Question

A person standing on a 50 m cliff throws a rock upward at 26.5 m/s. How long will it take for the rock to reach the ground? Hint: determine the final velocity first.

Answer 1

Equation of motion of the rock is given by

$$y(t)=50+26.5t-\frac{9.8t^2}{2}$$

At the instant when the rock reaches the ground

$$y(t)=0$$

Hence

$$50+26.5t-\frac{9.8t^2}{2}=0$$

Root

$$t=6.9\:\rm s$$