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Dynamics: impulse and momentum

  1. Oct 19, 2012 #1
    1. The problem statement, all variables and given/known data
    a 200-g projectile is fired with a velocity of 900m/s towards the center of the 15-kg wooden block, which rests on a rough surface. If the projectile penetrates and emerges from the block with a velocity of 300m/s, determine the velocity of the block just after the projectile emerges. How long does the block slide on the rough surface, after the projectile emerges, before it comes to rest again? the coefficient of kinetic friction between the surface and the clock is μk=0.2.


    2. Relevant equations

    conservation of momentum

    ∫ƩFdt=∫mdv

    Fr=N*μk

    N=W

    W=m*g

    and maybe ƩF=ma?


    3. The attempt at a solution

    Attempt is an attached pdf. I am stuck on how to find the force the bullet applied to the block. With that I could do Fb-Fr=ƩF, ƩF(t2-t1)=∫mdv where Δt would be my answer. Any guidance would be appreciated. Thank you!
     

    Attached Files:

    Last edited: Oct 19, 2012
  2. jcsd
  3. Oct 20, 2012 #2

    SammyS

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    Hello Mesmerr. Welcome to PF !


    For what length of time is the bullet passing through the block? You can answer this if you assume that the bullet experiences the same force throughout its path through the block.
     
  4. Oct 23, 2012 #3
    The question was how long does the block slide on the surface after the bullet leaves it. I noticed though that it said "after the bullet leaves the block". This means that the bullet is no longer acting on the block meaning you can use the equation of impulse to give you.

    ƩFblockΔt=mblockΔv where ƩF = Ffriction

    Thanks for the help though! I do see how if you are given the time through the block you can get impulse on the bullet. Then because of Newton's laws the block would experience the same force.
     
  5. Oct 23, 2012 #4

    cepheid

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    Why can't you just use conservation of momentum (for the bullet-block system) to solve this?

    EDIT: I see. Because external forces (friction) act on the system. OK. I don't see a problem with SammyS's approach. Using the impulse momentum theorem, you can at least figure out how much force the bullet applies to the block.
     
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