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Kinetic Energy Question

  1. Oct 6, 2004 #1
    Hi, I have a question about this problem:

    An air track glider of mass m1=0.300 kg moving at a speed of 0.800 m/s to the right collides with a glider of mass m2=0.300 kg moving at a speed of 0.400 m/s in the opposite direction. After the collision m1 rebounds at speed 0.200 m/s to the left. After the collision what is the speed and direction of m2? Calculate the total kinetic energy of the system before the collision. Calculate the total kinetic energy after the collision. Where did the lost energy (if any) go?

    I found the momentum of m1 and m2 before they collided and found the momentum of m1 after they collided. Then I used the formula pi1 + pi2 = pf1 + pf2 to find the momentum of m2 after the collision and plugged in the weight to p=mv to find the velocity.

    However, I'm stuck on how to find the total kinetic energy of the system before and after the collision? I used KE = 1/2mv^2 and plugged in the numbers for m1 and m2 before the collision separately and then added them together to find the total kinetic energy. I did the same for finding the KE after the collision and got KEbefore = 0.12J and KEafter= 0.06J. I thought the KE's were supposed to be the same though so I don't know where I went wrong...Please help!
  2. jcsd
  3. Oct 6, 2004 #2
    Your calculation is correct, and still, energy is conserved. Difference in KE before and after heated gliders a bit.
  4. Oct 6, 2004 #3
    kinetic energy and collision

    Your calculations are correct.

    It is not necessary for the initial and final Kinetic energies to be same as some energy is lost in collisions as a result of conversion to heat, sound or change in shape of the colliding objects. ( so although total energy is conserved, K.E. may not be the same ).

    The olnly thing that is always conserved is momentum.

    However, there are collisions called perfectly elastic collisions where K.E. is also conserved. But the case you have solved for is different.

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