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: Impulse-Change in Momentum Lab

  1. Dec 3, 2006 #1
    1. The problem statement, all variables and given/known data
    Im doing an impulse-change in momentum lab, and somethings not adding up. My data is as follows:

    Trial Vmin Vmax Mass Impulse
    1 -.6447 .5766 .548 .9952
    2 -.9044 .7803 .598 2.281
    3 -.9416 .8343 .548 1.597
    4 -.5553 .4425 .598 1.246
    5 -1.054 .9313 .498 .9239
    6 -.4706 .3409 .498 .3408

    Then I figured Momentum by multiplying each Vmin and Vmax by its respective mass, then found the change in momentum by subtracting each Pmin from its respective Pmax. So the final data table looks like this:

    Trial Change in Momentum Impulse
    1 .6692 .9952
    2 1.0074 2.281
    3 .9732 1.597
    4 .5966 1.246
    5 .9886 .9239
    6 .4041 .3408

    The lab handout tells me that there should be some sort of linear relationship here, but when I graph it theres no rhyme or reason, let alone anything linear. Anyone know what I've done wrong?

    2. Relevant equations
    none, really, except that Momentum=m*v
    Impulse was found with the computer program that we collected the data with

    3. The attempt at a solution
    its up there in part one, kind of integrated with everything else. sorry about the lack of organization
    Last edited: Dec 3, 2006
  2. jcsd
  3. Dec 3, 2006 #2
    Allright, I ran a best-fit line anyway and got a slope of 1.9 and a y intercept of -.23

    The slope is supposed to be 1 and the y-int is supposed to be 0, since impulse=change in momentum. I would understand if there were a small error, but 1.9 is way off.

    I switched axises (axes?) and got a slope of .277 and a y-int of .432.

    Any ideas?
  4. Dec 3, 2006 #3


    User Avatar
    Science Advisor
    Homework Helper

    It would help if you told us what you did in the experiment. What was moving? What are the min and max velocities in your table? What produced the impulse and how was it measured?
    Last edited: Dec 3, 2006
  5. Dec 3, 2006 #4
    Sorry, it was a cart and track setup.

    |force sensor|--elastic-cord-----o|cart|o <[motion detector]

    We used a Vernier LabPro, and we found Impulse with the computer by finding the area under the curve on the graph.

    we pushed the cart away from the force sensor and it rebounded.
  6. Dec 3, 2006 #5
    is my data just really off, or is there some calculation problem that I'm making?
  7. Dec 3, 2006 #6


    User Avatar
    Science Advisor
    Homework Helper

    So vmin and vmax are velocities before and after the elastic has turned the cart, and hopefully the times at which those data were recorded correspond to the ends of the interval you used to calculate the impulse. I don't know how you matched up the times of velocity measurement with your force curves, but that is a potential source of error. You appear to have computed your momentum changes correctly, so I guess your data is very scattered. Fitting the curve to a line is the bast you can do.

    It would be appropriate in this case to do a fit that forces the line through the origin, since you expect zero change in momentum to correspond to zero impulse. I'm not sure if your calculator or computer program gives you that option, but if your fitted curve is far from the origin and you have the option to force it to the origin you probably want to use it.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook