Answer to Question #262467 in Classical Mechanics for OMAR

Question

A girl is standing at the edge of a cliff 100. m above the ground. She reaches out over the edge of the cliff and throws a rock straight upward

with a speed 8.00 m/s.

a) How long does it take the rock to hit the ground?

b) What is the speed of the rock the instant before it hits the ground?

I think the answers are:

a) 6.4s

b) 45.0m

Answer 1

$h'=\frac{v^2}{2g}=3.3~m,$

$H=h+h'=103.3~m,$

a)

$t=t_1+t_2=\sqrt{\frac{2h'}g}+\sqrt{\frac{2H}g}=5.4~s,$

b)

$u=\sqrt{2gH}=45~\frac ms.$