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: Rigid body/perfectly plastic collision question

  1. Dec 4, 2008 #1


    User Avatar
    Gold Member

    1. The problem statement, all variables and given/known data
    Today a friend of mine came to my apartment and we discussed a problem. I realized something that surprises me A LOT.
    Consider the following situation. There are 2 disks of radius R and mass M on a frictionless table. One doesn't move while the other suffers a translation with speed v and will hit the quiet disk such that its lowest point will touch the upper point of the other disk. When it hits it, the two disks remain attached forming a new rigid body.
    Calculate the angular velocity of the rigid body.
    I think one gets the answer using the conservation of angular momentum or something like that. The result is [tex]\frac{v}{6R}[/tex]. I didn't realized it and thought that the kinetic energy would be conserved and I got a result of [tex]\omega=\frac{v}{\sqrt{6}R}[/tex]. As [tex]\omega=\frac{v}{\sqrt{6}R} > \frac{v}{6R}[/tex] it means the system loses kinetic energy by a factor [tex]\frac{1}{\sqrt 6}[/tex]. Why such a number? Is that true with any plastic collision?
    And another question : How does this energy converts itself into heat? There's no friction nor elastic deformation due to compression since we're talking about rigid bodies. I don't see any way to convert the energy. Hence I'm missing something... can you help me answering some (or all) these questions ? :smile:
  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted