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Homework Help: A conceptual question regarding collision/conservation of energy.

  1. Nov 13, 2011 #1
    Say there's the following situation:

    A bullet with some velocity strikes a block connected to a spring. The bullet passes right through the block and the spring is compressed by x cm to the right of the block after the impact. Some internal energy is lost due to deformation of the block while bullet passes through it.

    Because the question states "compressed AFTER the impact", i was able to assume that the only energy being converted into spring energy is the kinetic energy of the block (When it begins to compress when the bullet has already exited the block), so I solved the problem.

    HOWEVER, WHAT IF the question had stated "the block compresses the spring DURING the impact", in other words, it compresses WHILE the bullet is passing through the block? Then would the spring energy come from BOTH the kinetic energy of the block and some of the kinetic energy of the bullet while it is moving through the block?
    Last edited: Nov 13, 2011
  2. jcsd
  3. Nov 14, 2011 #2


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    Homework Helper

    Good question. But the usual assumption when treating collision problems is that the collision happens instantaneously, that is so fast, that only the velocities change during the impact, nothing else. Of course, this is not true, the bullet needs some time to traverse the block, pushing material aside along its path, initialising reversible and irreversible effects on the block and on itself: exciting elastic waves, producing heat, sound, melting some material...
    Ignoring the spring during collision, and treating compression of the spring by the block when the bullet left - it is a correct approximation.
    The problem can not be solved otherwise, you would need to calculate the motion both the bullet and block during the impact, and for that you would need details of the interaction between bullet and block.

  4. Nov 14, 2011 #3
    Thank you so much! That was an excellent explanation. May I ask further that if the situation had been "bullet strikes block and bounces back (elastic)", then is it also valid for me to assume that the block starts to move AFTER the bullet starts to bounce back? So I would ignore the spring's compression at the instant where the block-bullet moves together during the collision?

  5. Nov 14, 2011 #4


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    Exactly. Ignore that short time when they move together, as you do not know how long it is.
    There are some problems, when the time of interaction is given or can be computed - but you would notice if that is the case.

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