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Homework Help: Need some help on 2D Momentum Problems

  1. Nov 23, 2004 #1
    A steel ball of mass 10kg moves due east at 5.0m/s. It collides with a rubber ball of mass 5.0 kg moving at 10m/s due north. After the collision the steel ball moves at an angle of 60 degrees East of North with a speed of 4.0 m/s.
    If the collision were perfectly elastic what would be the speeds of the two balls after the collision?

    Where do I start. I know total momentum before = total momentum after. But there seems to be 2 variables. vafter for steel ball and v after for rubber ball.

    Can an elastic collision have the 2 objects even stick together?
    If so then i got 1.92m/s.
  2. jcsd
  3. Nov 23, 2004 #2
    If the collision is elastic the 2 objects won't stisk together.

    Try thinking of momentum as a vector. You can use triginometry for the angel, but I don't think it's realy necesry.

    P.S. don't forget that the mass of the 2 balls is constant
  4. Nov 23, 2004 #3


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    You have two unknowns so you need two equations. Yes, one of the equations is "momentum before= momentum after", conservation of momenum. Since this collision is "elastic", the other is "kinetic energy before= kinetic energy after": conservation of energy.

    Momentum is a vector quantity so you will need to consider x and y components separately. Kinetic energy is a "scalar" quantity so you really have 3 equations for 4 unknowns (x and y components of both velocities) but fortunately you are given the angles.
  5. Nov 24, 2004 #4
    I need help on another question
    A ball of mass 3.0kg moving at a speed of 3.0ms has a head-on collision with a stationary ball of mass 4.0kg.

    If the collision were perfectly elastic what would be the speeds of the two balls after the collision?

    I used the momentum before=momentum after equation and the KE before= Ke after equation.

    I get weird answers. I get 2 answers. I get the velocity of the 3kg ball at -.42m/s and 3m/s. So whats the answer? If i know the velocity of the of this ball I can find the velocity of the other.
  6. Nov 25, 2004 #5


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    Using only considerations of energy&momentum conservation before and after collision will always yield two sets of possible answers (solutions):
    1) There hasn't been any interacting force between the two objects in the collision period.
    That is, there wasn't any collision after all.
    2) There has been a net momentum transfer (through a non-zero force) during collision.

    Your 3m/s possibility is clearly of the trivial 1)-type.
    Last edited: Nov 25, 2004
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