Question

Question 7
A ball is gently dropped from a height of 20 m. If its velocity increases uniformly at the rate of 10 $m{s}^{-2}$, with what velocity will it strike the ground? After what time will it strike the ground?

Answer 1

Let the velocity with which the ball strikes the ground be 'v' and time it takes be 't'.
Initial velocity of the ball, u = 0
Distance or height of fall, s = 20 m
Downward acceleration, a = 10 $m{s}^{-2}$
As we know from the third equation of motion, $v^2=u^2+2as$
$=0^2+2\times 10\times 20$
$=400$
$\therefore$ Final velocity of the ball, $v=20m{s}^{-1}$
From the first equation of motion, $t=\frac{v-u}{a}$
$=\frac{20-0}{10}$ $=\frac{20}{10}$ $=2s$