Answer:
Explanation:
Given
Speed of rock [tex]v_0=24\ m/s[/tex]
another person at a height of [tex]y=15\ m[/tex]
time taken by the rock to cover 15 m from base is given by
[tex]y=v_0t+\frac{1}{2}at^2[/tex]
where
y=displacement
[tex]v_0[/tex]=initial velocity
a=acceleration
t=time
[tex]15=24t-4.9t^2[/tex]
on solving
[tex]t=\frac{24\pm \sqrt{24^2-4\cdot 4.9\cdot 15}}{2\cdot 4.9}[/tex]
[tex]t=\frac{24\pm 16.79}{9.8}[/tex]
so [tex]t=0.735\ s\ or\ t=4.16\ s[/tex]
[tex]t=4.16\ s[/tex] is the time for which person see the rock second time
thus at [tex]t=0.735\ s[/tex] is the time when person see the rock first time
