1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Conceptual momentum problem

  1. Jul 27, 2011 #1
    1. The problem statement, all variables and given/known data

    An empty sled is sliding on frictionless ice when Susan drops vertically from a tree above onto the sled. When she lands, does the sled speed up, slow down, or keep the same speed?
    The sled slows down.
    Later, susan falls sideways off the sled. When she drops off, does the sled speed up, slow down, or keep the same speed?

    3. The attempt at a solution

    I would think that when she falls off the sled, the speed of the sled should increase because m2 got smaller, so to conserve momentum, v2 should get larger
    m1v1 = m2v2

    however, the answer is that the sled keeps the same speed... why?
  2. jcsd
  3. Jul 27, 2011 #2


    User Avatar
    Homework Helper

    Remember she fell sideways off the sled, taking her momentum with her.
  4. Jul 27, 2011 #3
    Would she not have "taken her momentum with her" if she had fallen off the back instead of sideways?
  5. Jul 28, 2011 #4


    User Avatar
    Homework Helper

    Yes she would. BUT if she took her life in her hands, stood up, then kicked forward on the sled so that she "left the sled" with zero momentum, the sled would have continued with all the momentum - back to its original speed.

    That "kicking off" is the way people commonly dismount from a skate board, on which they are already standing.


    By the way, you were only asked what would happen if she fell out sideways!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook