Question

A ball is thrown with an initial velocity of 20 m/s. Find the time of flight and the maximum height reached by the body.
(Assume acceleration due to gravity $g=10m/s^2$.

• A. 4 s and 20 m
• B. 5 s and 25 m
• C. 6 s and 36 m
• D. 7 s and 49 m

Answer 1

The correct option is A

4 s and 20 m

Since acceleration due to gravity is opposite to the motion. At some point of motion, velocity will become zero. At that moment, the ball will change the direction of motion and fall downwards. Thus at maximum height, velocity will be zero.

Given,
initial velocity $u=20m/s$
Since, acceleration of gravity is opposite to motion, $a=-g=-10m/s^2$
Final velocity $v=0$ at maximum height.
Time taken $t$ can be obtained from
$v=u+at$
$0=20+(-10)t$
$t=2s$

Thus the ball takes 2 s to reach the top. Similarly to reach the ground from the top point is also 2 s. Thus total time or time of flight is 4 s.

The maximum height $s$ can be obtained using
$s=ut+\frac{1}{2}a{t}^2$​
$s=20\times 2+\frac{1}{2}(-10)\times 2^2=20m$