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Homework Help: Work-Energy Incline Problem with Two objects

  1. Nov 1, 2012 #1
    1. The problem statement, all variables and given/known data

    Block 1 is moving to the right (with velocity v1) when block two is released from rest on
    an incline, as shown (from height hi). They meet on the horizontal section of track. After
    they collide they move in opposite directions, but with the same speed. Block 2 moves
    back up the incline and comes to rest at height hf. Find m2.

    m1 = 4.5 kg
    v1 = 0.3 m/s
    hi = 0.5 m
    hf = 0.3 m


    2. Relevant equations

    3. The attempt at a solution

    I wasn't really sure where to begin with this problem. I began by trying to find m2's velocity at the bottom of the incline with kinematics equations, getting vf = 2.38 m/s.

    mgsinθ = ma (y direction)
    a = 4.9 m/s^2

    vf^2 = vi^2 + 2ad
    vf = 2.38 m/s

    If this collides with m1 moving at .3 m/s, and m2's final PE = mgh, where h = .3 m, I'm not sure how to find the mass using only energy equations, not kinematics... So I got stuck here and there's probably a more efficient way to do the problem to begin with.

    Attached Files:

  2. jcsd
  3. Nov 1, 2012 #2


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    Science Advisor
    Homework Helper

    welcome to pf!

    hi mcmahc! welcome to pf! :smile:
    so, for the collision on the ground, (energy is irrelevant but …) you can use conservation of momentum, combined with vf1 = vf2 :wink:
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