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Momentum/velocity questions (very simple)

  1. Dec 3, 2005 #1
    A 10 kg block is hanging by a string. How high will it go up if a 2 kg rock going up at 60 m/s hits it and "sticks to it"? (acceleration of 9.8 m/s^2) this has to relate with momentum i think

    a 2kg cart and a 4 kg cart are moving together at 4m/s velocity... a spring compressed between them expands rapidly, the front cart continuse at 12m/s in the same direction

    a) velocity of 2kg cart after "explosion"
    b) if explosion took 0.25s, whats the force of the explosion ( id ont know what mass to use, what acceleration to use)

    c) velocity of front cart to acquire in order for second cart to remain stationary after explosion

    formulas supposedly used are

    F= ma (also is [mass * (change of velocity) ] / change of time )
    impulse = (F)(change of time)

    thanks for any help


    i tried doing them, for the carts question..
    a) p=mv
    p = (6)(4) 6 kg being total masss of the carts
    p = 24 kgm/s

    so momentum after is
    24 = (4kg . 12 m/s) + (2kg . v) 4kg cart moves at 12 m/s after (its given)
    -24 = 2v
    v = -12 m/s

    so 2kg cart moves -12 m/s after ? i dont know but its a guess

    b) if explosion took 0.25s whats the force of it?

    well i used the impulse = force * change of time formula..

    24 kgm/s = F * 0.25
    F = 24 / 0.25
    F = 96 N? <--- is that correct? i dont know hope so =D

    the last one i dont know because i think the 4kg needs to move really really fast in order for the 2kg cart to not move the opposite way at all.. but no clue how to do it
    Last edited: Dec 3, 2005
  2. jcsd
  3. Dec 3, 2005 #2


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    Homework Helper

    The 2 [kg] cart's negative velocity tells you the direction of its motion.
    good job.

    Impulse ([tex]\vec{F} \cdot \Delta t[/tex]) is the quantity that CHANGES an object's momentum. You should get the same strength for the Force on the front car as you get on the back car (just opposite directions). Find the change of momentum for EACH car, and see for yourself.

    The total momentum is 24 kgm/s, before the explosion and after the explosion ... I think you need a WEAKER explosion for the little car to not go in the opposite direction.

    Your first problem has two parts : a collision (p conservation), then a rise (E conservation).
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