Question

A cricket ball is dropped from the height of 20 m. (a) calculate the speed of the ball when its hit the ground? (b) claculate the time it takes to fall through this height?[g=10m/s^2]

Answer 1

this is the perfect answer of the this question

Answer 2

Given :

A cricket ball is dropped from the height of 20 m.

To Find :

(a) the speed of the ball when its hit the ground

(b) the time taken by the ball to fall through height 20 m

Solution :

(a) We know for a motion with uniform acceleration,

          $v^{2} = u^{2} + 2as$  

(where v = final velocity; u = initial velocity; a = acceleration and s = distance travelled)

The ball was initially at rest. So, u = 0

         $v^{2}$ = 2as

or,      v  = $\sqrt{2as}$

or,      v  =  $\sqrt{2*10*20}$    (as the ball is falling under gravity, a=g)

∴   v  = 20 m/sec (Answer)

(b)  We know, s = ut + $\frac{1}{2} *gt^{2}$  (where t = time taken to hit the ground)

or,                20 =  0 + $\frac{1}{2} * 10 * t^{2}$

or.                20 = 5$t^{2}$

or,                   $t^{2}$ = 4

or,                   t  = $\sqrt{4}$

∴                     t  = 2 sec (Answer)