Question

A ball is dropped from a height of $5\text{m}$. Find the velocity with which it hits the ground.

• A. 10 m/s
• B. $\sqrt{-100}$m/s
• C. 5 m/s
• D. - 10 m/s

Answer 1

The correct option is D

- 10 m/s

Since the ball is dropped

So the initial velocity $u=0\text{m/s}$

$a=-10{\text{m/s}}^2$ as in the negative $y$ axis (remember the conventions)

$s=-5\text{m}$ as the body is getting displaced in negative $y$-axis or downwards.

We need final velocity $v$

$v=u+at$ .....(1)

$s=ut+\frac{1}{2}a{t}^2$ ......(2)

$v^2=u^2+2as$ ..........(3)

We will need equation 3 "why” you ask

Because we know 3 of variables in equation 3 and need to find the fourth variable

$v^2=0^2+2(-10)(-5)$

$v^2=100$

So, $v=\pm 10$ which to pick we know when the particle hits the ground the velocity is going to be in downward direction. So we choose v = $-10\text{m/s}$.