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Homework Help: Conservation of momentum

  1. Sep 3, 2015 #1
    1. The problem statement, all variables and given/known data

    Is the momentum going to be the same before and after the collision?
    (a car collides with a plastic thing) see pic

    2. Relevant equations

    I had this on a lab and I connected the machine to my computer. Then I calculated the momentum and it turned out to not remain the same. Why is that?

    3. The attempt at a solution

    Attached Files:

  2. jcsd
  3. Sep 3, 2015 #2
    Hey Drizzy, before you begin looking at the conservation of momentum, you need to define a system. Are you looking at the momentum of just the car, or the system containing both the car and the plastic thing? (also please label the diagram)

    Also what were your results? did the car bounce back at the same speed or slower than the incident speed?

    By law of conservation of momentum, a system of 2 colliding objects will always have the same momentum before and after they collide.
  4. Sep 3, 2015 #3
    I really din't know if tha plastic thing is a part of the system.

    Aftere the collision the car's velocity decreased

    mass of the car: 0,753kg
    Initial velocity of the car: 0,191m/s
    Final velocity(not relly final cuz it is going to stop eventually but right after the collision)= -0,173

    A calculated the momentum of the car:

    Before: 0,144
    After: -0,130

    The diagram that i had was a position-time and i derived it so i got the velocity. And the other diagram i had was a force-time. One of the conclusions that i came to is that delta momentum is the same as the impulse.
  5. Sep 3, 2015 #4
    what if the plastic thing is not in the system? What nocnlusion can we get?
  6. Sep 3, 2015 #5
    Is the plastic thing free to move like the car is, or is it grounded to the table?

    Also if you just look at the momentum of just the car alone, then yes it will be different after the collision, i mean it changes direction completely making it go from positive to negative.
    Momentum is only conserved within a system of colliding objects. So when the car collided with the plastic thing, it lost some momentum but the plastic thing gained some momentum, so if we look at the car & plastic-thing system, momentum is conserved.
    But the car alone lost momentum, leading to your results.

    And yes, by definition, impulse = delta momentum. so yours would be m(-0.130) - m(0.144)
  7. Sep 3, 2015 #6
    oh now I get it!!! the plastic stick is stuck to the table so it can't move! Thank you :)
  8. Sep 3, 2015 #7
    no prob!
  9. Sep 3, 2015 #8
    Is it correct to say "the car loses some of its momentum but the stick gets an impulse "
  10. Sep 3, 2015 #9
    well by law of conservation of momentum, the stick gains as much momentum as the car looses.

    The reason why the stick does seem to move after the collision is that it is connected to table which is connected to the earth which has a LARGE mass. When you hit the earth with something, it moves a very little but you can't see it
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