Question

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

• A. $4m$ and $5m{s}^{-1}$
• B. $5m$ and $5m{s}^{-1}$
• C. $6m$ and $6m{s}^{-1}$
• D. $7m$ and $7m{s}^{-1}$

Answer 1

The correct option is B

$5m$ and $5m{s}^{-1}$

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

Let height of building be $H$
Using equation of motion,
$s=ut+\frac{1}{2}{at}^2$​
$H=0\times 1+\frac{1}{2}\times 10\times 1^2$​​
$H=5m$

Let average speed of toy be $v_a$
From definition of average speed,
$v_a=\frac{\text{Total distance travelled}}{\text{Total time taken}}$
$v_a=\frac{5}{1}=5m{s}^{-1}$