Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Collision & Momentum

  1. May 25, 2008 #1
    The assignment was to observe the collision between two magnetic air pucks and determine the momentums and velocities at various points. This type of collision was elastic, and i had to prove this. So since this collision should be elastic, the total momentum before should equal the total momentum after the collision.

    I ended up with the total momentum before the collision to be 0.493 m/s*kg and the total momentum after the collision to be 0.520 m/s*kg. This answer resulted in a 5.2% error. Following these calculations, I had to disscuss why this percent error occurred. Any ideas?
  2. jcsd
  3. May 25, 2008 #2
    Elastic means that energy is conserved. You did the wrong calculation.
  4. May 25, 2008 #3
    This was an actual experiment. Two air pucks were placed on an air table. Airpuck one starts moving and airpuck two is stationary. Soon airpuck one collides with airpuck two. Dots were made on a piece of paper to observe the acutal movement and track of the two pucks. And from this we had to calculate momentum and velocity. So since this was an acutal experiement, chances of getting exactly an elastic collision is rare. There is a high chance of error in the experiment. And this is what i got. A percent error of 5.2%. But i dont know how to explain why.. besides human error of calculating and measuring.
  5. May 25, 2008 #4


    User Avatar
    Homework Helper

    an elastic collision would imply kinetic energy is constant as DavidWhitbeck said.

    So, then all the energy in the system should be entirely kinetic. Meaning that the only type of energy in the system is kinetic energy.

    Why do you think the experiment was done on an air table instead of just sliding the puck on a regular table?
  6. May 25, 2008 #5
    I think it was done on an air table rather than a regular table because this would cause less friction to occur. If there was more friction, then more kinetic energy and momentum is lost.
  7. May 25, 2008 #6


    User Avatar
    Homework Helper

    Even though there is less friction, there still is friction and so energy was lost as heat due to friction. Which might have used up enough energy such that the kinetic energy before was not the energy after and that could be why you got such a high error
  8. May 25, 2008 #7
    Okay then

    (a) you do not "prove" as you put it anything in an experiment. There is no such thing as proof in scientific experiments.

    (b) you can not simply assume that the collision is elastic, but you can test it.

    I'm just saying that if you are trying to test to see if the collision was elastic, then you do not compute the total initial and final momentum, you instead compute the initial and final total kinetic energy.
  9. May 27, 2008 #8
    what if the air table was slanted? how would this affect the collision of the two air pucks?
  10. May 27, 2008 #9
    Since the collision took place on a plane, it doesn't matter if the table was slanted or not, as in, elastic collision would remain elastic.

    However, in our case it matters because, in case of a slanted table, an external force acts on both the airpucks [i.e. a net external force acts on the system] and hence conservation of momentum does not follow. Even though in the case of a slanted table, the frictional force acting is less, for considerably big angles [as in.. 6-7 degrees and greater], the gravitational force assisting motion would be considerably more than the frictional force [because the [itex]\mu[/itex] of the air table is very small].
  11. May 27, 2008 #10
    You can prove, what for normal puck, which is spinning, friction is
    where R - radius, v - linear velocity, [tex]\omega[/tex] - angular velocity...
  12. Jun 1, 2008 #11
    would air resistance being neglected affect the results in anway?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook