Question

A cricket ball is thrown vertically up with a speed of 20 m/s from ground. Find the speed of the ball when it has reached a height of 15 m.

Answer 1

According to the conservation energy law, the current energy of the ball (which is the sum of its kinetic and potential energies) is equal to its initial kinetic energy:

$$\dfrac{mv_0^2}{2} = \dfrac{mv^2}{2} + mgh$$

where $m$ is the mass of the ball, $v_0 = 20m/s$ is its initial speed, $h =15m$ is its current height, $v$ is its current speed, $g = 9.8 m/s^2$ is the gravitational acceleration. Expressing $v$, obtain:

$$v = \sqrt{v_0^2 - 2gh}\\ v = \sqrt{20^2 - 2\cdot 9.8\cdot 15} \approx 10m/s$$

Answer. 10 m/s.