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Linear momentum

  1. Oct 23, 2005 #1
    A gun is fired vertically into a 1.4 kg block of wood at rest directly above it. If the bullet has a mass of 21g and speed of 210 m/s how high will the block rise into the air after the bullet becomes embedded in it?

    the total momentum in the system is 210 x .021 = 4.41,
    after the collision, the 1.421 mass is travelling upward at 3.0964 m/s... how would i find how high the mass rises into the air?
  2. jcsd
  3. Oct 23, 2005 #2


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    y = y0 + v0t + ½at2
    v = v0 + at
    or energy conservation (it's conserved after the impact).
  4. Oct 23, 2005 #3


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    If one has a mass M, and it has an intial velocity V, it is a simple kinematics problem. For starters neglect air resistance.

    If something is traveling vertical, it will be decelerating due to gravity.

    Determine the equation that describes vertical motion in a gravity field.

    Alternatively, use conservation of energy. The initial energy is kinetic. The mass travels vertically until it stops (vertical speed = 0). Equate initial kinetic energy with the change in gravitational potential energy.
  5. Oct 23, 2005 #4
    I got the right answer using kinematic equations (.491m) but i tried using conservation of energy and got a totally different answer:

    KE initial = delta PE
    1/2(0.021)(210)^2 = 1.421 (9.8) h
    33.25 m = h
    how am i setting up the equation wrong?
  6. Oct 23, 2005 #5


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    Energy is not conserved during the collision (unless the collision is elastic, which it isn't in this problem), see my first reply.
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