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Homework Help: Elastic collision, unknown masses & velocities, known displacements

  1. Nov 17, 2009 #1
    I am struggling with a problem that I could really use some assistance on. The problem sounds simple enough, but I am lost. here it is:

    Block A is on a frictionless table with block B. Block B is at rest, block A heads toward B and collides in an elastic collision. Block B falls off of the table a horizontal distance "D" from the edge of the table. Block A flies off the table in the opposite direction and lands a distance of "2D" from the edge of the table.

    Block A seems to be much smaller than B due to the displacements and with no friction, I dont think the length of the table matters...If you know of an example or tutorial I could get help from that would also be appreciated. Thanks
  2. jcsd
  3. Nov 17, 2009 #2
    I don't see a question...just remember that by "elastic" it means there's no loss of KE, and since the time taken to reach the floor by both blocks will be equal, the speed of A will indeed be double the speed of B.
  4. Nov 17, 2009 #3
    I need to show the masses and velocities of the blocks, but I don't know hoe to start the problem with the given information. I think I am struggling to grasp the algebra needed for this problem. Do you know of any tutorials similar to this one that I could compare or how to get started on the problem? Thanks.
  5. Nov 17, 2009 #4
    You can work out ratios of speeds/velocities of course, but there's no way you can reach a numerical answer with the information you're given...

    if you do want to pursue the ratios though i'd advice using the coefficient of restitution (elastic so e = 1) and the conservation of momentum equations.
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