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Pressure of balls bouncing on area

  1. Apr 5, 2006 #1

    LoN

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    a 5g balls fall dropped from 3m bounces up 1m. if 100 such balls fall on an area of 2m^2 every second what is the average pressure exerted by the balls?

    ive tried to solve this problem 100 different ways and i cant get the answer (3Pa).
     
  2. jcsd
  3. Apr 5, 2006 #2

    dav2008

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    Gold Member

    LoN, you wouldn't happen to have a Physics 213 midterm tonight at 7pm, would you? ;)

    Pressure is force/area and force is (change in momentum)/time. You're provided with the area and time, so all you have to find is the change in momentum of the ball as it hits the surface.
     
  4. Apr 5, 2006 #3

    LoN

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    haha yes i do. small world eh :biggrin:

    i must not be setting this up right, heres what im doing.

    1/2 m v^2 = mgh solve for v for each height, 1 m and 3m.

    then find the change in momentum by multiplying the vs i have solved for by the mass of the ball and subtract. i get .0162 for the change in momentum. so then i multiply that by 100(?) and divide by 2m^2 and i end up with .81Pa :yuck:

    where am i screwing up :confused:
     
  5. Apr 5, 2006 #4

    dav2008

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    Remember that the momentum as the ball is going down is negative, and the momentum as the ball is going up is positive.

    Good luck tonight!
     
  6. Apr 5, 2006 #5

    LoN

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    thanks for the help, what a lame mistake i was making :grumpy:
     
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