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PE/KE-Bullet Hits a Hanging Block?

  1. Nov 8, 2011 #1
    PE/KE--Bullet Hits a Hanging Block?

    1. The problem statement, all variables and given/known data
    A .05 kg bullet hits a 4 kg block hanging 1.5 m from the ceiling, displacing it by a height of .50 m. What is the KE of the bullet before it hits the block, and what is the max PE of the block?
    (So basically, a bullet hits a hanging block and makes it swing upwards by .50 m)

    2. Relevant equations
    PE = mgh
    KE = 1/2 mv^2


    3. The attempt at a solution

    I wanted to use 1/2 mv^2 to find the KE of the bullet before hitting the box, but where would I get v from? I figured I could set something equal to the max PE of the block which I'm assuming is 4 (9.8) (.50), but how would I find v?
    I figured the max PE of the block would just be mgh, so: 4 (9.8) (.50) But somehow that felt too easy, and I thought there should be more too it?
     
  2. jcsd
  3. Nov 9, 2011 #2

    PeterO

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    Re: PE/KE--Bullet Hits a Hanging Block?

    Using conservation of momentum is the only way to find the speed of the bullet.
     
  4. Nov 9, 2011 #3

    BruceW

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    Re: PE/KE--Bullet Hits a Hanging Block?

    A lot of the KE of the bullet will be used for deformation and heating of the block. So only a small amount of the KE of the bullet will be turned into the GPE of the block.

    On the other hand, momentum is conserved. So you could use a momentum calculation to find out what the speed of the bullet was.

    Edit: of course, momentum isn't conserved in the vertical direction, since we probably want to model the gravitational field as approximately constant. You could shoot the block horizontally and measure how far it swings out to? The momentum of the bullet would be transferred to the block as if the block were a pendulum.
     
    Last edited: Nov 9, 2011
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