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: Momentum Question

  1. Sep 19, 2004 #1
    Hey everyone!
    I am having some trouble with this problem. I would appreciate any help/advice/hints.

    A child of mass 33 kg jumps off of a wagon. After jumping, the child's velocity is 4.0 m/s
    . If the mass of the wagon is 7.2 kg, determine the velocity of the wagon after the child jumps off.

    All I have figured is that the childs momentum is 132 Kgm/s right.
    I am confused as to how to get the velocity of the wagon now. I do not have enough info to use Momentum= Mass X Velocity.

    Any help would be appreciated.​
  2. jcsd
  3. Sep 19, 2004 #2
    Just before the child jumps off the wagon, do they have any velocity?
  4. Sep 19, 2004 #3
    I think you mean, is the wagon moving with the child in it?? and if so, i do not know. What i have here is what is given in the question. this is what is messing me up too. im not sure if i should assume the wagons V is 0.
  5. Sep 19, 2004 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Assume that the wagon is not moving. You know the momentum before (0) and you have the momentum of the child. What must be the momentum of the wagon after the child jumps if momentum is conserved?
  6. Sep 19, 2004 #5
    Yeah, well if the wagon just starts moving as the child jumps off, you do:

    momentum initial = momentum final
    (33kg+7.2Kg)(0) = (33Kg)(4)m/s) + (7.2Kg)(x)

    Then, just solve for x
  7. Sep 19, 2004 #6
    the momentum is therefore equal to the momentum of the child????
    v= 18.3 m/s
    Is that correct??
    thanks for the help guys
  8. Sep 19, 2004 #7

    If you assume the wagon has a velocioty to start with that is unknown, the proceed like this. (to the right is the positive direction)

    The momentum after is M(wagon) *V(wagon after) plus 132.

    This is equal to m(wagon plus child)*v(wagon and child before)

    You can then solve for V as a function of v.

    If v=0, Then V=132/7.2 to the left.

    Last edited: Sep 19, 2004
  9. Sep 19, 2004 #8
    ok i'm not sure if i understand. so you are assuming V(Wagon) before is 0 so v=132/7.2. (this is what i was doing) what if you assumed V(wagon) to be 1m/s. can you make an equation for this instance??
    Last edited: Sep 19, 2004
  10. Sep 19, 2004 #9
    I guess you must assume the initial velocity of the wagon and the child (before he jumps) is zero, unless it is stated in the problem. You can't just asssume the initial velocity is a random number.
  11. Sep 19, 2004 #10

    The general equation works out to be as follows.


    where V=vleocity of the wagon after the child jumps off

    and v=the velocity of the child and the wagon before the child jumps off.

    This equation results from equating the total momentum of the wagon and the child before to the total momentum of the wagon and child after.

    The velocities are considered positive to the right and negative to the left.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook