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Homework Help: Impulse and Momentum Q!

  1. Dec 9, 2009 #1
    I have two questions regarding this, any help would be GREAT!

    1st Problem:

    1. The problem statement, all variables and given/known data

    A conveyor belt lifts 1000kg of rocks per minute a vertical distance of 10m. The rocks are at rest at the bottom of the belt and are ejected at 5 m/s. The power supplied to this machine is:

    A) 1000W
    B) 1260W
    C) 1630W
    D) 1840W
    E) 2100 W

    2. Relevant equations

    P=Fv

    p=mv
    [tex]\sum[/tex]F[tex]\Delta[/tex]T=m(vf-vo)

    3. The attempt at a solution

    vo = 0m/s
    vf = 5m/s

    1000kg/min = 16.67kg/s

    sumF[tex]\Delta[/tex]T=m(vf-vo)
    sumF=m(vf-vo)/[tex]\Delta[/tex]T
    sumF=m(vf)-m(vo)/[tex]\Delta[/tex]T

    since vo = 0/ms

    sumF=m(vf)/[tex]\Delta[/tex]T
    sumF=m/[tex]\Delta[/tex]T (vf)

    subbing in values...

    m/dT = 16.67kg/s

    F = (16.67)(5) = 83.35N

    and now for Power...
    P = Fv
    P = 83.35(5) = 416.75W

    Now, I know i've done something wrong, especially as I haven't accounted for height. I feel like I am EXTREMELY off with attempting to solve this so i'd appreciate any help.


    2nd Problem:

    1. The problem statement, all variables and given/known data

    Water runs out of a horizontal pipe at the rate of 120kg/min. It falls 3.20m to the ground. Assuming the water doesn't splash up, what average vertical force does it exert on the ground?

    2. Relevant equations

    P=Fv

    p=mv
    sumF[tex]\Delta[/tex]T=m(vf-vo)


    3. The attempt at a solution

    SOLVED THIS ONE - if anyone else needs the solution though let me know and i'll post it.
    Still need help on the first though... workin' on it however.
     
    Last edited: Dec 9, 2009
  2. jcsd
  3. Dec 9, 2009 #2
    For the first problem, ignore the forces within the machine. Ask yourself what happens to the rocks every second (Think energy rather than momentum)
     
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