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!

Collision Question problem with question itself!

  1. Mar 12, 2006 #1
    This is a problem that though I've solved has had me racking my brains trying to figure out whether or whether not I actually understand what I'm doing..here goes:

    Particles A and B have mass m and are moving in the same direction along a line, A with speed 3u and B with speed u. They collide and after the impact they both move in the same direction, A with speed u and B with speed ku. The coefficient of restitution is e

    a) show that e = (k-1)/2
    b) Deduce that 1 =< k =< 3
    c) Find the loss in kinetic energy in terms of m, k, and u.

    As I said above...reaching the answers to these questions is not the problem...the bit thats bothering me is that before getting to the bit where I could solve part a) I had come to the conclusion that k can only = 3. My reasoning is as follows:

    The total momentum of the system before impact is 3mu + mu, and given that I know the momentum of A after impact (mu) I can say that:
    mu + kmu = 4mu...k = 3...the other way I look at it is as follows:

    B imparts an impulse on A that changes it's velocity from being 3mu to mu, in doing this A must impart the same impulse to B such that:
    -(mu - 3mu) = (kmu - mu)...k still = 3 (the minus sign is because J acts in different directions)

    The question implies however that e and k are not already determined and can take any value beween 0,1... 1,3 respectively.
    The only conclusion that I can reach however is that this is wrong and that given the mass of both objects, their initial speeds and either the value of e or just the speed of either A or B any other values are forced...this question stated otherwise :frown:
     
    Last edited: Mar 12, 2006
  2. jcsd
  3. Mar 12, 2006 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Your thinking seems correct to me. The problem--as stated--makes no sense. Since you are given the final velocity of one of the masses, all else is determined.
     
  4. Mar 12, 2006 #3
    ah cheers for backing me up Doc Al...I've spent too long torturing myself over this one :smile: (The question I have stated is quoted word for word from the book (forgot the period at the end of the paragraph though :smile: ))
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Collision Question problem with question itself!
  1. Collision Question (Replies: 5)

  2. Collision Questions (Replies: 3)

  3. Collision Question (Replies: 6)

Loading...