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!

Block hits a rod with axle in middle

  1. Apr 30, 2009 #1
    A cube of mass m slides without friction at speed v0 and undergoes a perfectly elastic collision with the bottom tip of a rod of length d and mass 2m. The rod is pinned through its center through a frictionless axle. And initially hangs straight down at rest. What is the cubes velocity both speed and direction after?

    Im kinda lost on this one on where to start.

    So i tried momentum equations
    mv0 =mv1+2mv1

    but the answer is v1=(1/5)v0

    Im guessing both objects do not receive the same speed so this would be where my equation fails...
     
  2. jcsd
  3. Apr 30, 2009 #2
    So this just popped in my head but i'd like to check if my idea is correct
    M/V is a constant. Thus the velocity for the rod must be 2v1

    Go back to the momentum equation
    mv0= mv1 +2m*2v1
    solve for v1
    v1=1/5 v0

    Thoughts?
     
  4. May 1, 2009 #3
    Why are you using a common value for the velocities after the collision? (v1)
    That's generally for an inelastic collision, where both objects 'stick together' and move off with a common v.
    Here the collision is perfectly elastic.
    What do you know about perfectly elastic collisions? What quantities are preserved?
     
  5. May 1, 2009 #4
    I tried pairing momentum and energy equations together and solving in terms of v0 and v1. but the answer was incorrect
    Kinetics:
    .5m v02 = .5mv12+.5Iw2
    where I=(1/12)MR^2
    M=2m
    and w= v2/R
    thus mv02=mv12+(1/6)mv22
    v02=v12+(1/6)v22
    Momentum:
    mv0=mv1+Iw
    v0=v1+(1/6)v2R
    With the R left i was unsure of what i could do here
     
    Last edited: May 1, 2009
  6. May 1, 2009 #5

    Doc Al

    User Avatar

    Staff: Mentor

    OK.
    No need to introduce v2; stick with ω.
    You want angular momentum, not just momentum. Fix two of those terms.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Block hits a rod with axle in middle
  1. Ball hitting a rod (Replies: 14)

  2. Block hitting spring (Replies: 27)

Loading...