1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: A bullet with a velocity of 575m/s Use work and energy considerations

  1. Oct 20, 2009 #1
    1. The problem statement, all variables and given/known data

    A 7.8g bullet moving at 575m/s penetrates a tree trunk to depth of 5.50cm. (a) Use work and energy considerations to find the average frictional force that stops the bullet. (b) Assuming that the frictional force is constant, determine hot much time elapses between the moment the bullet enters the tree and the moment it stops.

    2. Relevant equations

    I figure part (a) by:

    KEi - Fk = KEf
    KEi= kinestic energy initial, Fk=frictional force, and KEf= kinetic energy final

    For part (b) I thought that the t=D/r would be sufficient but it's not?? I don't really understand what other equations to use in this scenario... HELP!

    3. The attempt at a solution

    For part (a)

    1/2mV^2 - Fk*d = 1/2mVf^2
    1/2(.0078)(575)^2 - .055Fk = 0
    1289.44 - .055Fk = 0
    -.005Fk = -1289.44
    Fk = 2.34e4 N

    For part (b)

    t= .055/575
    t= 9.5e-4 --> which is not correct, book shows 1.91e-4, huh? :cry:
    1. The problem statement, all variables and given/known data
  2. jcsd
  3. Oct 20, 2009 #2
    What you are doing is assuming that the bullet travels at a constant velocity and suddenly stops after penetrating 5.50cm into the wood o.0, which definitely does not hold true especially when it is decelerating at a constant rate!
    Think along the lines about the relation between impulse and force-time.
  4. Oct 20, 2009 #3
    Ohhh... so this is a momentum question.

    F delta t = mVi - mVf


    F delta t = (.0078)(575)- (.0078)(0)
    2.34e4 (delta t)= 4.49
    delta t = .000191 = 1.91e-4

    Ah.. Thank you!!!!!! I would have never of seen that relationship... grrr.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook