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Homework Help: Momentum Physics Question?

  1. Feb 10, 2014 #1
    I'm confused on how to start this Physics problem. Please help me and go through the steps with me.
    Thank you.

    Two trucks collide. One weighed 1500 newton and was traveling 45.0 m/s at 105 degrees. The second weighed 1800 Newton and was traveling 27.5 m/s at 55 degrees. Find the resulting velocity of the trucks if they stuck together on impact

    Ok so the first step i did was get the mass which is the force (1500) divided by 9.8 m/s^2 and got 150 kg and did the same to truck #2 and got 180 kg.
    Because i know we have to use the mass to find the momentum which is force time velocity.

    However after that step what i do?
  2. jcsd
  3. Feb 10, 2014 #2
    The setup will be one of a completely inelastic collision:$$m_{1}v_{1} + m_{2}v_{2} = (m_{1} + m_{2}) v$$This means that the sum of the individual momenta will equal the momentum of the combined masses. However, since this problem is in two dimensions, you need to use the equation twice: once for the x-components of velocity, and again for the y-components.
  4. Feb 10, 2014 #3


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    Hello lover and welcome to PF. A special welcome, I might add, because the nickname fills us with (false?) pride.

    Somehow you managed to evade the template. But your rendering of these toy trucks smacking into one another gives me the impression you've seen it come by. Please use it next time.

    Now to business: One of the relevant equations would be F = mg which you used to calculate the m. Decently written exercise: weight as a force. We like that.
    Another would be conservation of momentum. You should have learned that momentum is conserved in accidents like this. Momentum is not force times velocity, but mass times velocity. masses you have, velocities you have also. Note that velocities are vectors: they have a magnitude (given) and a direction (also given). All you have to do is add these momentum vectors to get the momentum vector of the stuck-together wreckage, of which you can easily calculate the mass. Divide by mass and voila: the velocity vector !
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