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Relative velocity and momentum

  1. Dec 1, 2008 #1
    1. The problem statement, all variables and given/known data
    Tony (of mass 50kg)coasts on his bicycle (of mass 5 kg)at a constant speed of 6m/s, carrying a 12 kg pack. Tony throws his pack forward, in the direction of his motion, at 2m/s relative to the speed of the bicycle just before the throw. What is the initial momentum of the system (Tony, the bicycle, and the pack)?
    Answer in units of kgm/s.

    Does relative velocity here mean that the pack is actually moving at 8m/s?I'm a little confused as to what relative velocity is. To get initial momentum, I would just add all their momentums together, right?Because they have different speeds? Because I don't think I would be able to add their masses together and multiply by the velocity.
     
  2. jcsd
  3. Dec 1, 2008 #2

    turin

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    Yes, relative to the speed of the bike just means that you should imagine that the bike is at rest and the earth is moving 6 m/s backward. So, yes, the pack is moving at 8 m/s relative to the earth. However, are you sure this is relevant to the question that the problem asks?
     
  4. Dec 1, 2008 #3
    hmm i guess not b/c the pack should be moving at the same speed as the boy and the bike..but it would be if i were asked to find the momentum of the system after the pack was thrown i think..
     
  5. Dec 1, 2008 #4

    turin

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    Careful ...
     
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