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Homework Help: Impluse / momentum

  1. Feb 18, 2007 #1
    1. A dump truck is being filled with sand. The sand falls straight
    downward from rest from a height of 2.00 m above the truck bed, and the
    mass of sand that hits the truck per second is 55.0 kg/s. The truck is
    parked on the platform of a weight scale by how much does the scale
    reading exceed the weight of the truck and sand?

    2. J = Ft p=mv mv(f) - mv(0)

    3. i am completely lost.
  2. jcsd
  3. Feb 18, 2007 #2


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    The relevant equation is "force = rate of change of momentum".

    Start by finding the momentum of the sand just before it hits the truck.
  4. Feb 18, 2007 #3
    Mass per second = 55kg , h= 2m

    because mgh = ½ mv2

    Velocity of sand v=sqrt(2gh) , = 6.2 m/s

    I am going round and round.

    When sand hits the top of the truck it hits it with a force which is equal to change in momentum. Final velocity of sand is 6.2 m/s which becomes zero upon striking the truck.

    This change in momentum gives the force that cause apparent increase in weight.

    V1= 6.2 , V2= 0

    F = m a = (m V1- mV2) = 55 x 6.2 = 341 Newton

    Mass = weight/g = 341/9.81 = 34.7 kg

    i'm stuck from here on
  5. Feb 18, 2007 #4


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    Right. So why are you setting it equal to ma?? Weight is measured in newtons, not kilograms.
  6. Feb 18, 2007 #5


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    You got it. The answer to the question is 341 Newton, or 34.7 kgF.

    Most likely the truck will be on a scale calibrated in "Kilograms" which means kilograms force not kilograms mass.

    1 kgF = the force exerted by the earth's gravity on a mass of 1 kg = 9.81 Newtons.
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