Question

A stone thrown from ground level returns to the same level 4 s after. With what speed was the stone thrown? Take g=10ms−2

Answer 1

The time to reach maximum height is half the total time, and at the maximum height the velocity is zero, \[\begin{align*} \vec{v}_f &= \vec{v}_i + \vec{g}t \\ \vec{v}_i &= \vec{v}_f -\vec{g}t \\ & = 0 - (-9.80~\text{m/s}^2)(2.0~\text{s})\end{align*}\]