Conservation of Momentum

Discussion in 'Physics & Math' started by kingwinner, May 13, 2006.

  1. kingwinner Registered Senior Member

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    796
    I have some questions regarding the Law of Conservation of Momentum, can someone help me out? Thank you!

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    1) One air puck is pushed to collide another puck in the middle of the air table to make a head-on collision. A Velcro adhesive strip collar has been wrapped aroudn each puck so they stick together and move after the collision. When I calculate the total initial and total final momentums, I got 0.3473 and 0.3282 respectively.

    2) Two air pucks are pushed to make a glancing collision. A Velcro adhesive strip collar has been wrapped aroudn each puck so they stick together and move after the collision. When I calculate the total initial and total final momentums, I got 0.5455 and 0.4978 respectively.


    I have measured and done the calculations carefully, but both labs do not support the law of conservation of momentum well. I don't know why the final total momentum is getting lower. Is it because of some kind of friction? Will friction decrease the total momentum?

    Also, for the 2 collisions described above, are they completely inelastic collisions where the final total kinetic energy will be much lower than the initial total kinetic energy?
     
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  3. §outh§tar is feeling caustic Registered Senior Member

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    in #2, are those values for the momentum of each puck?

    possibly your air pucks are too close to the glider. i can't remember this lab too well. air currents in the room can also really affect things.
     
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  5. CANGAS Registered Senior Member

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    Will friction decrease momentum?

    When your car runs out of gas and you are coasting on the road, what decreases its momentum so that it probably eventually stops on the railroad tracks?
     
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  7. Zephyr Humans are ONE Registered Senior Member

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    Momentum can change, but not total momentum, right? So for rotational velocity of a car (around the earth) to decrease, rotational velocity of the earth, or the air around the earth, or somesuch, must be (minutely) affected.
     
  8. Zephyr Humans are ONE Registered Senior Member

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    BTW, did you record error estimates for these results? They're not that disparate. And did you check things like the slope of the air table?
     
  9. DaleSpam TANSTAAFL Registered Senior Member

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    I agree with Zephyr. To me it looks like momentum is conserved unless you have some incredibly accurate measurement system in your lab. Momentum is conserved even when KE is not conserved.

    How did you measure momentum? If you have any kind of error estimate for your measurements then you can look at the Propagation of Error to determine if you are getting agreement. If you really are not getting agreement then since the final is consistently less than the initial it is likely due to some of the system's momentum being transfered to the air and table.

    -Dale
     
  10. kingwinner Registered Senior Member

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    796
    The values I posted are the total momentum of both pucks before and after collision. The slope of the air table is horizontal because when I place a puck on it, it remains stationary. But the air table has some friction, byt will friction will decrease the total momentum? If so, the results make more sense...

    Ironically, the question in both labs is whether total momentum will be conserved or not...the % difference of the first lab is -5.47% and for the second lab is -8.74%. With such % difference, should I still say that total momentum is conserved?
     
  11. kingwinner Registered Senior Member

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    796
    For both labs, the two pucks stick together after collision, is this considered completely inelastic collision in which the total kinetic energy of both pucks will decrease after collision?
     
  12. przyk squishy Valued Senior Member

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    Friction will inevitably decrease the momentum of the pucks. It's just a question of by how much. It shouldn't really be noticeable on an air table.
    Depends on how accurate the measurements you made during the experiment were.
    Yes (but they heat up, which is another kind of more messed up kinetic energy).
     
  13. DaleSpam TANSTAAFL Registered Senior Member

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    Without some estimate of the errors it is not possible to say wether or not momentum was conserved.

    -Dale
     
  14. kingwinner Registered Senior Member

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    796
    But I know that if the net force on a "system" is zero, the initial total momentum must equal to the final total momentum. And even friction is involved, the net force on the "system" is still zero, right?
     
  15. przyk squishy Valued Senior Member

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    That's what the theory says. The point of an experiment is to test a theory. The results of your experiment are in agreement with the theory if the theory's prediction is within the experimental error.
    Depends on how you define your system. If your system is just the 2 pucks, then no because the friction is an external force and the net force on the system is not zero. If your system is defined as the two pucks + the table + anything the table is in contact with, then yes, but your system then becomes the entire planet Earth (which was outside the scope of your experiment).
     
  16. kingwinner Registered Senior Member

    Messages:
    796
    1) For the first lab, one air puck is pushed to collide another puck at rest in the middle of the air table to make a head-on collision. The two pucks stick together after the collision by some Velcro adhesive strip collar wrapped around each puck. The masses of the 2 pucks are 0.654kg and 0.469kg respectively.

    I was given this question "Why is it important for us to ensure that the 2 pucks have different masses than each other for this lab?" and I really have no idea. Why can't I perform such a lab with pucks of the same mass? What would happen? Can someone please give me some hints? Thank you!
     
  17. przyk squishy Valued Senior Member

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    I have no idea why the 2 pucks need different masses.
     

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