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Homework Help: Rubber bullet / metal bullet hitting wooden block

  1. Oct 31, 2009 #1
    The bullet hits near the top of block. Which will tip the block over? Which will do most damage?

    I believe the rubber will tip it over and the metal will do the most damage...but I'm not sure how to explain it. When the metal hits it enters the block. When the rubber hits there is a recoil. The rubber doesn't enter the block
  2. jcsd
  3. Nov 1, 2009 #2


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    Think about the momentum of the bullet before and after the impact...
  4. Nov 1, 2009 #3


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    Tipping over is done by putting momentum(vector) into the block, and doing damage is done by putting energy(scalar) into the block. If you consider that both quantities are the same in total before and after the impact, you know which bullet does what.
  5. Nov 3, 2009 #4
    Would the momentum and KE of both rubber and metal bullet be the same if they have the same mass, shape, & velocity?

    So in that case how would I distinguish the metal from the rubber? How do I explain the KE of the metal bullet enters the block and causes damage? While the rubber just tips it over?
  6. Nov 3, 2009 #5
    Hi there,

    The momentum is the same, even if the shape is not the same. The mass and the velocity are the momentum and kinetic energy's only parameters. Therefore, if you want to keep the same momentum, and increase the mass, the velocity has to be reduced.

  7. Nov 3, 2009 #6


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    The difference between rubber and metal is that rubber is much more elastic than metal. When the bullet hits the wood, there is some interaction between them for a very short time. During this interaction, the rubber bullet will be deformed, but it will get back its shape after the contact with the block ceases. It is like a spring, the KE is converted to elastic energy and converted back to KE when the bullet leaves the block. This is an elastic collision, and both the total momentum and KE is conserved.

    In case of metal bullet, it is solid and rigid enough to go further after hitting the block, and enters inside the block, the two of them moving together. This is an inelastic collision, momentum is conserved, but the KE does not. Part of the original KE of the bullet is used to break the block, to warm it and so on.

    As the rubber bullet recoils from the block, but the metal one goes together with it, the momentum of the block after collision is bigger if the collision was elastic. Moreover, when the top is hit, the block gets not only translational KE but also a torque which can tip it over. This torque is proportional to the momentum it gained, so it would be more likely that the rubber bullet tips the block over.

  8. Nov 4, 2009 #7
    Thanks ehild....thats very similar to how my prof explained it
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