Pressure of balls bouncing on area

[LoN]

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 can't get the answer (3Pa).

 

[dav2008]

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.

 

[LoN]

haha yes i do. small world eh

i must not be setting this up right, here's what I am 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

where am i screwing up

 

[dav2008]

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!

 

[LoN]

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