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Homework Help: Find momentum of two objects

  1. Jul 23, 2009 #1
    1. The problem statement, all variables and given/known data

    Object 1 has a mass m1 and a velocity 1 = (2.70 m/s) on the x-axis. Object 2 has a mass m2 and a velocity 2 = (2.95 m/s) on the y-axis. The total momentum of these two objects has a magnitude of 18.6 kg·m/s and points in a direction 66.5° above the positive x-axis. Find m1 and m2.

    3. The attempt at a solution

    I honestly don't even know where to begin. I am finding it impossible to determine mass 1 and mass 2 without knowing one or the other. How do I even set this one up?
    Last edited: Jul 23, 2009
  2. jcsd
  3. Jul 23, 2009 #2


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    Homework Helper

    Re: Momentum

    Hi MarineBio,

    Was there some more information given, perhaps in a diagram? (For example, showing that mass 1 goes along the x-axis and mass 2 goes along the y-axis, or something like that.)

    I'm surprised that they would say the velocity is 2.7m/s (for example) without giving a direction or a unit vector. If they did not give a direction, it seems like they would have referred to it as just the speed.
  4. Jul 23, 2009 #3
    Re: Momentum

    Shoot, when I copy/pasted it did not include the x and y hats.

    V1 is on the x axis
    V2 is on the y axis
  5. Jul 23, 2009 #4


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    Gold Member

    Re: Momentum

    That makes things relatively simple! Since momentum is a vector quantity the component in the x and y directions should be: 18.6cos(θ) and 18.6sin(θ) respectively. Just resolve the components!

    Since V1 is on the x-axis: m1V1 = 18.6cos(θ)

    A similar argument can be made for the y-axis component.
  6. Jul 23, 2009 #5


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    Homework Helper

    Re: Momentum

    Okay, then you can add up the momentum: the momentum of m1 + momentum of m2 = total momentum.

    Remember though that since momentum is a vector quantity, and this is a 2-D problem, you'll end up needing to have a momentum equation for the x-direction, and a momentum equation for the y-direction.

    Does that help? If not, please post what you are trying.
  7. Jul 23, 2009 #6
    Re: Momentum

    Ha! I was making that one WAY more difficult than it actually was. Thank you, problem solved!

    (this is such a great forum) :smile:
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