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A stone is shot straight upward with a speed of $$20 \ m/sec$$ from a  tower $$200 \ m$$ high. The speed with which it strikes the ground is approximately  （ ）
A. $${60} \ {m}/{\sec{}}$$
B. $${65} \ {m}/{\sec{}}$$
C. $${70} \ {m}/{\sec{}}$$
D. $${75} \ {m}/{\sec{}}$$
Speed
00:00
04:22

## QuestionPhysicsClass 11

A stone is shot straight upward with a speed of $$20 \ m/sec$$ from a  tower $$200 \ m$$ high. The speed with which it strikes the ground is approximately  （ ）
A. $${60} \ {m}/{\sec{}}$$
B. $${65} \ {m}/{\sec{}}$$
C. $${70} \ {m}/{\sec{}}$$
D. $${75} \ {m}/{\sec{}}$$

B
4.6
4.6

## Solution

Speed of stone in a vertically upward direction is $$20 \ m/s$$.
So, the speed of stone at the point of projection will be same for upward or downward motion ,$$u=-20 \ m/s$$
Using third equation of motion,
$$v^2=u^2+2gh$$
$$v^2=(-20)^2+2\times 9.8\times 200=4320 \ m/s$$
$$v^2=4320 \ m/s$$
$$v=\sqrt{4320}=65.72 \ m/sec$$
So,option $$B$$ is correct.