Question

A ball is thrown upward from the top of a tower of height $50\text{m}$ with a velocity of $15\text{m/s}$. Find the time when it strikes the ground. Take $g=10\text{m}/s^2$.

• A. $2\text{s}$
• B. $3\text{s}$
• C. $4\text{s}$
• D. $5\text{s}$

Answer 1

The correct option is D $5\text{s}$

The given problem can be shown as

We have
$u=+15\text{m/s}$ , $a=-10\text{m}/s^2$
and $s=-50\text{m}$
From the equation of motion we have
$s=ut+\frac{1}{2}a{t}^2$
$\Rightarrow -50=15t-5t^2$
$\Rightarrow 5t^2-15t-50=0$
$\Rightarrow t^2-3t-10=0$
On solving this, we have $t=5\text{s}$ and $t=-2\text{s}$
Taking the positive value of time $t=5\text{s}$.
Hence, the time when it strikes the ground is $5\text{sec}$