# 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