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Math Help - Momentum

  1. #1
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    Momentum

    A railway truck of mass 4 x 10exp4 Kg moves at 3ms-1 towards a stationary truck of mass 2 x 10exp4 Kg. the trucks collide and join calculate the total momentum of the two before the collision. What is the total momentum of the two trucks after the collision? Find the speed at which the joined trucks move off after their collision. Compare the kinetic energy after the collision with that before the collision.
    Momentum= mass x velocity


    4 x 10exp4 x 3ms-1 = 120000 Kg ms-1


    2 x 10exp4 x 0ms-1 = 0 Kg ms-1 ( by stationary I take it, it means 0 ?)


    Total momentum = 120000 + 0 = 120000

    Total momentum after collision is equal to total momentum before collision if you exclude all other factors?

    Speed ?
    KE = ½mv2

    If this is all wrong, which I am pretty sure it is, can you please show your full working.

    Thanks.
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  2. #2
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    4 x 10exp4 x 3ms-1 = 120000 Kg ms-1
    That's the momentum of the moving truck before collision.

    2 x 10exp4 x 0ms-1 = 0 Kg ms-1 ( by stationary I take it, it means 0 ?)
    That's the momentum of the stationary truck before collision.
    Yes, it is zero.

    Total momentum = 120000 + 0 = 120000
    Before collision.

    Total momentum after collision is equal to total momentum before collision if you exclude all other factors?
    Yes. Momentum is always equal before and asfter collisions.

    Speed ?
    The total mass now of the combined two trucks is
    (4*10^4 +2*10^4) = 6*10^4 kg
    So, mV = (6*10^4)V
    And that is equal to 120000 or 12*10^4.
    Hence,
    (6*10^4)V = 12*10^4
    V = (12*10^4)/(6*10^4) = 2 m/sec.

    ----------------------------------------
    Here, I am not very sure because I am rusty in Physics.

    KE = (1/2)mV^2
    Before collision,
    KE = (1/2)(4*10^4)(3^2) +0 = 18*10^4 joules

    After collision,
    KE = (1/2)(6*10^4)(2^2) = 12*10^4 joules.
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  3. #3
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    what equation did you use to work out speed?
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  4. #4
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    Quote Originally Posted by hana_102 View Post
    what equation did you use to work out speed?
    It is shown there:
    momentum = mV
    120000 = (60000)V

    V is velocity. It is speed with direction.
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