# A stone projected vertically up from the ground reaches a height y in its path at t1 seconds and after further t2 seconds reaches the ground…..

Q. A stone projected vertically up from the ground reaches a height y in its path at t1 seconds and after further t2 seconds reaches the ground. The height y is equal to

(a) $\displaystyle \frac{1}{2} g(t_1 + t_2)$

(b) $\displaystyle \frac{1}{2} g(t_1 + t_2)^2$

(c) $\displaystyle \frac{1}{2} g(t_1 . t_2)$

(d) $\displaystyle g(t_1 . t_2)$

Ans: (c)
Sol:
$\displaystyle h= ut -\frac{1}{2}gt^2$

$\displaystyle gt^2 -2ut + 2h = 0$

$\displaystyle t_1 + t_2 = \frac{2u}{g} and \quad t_1 t_2 = \frac{2h}{g}$

Put h = y
$\displaystyle y = \frac{1}{2}g t_1 t_2$