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: Collision with friction

  1. Nov 4, 2004 #1
    I'm having trouble solving for the correct answer for this problem:

    A block of mass M1 = 3.5 kg moves with velocity v1 = 6.3 m/s on a frictionless surface. It collides with block of mass M2 = 1.7 kg which is initially stationary. The blocks stick together and encounter a rough surface. The blocks eventually come to a stop after traveling a distance d = 1.85 m . What is the coefficient of kinetic friction on the rough surface?

    With the help the problem provided, I get that KE = (1/2)m*v^2 which also equals momentum squared over 2 times the two masses added together; this equals the post-collision KE. I also find out that the Force of friction = (friction constant)g(m1 + m2) . Then through the vector dot product you can figure out that delta KE = F*d = (sliding friction coefficient)gd(m1 + m2)cos(180) . I calculate the initial KE as 69.4575 and the post-collision KE as 46.75 leaving delta KE as 22.7073. Then when I solve for the sliding friction coefficient, from the equation
    delta KE = (friction coefficient)gd(m1+m2)cos(180).
    After my numbers are plugged in and I solve, I get 0.240859 as the answer. However, the online program I'm using says it is incorrect. Am I doing something wrong?
  2. jcsd
  3. Nov 4, 2004 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    It is not meaningful to use (delta KE) here!
    What you then actually are saying, is that all of your initial KE is distibuted between post-collision KE, and the work of friction.
    This is patently untrue, since some of the initial KE were lost in the collision.

    What IS true, is that ALL post-collision KE is dissipated by friction.

    So, equate post-collision KE with work of friction!
  4. Nov 4, 2004 #3
    Thank you!!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook