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    illinihawks's Avatar
    illinihawks Posts: 10, Reputation: 1
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    #1

    Mar 29, 2011, 01:25 PM
    Bullet in Block
    A 0.001 kg bullet is fired from a gun and lodges inside a wooden block of mass 0.2 kg. The block and bullet then slide on a rough floor with a coefficient of kinetic friction μk = 0.4 before coming to rest after sliding a distance of 3 m.

    Compare KE_initial, the initial kinetic energy of the bullet, with W_friction, the macroscopic work done by the frictional force between the block and the floor in stopping the block.

    Not sure on this but, is KEi greater than W_friction because it is positive while w_friction is negative?

    The initial velocity of the bullet was:

    And this is the question I'm most stuck on. Thanks!
    illinihawks's Avatar
    illinihawks Posts: 10, Reputation: 1
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    #2

    Mar 29, 2011, 06:59 PM
    Only idea I could think of was setting the KE_initial equal to W_friction but I doubt that's write as I got 68.79 m/s which is wrong as the answer is supposed to be 975 m/s.
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #3

    Mar 29, 2011, 10:41 PM

    Use first the principle of conservation of momentum.



    m being the mass of bullet,
    u being initial velocity of bullet,
    M being mass of block
    U being initial velocity of block,
    v being final velocity of bullet,
    V being final velocity of block.

    U = 0,
    v = V

    Then, find the work done by friction, to get v the common speed of the block and bullet.

    Then use this value with your momentum equation to get u.
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