1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Staff Emeritus
    Science Advisor

    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


    User Avatar
    Science Advisor

    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?>
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Frictional Force, PLEASE HELP