### Chapter - 1 Motion

Q
##### Motion

Question:

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?

Distance covered by the ball, S = 20 m

Acceleration, a = 10 ms-2

Initial velocity, u = 0 (since the ball was initially at rest)

Let final velocity of the ball with which it strikes the ground is v.

According to the third equation of motion:

$v^2=u^2+2as$

$v^2=(0)^2+2 \times 10 \times 20$

$v^2=400$

$v=\sqrt{400}=20\ m/s$

$v=20\ m/s$

According to the first equation of motion:

$v=u+at$

Where,

Time, t is taken by the ball to strike the ground is,

$20=0+10 \times t$

$t=2\ s$

Hence the ball strikes the ground after 2 s with a velocity of 20 m/s.

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