Question

A man throws a ball vertically upward and it rises through $20$ cm and returns to his hands. What was the initial velocity $\left(u\right)$ of the ball and for how much time $\left(T\right)$ it remained in the air? $(g=10m/s^2)$

• A. $u=10$ m/s, $T=2s$
• B. $u=10$ m/s, $T=4s$
• C. $u=20$ m/s, $T=2s$
• D. $u=20$ m/s, $T=4s$

Answer 1

Answer: (D)

$u=20$ m/s, $T=4s$

Given final velocity v= 0 m/s and displacement h= 20 m; acceleration due to gravity = 10 ${\text{m/ s}}^2$

From equation of motion

$v^2=u^2+2gs-u^2=-2\left(10\right).20u=20m/s$

and time $t$ can be determined by the formula

$t=\frac{v-u}{g}=\frac{-20}{-10}=2s$

total time = $2\times$ time of ascend=$2\times 2=4s$