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Homework Help: Momentum Word Problem.

  1. Jul 16, 2006 #1
    Two carts with masses of 4.3 kg and 3.2 kg
    move toward each other on a frictionless track
    with speeds of 5.8 m/s and 4.5 m/s, respec-
    tively. The carts stick together after colliding
    Find their final speed. Answer in units of

    I plugged in the numbers into this equation

    (4.3)(5.8) + (3.2)(4.5) = (4.3+3.2)Vf
    39.34 = 7.5Vf
    5.2453 = Vf

    That apparently isn't the correct answer =/
    Last edited by a moderator: Apr 22, 2017
  2. jcsd
  3. Jul 16, 2006 #2
    Remember that momentum is a vector it has magnitude and direction, so you need to take this into account when you work the problem. Notice that the problem says the two carts ae heading towards each other, so they have to be moving in different directions, so take velocity in one direction as positive and the other as negative and you should be able to get the correct answer.
  4. Jul 16, 2006 #3
    Okay cool, I got it . Thanks!
  5. Jul 16, 2006 #4
    No problem, glad I could help.
  6. Jul 16, 2006 #5
    I don't want to make a new thread so I'll ask this one.

    A 37.9 kg girl is standing on a 98 kg plank.
    The plank, originally at rest, is free to slide on
    a frozen lake, which is a flat, frictionless sup-
    porting surface. The girl begins to walk along
    the plank at a constant speed of 1.54 m/s to
    the right relative to the plank.
    What is her velocity relative to the ice sur-
    face? Answer in units of m/s.

    Am I using conservation of momentum or force here? I don't know where to start.
  7. Jul 16, 2006 #6
    Conservation of momentum, I'm pretty sure there is no such thing as conservation of force. Since she and the plank are initially at rest there is no initial momentum, so when she starts moving the plank must move in the other direction in order for momentum to be conserved.
  8. Jul 16, 2006 #7
    I meant using conservation of momentum or just "FORCE"...
  9. Jul 17, 2006 #8
    Yes, use coservstion of momentum .
    Why did the doubt arise ?
  10. Nov 27, 2006 #9

    Hey i need help i have a web assignment due soon and i could really use some guidence.

    A 10 metric ton train moves toward the south at 70 m/s. At what speed must it travel to have four times its original momentum? Answer in units of m/s.
  11. Nov 27, 2006 #10
    What do you think? What have you tried?

    You know the equation that describes its current momentum. If you want it to be four times greater, what do you do to it?
  12. Nov 27, 2006 #11
    i've solved it already thanks
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