Question

19. A ball is gently dropped from a height of 20m. If its velocity increases uniformly at the rate of 10m/s, with what velocity will it strike the ground after what time will it strike the ground (a) 2m/s, 5sec (b) 20m/s,2 sec (c) 25m/s, 5sec (d) 10m/s,2 sec ​

Answer 1

Answer:

B. 20m/s , 2sec

Explanation:

Here ball is dropped so initial velocity(u) = 0 and s= 20m

$by \: newton \: third \: law \: of \: motion \\ v = \sqrt{ {u}^{2} + 2as } \\ = \sqrt{{0}^{2} + 2 \times 10 \times 20} \\ = \sqrt{400} \\ = 20 \frac{m}{s} \\ \\ s = ut + \frac{1}{2} a {t}^{2} \\ 20= 0 + \frac{1}{2} \times 10 {t}^{2} \\ 40 = 10 {t}^{2} \\ \frac{40}{10} = {t}^{2} \\ \sqrt{4} = t \\ t = 2sec$