Mechanics - Newton

Hi,

I'm struggling with this problem, I must be missing something.

As far as I can see, F=ma and m=0.004 and a=9.8. I'm sure there's more to it than multiplying this by 6000*30.

Thanks

Original post by Substitution

Hi,

I'm struggling with this problem, I must be missing something.

As far as I can see, F=ma and m=0.004 and a=9.8. I'm sure there's more to it than multiplying this by 6000*30.

Thanks

I do not think the problem is very well defined.

my plan

Find the total mass of the hailstones which fall on the roof in the the 5 minute period

then use the impulse momentum equation (very crude here)

Ft = mv - mu

Reply 2

Substitution

OP

3

Original post by TeeEm

I do not think the problem is very well defined.

my plan

Find the total mass of the hailstones which fall on the roof in the the 5 minute period

then use the impulse momentum equation (very crude here)

Ft = mv - mu

Thanks for the reply, I can only agree with you about the problem being ambiguous.
Would I be correct in thinking we calculate impulse as opposed to force using the impulse equation as we wouldn't know t?

Thanks again

Reply 3

Gregorius

14

Original post by Substitution

Hi,

I'm struggling with this problem, I must be missing something.

As far as I can see, F=ma and m=0.004 and a=9.8. I'm sure there's more to it than multiplying this by 6000*30.

Thanks

This is one of those cases where you can use

\frac{d}{dt}(mv) = v\frac{dm}{dt}

if you fiddle the question a bit. That is, the question is equivalent to the hailstones arriving at v = 15m/s and not rebounding (but also not applying any pressure to the roof after they've landed). Then you just need to calculate the mass flux

\frac{dm}{dt}

Reply 4

TeeEm

19

Original post by Substitution

Thanks for the reply, I can only agree with you about the problem being ambiguous.
Would I be correct in thinking we calculate impulse as opposed to force using the impulse equation as we wouldn't know t?

Thanks again

Impulse contains the force of the roof on the hailstones, which is the same as the force of the hailstones on the roof.

Mechanics - Newton

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