan 8, 2005 #3


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    What are the positions of the two balls as you drop them ? Are they side-by-side or one over the other or something else ?
  5. Jan 8, 2005 #4
    one over the other, it's like an inelastic collision, where the two ball come down as one
    and then seperate.

    To prove:
    mass total * v = mass basketball * vb + mass tennisball vt
  6. Jan 8, 2005 #5
    Okay, I did what my teacher said. I proved that the inital PE is equal to the inital KE of the tennis ball after it hits the floor, plus the inital KE of the basket ball after it hits the floor, plus the theoretical loss of energy when dropped from a height of 1.6m. How does this give me the initial velocity right before the collision?
  7. Jan 8, 2005 #6


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    What did you do, urban?

    [tex] \sqrt{2gh} = v [/tex] ?

    Althought there will be loss of energy due to air drag.. you probably got an approximation. that [itex] mgh = \frac{1}{2} mv^2 [/itex]

    I believe you took the balls together as the system baketball+tennisball?

    Because on inelastic collisions Kinetic energy is not conserved thus mechanical energy is not conserved for the bodies, but yes for the system when they are together.
  8. Jan 8, 2005 #7
    It's a very long story. Basically, I have a video clip of a guy dropping a tennis ball and a basketball at the same time from a height of 1.6 m. I need to prove conservation of mementum though video analysis with Logger Pro, however, the clip when the balls collide with the table is not there. Only the clip before and after the collision. My teacher said that I cannot prove Conservation of Momentum without the clip of the contact. He told me to prove that: PE inital = KE (tennis ball after collision) + KE (basket ball after collision) + Energy loss. The loss energy was calculated by finding how much less the ball bounced each time it hit the floor. I plotted this and now I have a linear equation for the loss of energy. I added all this up and go an answer very similar to the PE total. Now I have no clue where to go to prove conservation of momentum. My teacher said that I no can find the velocity right before the basketball and tennisball hit the floor. How?

    yes I did
  9. Jan 8, 2005 #8


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    What are all your known values?? Have you thought about weighting a basketball and a tennisball to get their mass, so then you can use the relationship you proved with the energies to calculate the speed? You could do an average mass of the balls.
  10. Jan 8, 2005 #9
    yes, I did all that. I measured the weight, I have the velocities. I have everything except the clip OF the collision. My teacher said that it was invalid without the clip of teh actaul collision. What does he mean by that? And how do I calculate the final velocity?
  11. Jan 8, 2005 #10
  12. Jan 8, 2005 #11


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    I am not sure why he needs the clip of the collision, maybe he wants to see how much "perfectly inelastic" it was.
  13. Jan 8, 2005 #12
    I need to prove Conservation of Momentum outside the ideal labs of science (including outside forces, such as drag+friction). Maybe since proving that the energy is conserved, I can plug in numbers to find the velocity RIGHT before the collision.

    so then...v(m1+m2)=m1v1+m2v2
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