Question

A cliff is 20m above the ground. A boy rolls a ball off the edge with an unknown initial velocity, and its strikes the ground 3m horizontally away. Calculate the initial velocity of the ball 

Answer 1

Let's first find the time that the ball takes to reach the ground:

$$h=\dfrac{1}{2}gt^2,$$ $$t=\sqrt{\dfrac{2h}{g}}=\sqrt{\dfrac{2\cdot20\ m}{9.8\ \dfrac{m}{s^2}}}=2.0\ s.$$

Finally, we can find the initial velocity of the ball:

$$x=v_0t,$$ $$v_0=\dfrac{x}{t}=\dfrac{3\ m}{2.0\ s}=1.5\ \dfrac{m}{s}.$$