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During heavy rainfall, spherical hailstones of average diameter 1.0 cm fall at an average speed of 20 m/s. If 2000 hailstones hit each square metre of a flat 10 m x 10 m roof every second, striking perpendicularly and not bouncing back, find the average force the hailstones exert on the roof. Take the density of a hailstone as 900 kg/m³.

1. About 1.9 x 10³ N
2. About 9.4 x 10³ N
3. About 1.9 x 10⁵ N
4. About 3.8 x 10⁵ N

Correct answer: About 1.9 x 10³ N

Solution

Each hailstone loses all its momentum on impact, so the roof must supply force equal to the total momentum delivered per second.

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