Hailstones with an average mass of 4g fall vertically and strike a flat roof at 12 metres per second.In a period of 5 minutes 6000 hailstones fall on each square metre of roof and rebound vertically at 3 metres per second. Calculate the force on the roof if it has an area of 30 square metres.

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This problem is based on the equivalence of impulse and change of momentum, and involves an assumption that the force on the roof is equal to the weighted average of force per unit time. So you're really calculating the average force on the roof. Post back with your attempt at solving this and we'll check it for you.