A boy stands at the base of a building and throws a rock vertically upward (+y direction) with an initial speed of v0 = 24.0 m/s. How long after the rock is initially thrown will a person 15.0 m above him see the rock for the first time?

Explanation:

Given

Speed of rock

another person at a height of

time taken by the rock to cover 15 m from base  is given by

where  

y=displacement

=initial velocity

a=acceleration

t=time

on solving

so

is the time for which person see the rock second time

thus at is the time when person see the rock first time