Question

A child drops a toy from the top of a building, it takes 1 s to reach the ground. Find the height of the building and the average speed of the toy respectively.
(Take g = $10m{s}^{-2}$)

• A. $4mand5m{s}^{-1}$
• B. $5mand5m{s}^{-1}$
• C. $6mand6m{s}^{-1}$
• D. $7mand7m{s}^{-1}$

Answer 1

The correct option is B $5mand5m{s}^{-1}$

Given:
Initial velocity, $u=0$
Acceleration due to gravity, $g=-10m{s}^{-2}$
Time taken, $t=1s$
Upward direction is taken as positive.

Let the height of the building be $H$. Displacement will be:
$s=-H$

Using second equation of motion,
$s=ut+\frac{1}{2}a{t}^2$​
$-H=0+\frac{1}{2}\times (-10)\times 1^2$​​
$H=5m$

Average speed of a body is the ratio of total distance travelled to the total time taken.
$<v>=\frac{|s|}{t}$
$<v>=\frac{5}{1}=5m{s}^{-1}$