Raindrops of radius $1 \mathrm{~mm}$ and mass $4 \mathrm{mg}$ are falling with a speed of $30 \mathrm{~m} / \mathrm{s}$ on the head of a bald person. The drops splash on the head and come to rest. Assuming equivalently that the drops cover a distance equal to their radii on the head, estimate the force exerted by each drop on the head.