Question

Raindrops of radius $1mm$ and mass $4mg$ are falling with a speed of $30m/s$ 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.

Answer 1

Given

$r=1mm={10}^{-3}m$

$m=4mg=4\times {10}^{-6}kg$

$u=30m/s$

we know

$v^2-u^2=2as$

$0-(30{)}^2=2a\times {10}^{-3}$

$a=-450\times {10}^{3}m/s^2$

now from above equation

$F=ma$

$F=1.8N$