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Help with a collisions question!

  1. Oct 30, 2005 #1
    Two blocks can collide in a one-dimensional collision. The block on the left hass a mass of 0.50 kg and is initially moving to the right at 2.4 m/s toward a second block of mass 0.80 kg that is initially at rest. When the blocks collide, a cocked spring releases 1.2 J of energy into the system.
    (a) What is the velocity of the first block after the collision?
    (b) What is the velocity of the second block after the collision?

    I don't know what to do. I assume equal momentum is given by the spring to each block, but i tried working out the momentum given to the system by pretending it was 1 big block and working out the KE and so the momentum. I though this was the momentum given to each block so worked out the velocity given to each block by the spring and adding it on to the two velocities i found using simultaneous equations of momentum and kinetic energy. It didn't work :(. I don't know what else to do without knowing how the energy is distributed between the two blocks.
     
    Last edited: Oct 30, 2005
  2. jcsd
  3. Oct 30, 2005 #2

    Doc Al

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    Staff: Mentor

    Hints:
    (1) Is momentum conserved during the collision?
    (2) How does the total kinetic energy after the collision compare to the total KE before the collision?

    Translate your answers into equations and solve.
     
  4. Oct 31, 2005 #3
    The kinetic energy after must be 1.2J greater than the kinetic energy before right? With the momentum equation, momentum can't be conserved if you release more energy into the system, because each block will have a greater momentum than they would have were the spring not there. Will overall momentum of the system be the same with or without the spring - do the extra momentums in opposite directions caused by the spring cancel?
     
  5. Oct 31, 2005 #4

    Doc Al

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    Staff: Mentor

    Right.
    Conservation of momentum refers to the total momentum of the system, not the individual momenta of each block. And remember that momentum is a vector.
    The total momentum of any system remains the same as long as no external forces act on it. (The spring is an internal force to the two-block system.)
     
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