Homework Help: Rate of change of momentum. newton 2nd law

1. Jul 10, 2012

roovid

rate of change of momentum. newton 2nd law (solved)

hailstones with an average mass of 4grams fall vertically and strike a flat roof at 12ms-1 .
In a period of 5 minutes, 6 000 hailstones fall on each square meter of roof and rebound vertically at 3ms-1 .
Calculate the force on the roof if it has an area of 30m2 .

the ans is 36 N

plz help

Last edited: Jul 10, 2012
2. Jul 10, 2012

truesearch

Do you know that Force = change in momentum per second?
Can you use these figures to calculate the change in momentum in 1 second over tha area of 30m^2?

3. Jul 10, 2012

roovid

this was what i did so far

mass for 1 second:
600 / (5x60)
= 20 stones s-1

initial momentum
mv
(20)(.004)(12)
=.96

final momentum
20(.004)(3)
=.24

rate of change of momentum
(.96) - (.24)
=.72
=F

stuck

4. Jul 10, 2012

TSny

Remember: momentum is a vector quantity. It has direction! If you take positive as representing upward direction, what would be the sign of the initial momentum?

5. Jul 10, 2012

roovid

so will it be
change of momentum
-.96 - (+.24)
= - 1.2 N
?
now what?

6. Jul 10, 2012

TSny

Check the location of that decimal point in your answer. I guess you decided to take downward as the positive direction. That's ok.

So, you get a force. But is this the force for 30 m^2?

7. Jul 10, 2012

roovid

i dont understand what to do with the force and the area 30m^2
sorry
can u explain some more

8. Jul 10, 2012

truesearch

You are almost there. What you have calculated is the force on 1m^2
The area of the roof is 30m^2.......

9. Jul 10, 2012

ohh....
thankkk uu
1.2x30
36 yayy