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Math Help - Momentum, Mechanics 1

  1. #1
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    Momentum, Mechanics 1

    Two boys, Jason and Keith are travelling on small trucks which are both coasting at 3ms-1 in the same direction along a straight horizontal track. Each boy and the truck he is standing on have a total mass of 120 kg. jason on the trailing truck has a medicine ball of additional mass of 5 kg which he is going to throw to Keith. The trucks are taken to have small frictionless wheels of negligible mass and air resistance is neglected.

    (a) Find the total momentum of Jason, his truck and the medicine ball

    (b) Jason throws the ball forwards so that it travels through the air with a horizontal component of 9 ms-1 towards Keith. Find the velocity of Jason after he has thrown the Ball.
    (c) keith catches the ball. Find out how much faster than jason he is travelling after he has caught it.


    My Attempt:

    (a) 125 x3= 375N

    (b) 120x3+5x9= 125V
    405=125v ( I am not sure, Please help me)
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  2. #2
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    Re: Momentum, Mechanics 1

    (a) you've calculated the correct value, but note that Newtons are units of force, not momentum.

    (b) conservation of momentum ...

    Jason ...

    (125 \, kg)(3 \, m/s) = (120 \, kg)v_f + (5 \, kg)(9 \, m/s)

    Keith ...

    (120 \, kg)(3 \, m/s) + (5 \, kg)(9 \, m/s) = (125 \, kg)v_f
    Last edited by skeeter; November 24th 2012 at 11:09 AM.
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  3. #3
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    Re: Momentum, Mechanics 1

    Initially, Jason has momentum of 125*3= 375 "kg-m/s", not "Newtons", as skeeter said. If he throws the ball forward, with velocity v (relative to the ground), The ball has momentum 5*9= 45 kg-m/s and, by conservation of momentum, Jason has momentum 375- 45= 330 and so his new velocity is 330/120 = 2.75 m/s.

    Keith's initial momentum was 120*3= 360 kg-m/s. After he catches the ball his momentum will be 360+ 45= 405 kg-m/s and the total mass of Keith, truck, and ball is 125 kg so his new velocity is 405/125= 3.24 m/s.

    Of course, the problem asks for the difference in their speeds.
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  4. #4
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    Re: Momentum, Mechanics 1

    Thank you Skeeter and Halls of Ivy for your time and detailed explanation. Thank you Thank you Thank you
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