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Homework Help: Does the Mass of an Object Fluctuate?

  1. Sep 9, 2007 #1
    I'm working on a momentum lab right now and I'm having a hard time finding the sources of error.

    What we did was take two cars. One was moving and one wasn't, and we had them collide and stick together. Then we had to figure out the change in momentum for each car.

    I thought my teacher mentioned something about having to assume that the masses stay constant thoughout the experiment. I don't know if this seems like a valid source of error or not.

    Also, does it matter if the moving car wasn't travelling at a constant velocity? Since there is no acceleration in the formula p=mv, I want to say no, but I'm not sure. (We measured the velocity at different time intervals and at the end used the average momentum).
  2. jcsd
  3. Sep 9, 2007 #2


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    This could be your problem - you should try to estimate the momentum at the time of collision.
  4. Sep 9, 2007 #3
    We did, but we wanted to see how an object's momentum compares before collision, during, and after. Because the velocity wasn't staying constant before collision, we had to use an average momentum there.
  5. Sep 9, 2007 #4
    what are you 'averaging' when you are 'averaging momentum'?
  6. Sep 9, 2007 #5
    Regards the question... no, with usual apparatus you should certainly not be able to detect the mass fluctuate, but nonetheless you should be highly aware of all assumptions you make. In principle... the cart is probably heavier after the collision.
  7. Sep 9, 2007 #6


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    - no parts fly off in the collision
    - no emissions or loss in fuel
  8. Sep 9, 2007 #7
    Is the car a "rigid" object, do you assume that the body of the car didn't crumple. If so, then you have to put the coefficient of restitution into a consideration. If you're wondering how acceleration be put into the equation, well, p = mv, maybe what you're talking about is the force, where F = dp/dt = ma.

    besides your teacher may say that you have to assume that the mass is constant, that is to remove all the complexities in your lab experiment.
  9. Sep 10, 2007 #8
    In the case of rocket propulsion, one must consider the rocket equation


    In other words, mass expelled contributes to the impulse of a rocket beyond the conventional Newtonian consideration.
  10. Sep 10, 2007 #9


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    I'm sayin':
    - do not consider the cars to be rigid objects (they stick together)
    - do not consider the mass of lost fuel (the teacher explicitly said not to, and besides: hello? they're CARS)
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