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Homework Help: Frictional Force, PLEASE HELP

  1. Feb 28, 2005 #1
    Frictional Force, PLEASE HELP!!!

    A rifle with a barrel length of 55.0cm fires a 12.0g bullet with a horizontal speed of 350m/s. The bullet strikes a block of wood and penetrates to a depth of 11.0cm. What frictional force (assumed to be constant) does the wood exert on the bullet?

    heres what i do but its wrong:
  2. jcsd
  3. Feb 28, 2005 #2


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    That formula certainly won't work. It's for something moving over a rough surface.

    Do this instead: assuming (as the problem says) that the friction force is a constant, F, so that it has acceleration a= F/m, the velocity, t seconds after the bullet hits the wood, is v(t)= v0- (F/m)t and the distance moved in t seconds is x(t)= v0t- (F/2m)t2.

    We know that v0[/sub= 350 m/s and that the bullet penetrates 11 cm into the wood. Of course, the velocity when it has reached 11 cm is 0.

    Solve v(t)= 350- (F/55)t to find the time, t, it takes the bullet to penetrate the wood (depending on F of course). Now put that value of t into 350t- (F/2(12))t2= 11.0 and solve for F.
  4. Feb 28, 2005 #3


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    Conservation of Energy can also be used:

    {Delta Kinetic Energy} = {Frictional Force Work}

    {Delta Kinetic Energy} = (1/2)*m*(v_f)^2 - (1/2)*m*(v_i)^2 =
    = 0 - (1/2)*(12x10^(-3) kg)*(350 m/sec)^2
    = (-735 Joules)

    {Frictional Force Work} = {Friction Force}*(11x10^(-2) meters)

    ⇒ {Friction Force}*(11x10^(-2)) = (-735 J)
    {Friction Force} = (-6682 N)

    Last edited: Feb 28, 2005
  5. Feb 28, 2005 #4
    finding F is the hardest part for me. how do i solve for F?>
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