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Homework Help: Speed Of A Bullet Before It Strikes A Block Of Wood

  1. Aug 28, 2011 #1
    1. The problem statement, all variables and given/known data

    A 10g bullet strikes and becomes embedded in a 0.200kg block of wood placed on a horizontal surface just in front of the gun. If the coefficient of sliding friction between the block and the surface is 0.4 and the impact drives the block a distance of 8m before it comes to rest, what is the velocity of the bullet before it strikes the block of wood?



    2. Relevant equations

    E(kinetic)=E(frictional)
    1/2(m1+m2)v2^2=μ(m1+m2)gs

    or

    ma=μmg

    a=μg

    v^2=u^2+2as

    v1(m1)=v2(m1+m2)


    3. The attempt at a solution

    I've tried 2 different ways to do it which are:
    1.)

    m1=0.010kg
    m2=0.200kg
    s=8m
    μ=0.4

    1. E(kinetic)=E(frictional)
    1/2(m1+m2)v^2=μ(m1+m2)gs
    1/2(0.210)v^2=0.4(0.210)(9.8)(8)
    v2=7.92m/s

    m1v1=(m1+m2)v2
    0.010v1=0.210*7.92
    v1=166.32m/s

    2.)

    ma=μmg

    a=μg
    a=0.4*9.8
    a=3.92

    v^2=0
    a=-3.92
    s=8m
    u^2=?

    v^2=u^2+2as
    0=u^2+2(-3.92)(8)
    u^2=7.92

    m1v1=(m1+m2)v2
    0.010v1=0.210*7.92
    v1=166.32m/s

    I'd love to know if I'm on the right track here. Thanks a million in advance.
     
  2. jcsd
  3. Aug 28, 2011 #2

    PeterO

    User Avatar
    Homework Helper

    I would have used your method 1. and agree with your calculations.
    There is a chance, that if this was an introductory physics question, that you were only required to use g = 10. This makes calculations much simpler and able to be done without a calculator. In that case that first velocity is 8, and the initial velocity this 168 m/s,
     
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