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Homework Help: Collision experiment

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

    My class had a lab where we had to push a car toy and then let it bounce back. The on the computer we got a diagram on the impulse and then another diagram with position on y axel and time on the y axel. Then we had to make a chart on the impulse and the momentum before and efter the collision. We did this experiment with cars of different mass.

    now er have to have a conclusion.

    2. Relevant equations
    I dont have any equations

    3. The attempt at a solution

    So the conclusion I have is that the delta P is the same as the impulse and that the collision was non elastic.

    I dont know if this is the right conclusion. What do you think?

  2. jcsd
  3. Sep 1, 2015 #2
    Need more information.

    I will guess that you bounced cars off a rubber bumper which was attached to a strain gauge so the computer could read the force against the bumper in real time.
  4. Sep 1, 2015 #3
    Yes you are correct.

    delta p = 0,283

    impulse = 0,286

    this is what i got. So what is the conclusion?
  5. Sep 1, 2015 #4
    If the magnitude of velocity of a car immediately after the bounce was less than immediately before, your conclusions sound correct.
  6. Sep 1, 2015 #5
    yaay thanx :) should I write something more?
  7. Sep 1, 2015 #6
    I don't think so. Have a nice day.
  8. Sep 1, 2015 #7
    You too :)
  9. Sep 1, 2015 #8


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    You wrote that it was done with cars of different mass. That suggests you should look at how this affected the results. Can you find a quantitative result that was independent of mass?
  10. Sep 2, 2015 #9
    well the momentum is higher if we have higher mass but I don't know I can't see a pattern except for that the change in momentum is equal to impulse.
  11. Sep 2, 2015 #10
    Can you please help me?
  12. Sep 2, 2015 #11


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    I don't know what data you collected.
    Mass of car
    Impact speed?
    Rebound distance?
  13. Sep 2, 2015 #12
    Here are my data:

    TEST 1:
    Delta P=0,368

    TEST 2:
    Delta P=0,247

    I had different mass in the two tests.
    The first conclusion that i have is that delta p is the same as the impulse. But now i have a question. The momentum before and after a collision should be the same. Why did it change in my tests?
  14. Sep 2, 2015 #13


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    Not sure what you are saying there.
    You concluded that ##\Delta P## was "the same as the impulse". I presume you mean that you know the impact velocity of the car, the rebound velocity of the car, and its mass, and from these you compute a ##\Delta P##. Separately, the instrumentation tells you the impulse experieced by the wall. You say they are observed to be the same, but I note a significant discrepancy in the second test.
    When you say "The momentum before and after a collision" was not the same, do you mean that the rebound speed was lower than the impact speed? All that means is that the impact was not completely elastic.

    The further analysis I was hinting at is what determines how much speed is lost? Is it related to the original speed, to the mass, or both? However, seeing you only have two datapoints you cannot really go into that. As a general comment, were you only expected to run two tests? I would have thought that rather inadequate for drawing any conclusions.
  15. Sep 3, 2015 #14
    I was supposed to do 3 tests but I didn't have time. The speed of the car before the collision was higher than after. and that affected the momentum. cuzz the mass was the same but the velocity decreased. It wasnt really a wall, it was a plastic thing but maybe that doesn't matter.

    But why did it get an impulse from the "wall"? according to conservation of momentum the momentum befor and after should remain the same
  16. Sep 3, 2015 #15


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    Momentum is only sure to be conserved in a closed system - no external forces. The 'wall' would be joined to a bench or whatever, and ultimately to the Earth.
  17. Sep 3, 2015 #16
    hmm i'm not sure if I understand what you mean. If two cars collide (in real life, not this expretiment) Does that mean that the momentum is not going to be the same just because the cars are on the ground which is connected to earth? Isnt everything here connected to earth?
  18. Sep 3, 2015 #17


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    That's rather different. If the cars collide 'freewheeling' then there is no horizontal force between cars and ground, so for the purposes of horizontal momentum they form a closed system. As I wrote, the key is "no external forces", but I perhaps should have added "in the direction of interest". Typically, the cars would be braking at approach to impact, but during the impact the braking force is small compared with the collision force and can be largely ignored.
    In your car rebound experiment, the external force is almost entirely responsible for the change in momentum.
    Reading back through your posts, I wonder if you understand what 'change in momentum' means in this context. Momentum and velocity are vectors. If the car mass m approaches the wall with velocity ##\vec u## and rebounds, straight, at velocity ##\vec v## then the change in velocity is ##\vec v -\vec u## and will have magnitude |u|+|v|. The magnitude of the change in momentum is m(|u|+|v|). All of that momentum change comes from an impulse from the wall.
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