Question

A body is dropped from a tower of height 45 m. Another body is thrown up at the same instant with speed u from a height of 30 m if they reach the ground simultaneously, u is (g = 10 m$s^{-2}$)

• A. 5 m$s^{-2}$
• B. 10 m$s^{-2}$
• C. 15 m$s^{-2}$
• D. 20 m$s^{-2}$

Answer 1

The correct option is A 5 m$s^{-2}$

Given,

for 1st body

$h=45m$

$u=0m/s$

$g=10m/s$

From the 2nd equation of motion,

$s=ut+\frac{1}{2}g{t}^2$

$45=0+\frac{1}{2}\times 10t^2$

$t^2=9$

$t=\sqrt{9}$

$t=3sec$

From the relative motion of the body,

Relative velocity of 2nd body with respect to 1st.

$v_r=v_2-v_1=v-0=v$

$vt=h-h^{\prime }$

$3v=45-30$

$3v=15$

$v=5m/s$

The correct option is A.