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: Conservation of momentum with friction

  1. Nov 3, 2008 #1
    1. The problem statement, all variables and given/known data

    A bullet of mass 1.9×10^−3 kg embeds itself in a wooden block with mass 0.991kg
    which then compresses a spring (k= 100N/m ) by a distance 4.5×10^−2m before coming to rest. The coefficient of kinetic friction between the block and table is 0.55.

    a) What is the initial speed of the bullet?

    b) What fraction of the bullet's initial kinetic energy is dissipated (in damage to the wooden block, rising temperature, etc.) in the collision between the bullet and the block?

    2. Relevant equations
    normal force x Mu,

    3. The attempt at a solution
    .5kx^2= .5mv^2- (Mu x F(g))
    2.25J= .5(.0019kg)v^2 - (.55)1.036kg(9.8m/s^2)
    v=91 m/s

    I don't know what I did wrong. Also I can answer the second part if I know the first part so just help me out with the first part.
  2. jcsd
  3. Nov 3, 2008 #2


    User Avatar
    Homework Helper

    Calculate the PE put in the spring:
    1/2*100(.045)2 = .10 not the 2.25 J you used. (You apparently used 1/2*k*x not x2 )

    Also m1 + m2 is .9929. Where did the 1.036kg come from?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook