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Momentum Concervation and Collisions

  1. Mar 13, 2008 #1
    1. The problem statement, all variables and given/known data

    A bullet with a mass of 7.00 , traveling horizontally with a speed of 400 , is fired into a wooden block with mass of 0.850 , initially at rest on a level surface. The bullet passes through the block and emerges with its speed reduced to 200 . The block slides a distance of 48.0 along the surface from its initial position.

    What is the coefficient of kinetic friction between block and surface?

    What is the kinetic energy of the block at the instant after the bullet passes through it?

    2. Relevant equations

    P=m1v1 + m2v2

    KE = 1/2mv^2

    ...I think, I'm not completely sure on if it is elastic or inelastic. Our physics book is honestly terrible and has no examples relating to this.

    3. The attempt at a solution

    I really have no idea where to start. I've tried setting the momentums equal to eachother and solving for v, but that doesn't work.

    Second question:

    A blue puck with a mass of 3.80×10−2 , sliding with a speed of 0.200 on a frictionless, horizontal air table, makes a perfectly elastic, head-on collision with a red puck with mass , initially at rest. After the collision, the velocity of the blue puck is 2.0×10−2 in the same direction as its initial velocity.

    Find the magnitude of the velocity of the red puck after the collision.

    Find the mass of the red puck.

    Once again, no idea where to start.
  2. jcsd
  3. Mar 13, 2008 #2


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    Homework Helper

    The letters in bold are what I assumed to be units.
    Before collision:
    Block has mass 0.850kg with a velocity of 0ms^-1
    Bullet has mass 7g with a velocity of 400ms^-1

    After collision:
    Block moves with a velocity of Vms^-1
    Bullet moves with velocity of 200ms^-1

    Using the law of conservation of linear momentum
    [tex]\frac{7}{1000}\times 400 + 0.850\times 0 = 0.850V+\frac{7}{1000}\times 200[/tex]

    Is that what you did and it didn't work out?

    Also an elastic collision is one in which kinetic energy is conserved.
  4. Mar 13, 2008 #3
    No that doesn't work. I have one more question, this section seems to be particularly bad at explaining the concepts.

    A 22.00 lead sphere is hanging from a hook by a thin wire 3.80 long, and is free to swing in a complete circle. Suddenly it is struck horizontally by a 5.00 steel dart that embeds itself in the lead sphere.
  5. Mar 13, 2008 #4


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    The velocity of the block is not obtained in that way? Well I don't see any other way to get it.

    What exactly are you supposed to do in this one?
  6. Mar 13, 2008 #5
    I though I edited in the question, sorry. It's too late now though, the assignment is due in 15 minutes. Thanks for your help.
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