Question

A body is dropped from a certain height $h$ ($h$ is very large) and second body is thrown downward from the same height $h$ with velocity of $5\text{m/s}$ simultaneously. What will be difference in heights of the two bodies after $3s$ ?

• A. $5\text{m}$
• B. $10\text{m}$
• C. $15\text{m}$
• D. $20\text{m}$

Answer 1

The correct option is C $15\text{m}$

Taking upward as positive,
$u_{\text{rel}}=u_1-u_2=0-(-5)=5{\text{ms}}^{-1}$
$t=3s$
$a_{\text{rel}}=a_1-a_2=-g-(-g)=0{\text{ms}}^{-2}$
$s_{\text{rel}}=u_{\text{rel}}t+\frac{1}{2}{a}_{\text{rel}}{t}^2$
$\Rightarrow s_{\text{rel}}=5\times 3=15\text{m}(\because a_{\text{rel}}=0)$
So, $s_{\text{rel}}=15\text{m}$