Question

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
​

Answer 1

Final speed v= 0

Initial speed u = 20m/s

acceleration g= -g=-10 (approx.)

Apply third law of motion,

v^2-u^2=2as

so distance/height covered..

s=(v^2-u^2)/2a

so s=(0-400)/2x(-10)=-400/-20=20m

(the top height from the tower)

now to hit the ground=u =0, s=220m

again by 3rd law,

v^2=0^2+2×10×220

v^2=4400

v=66m/s (approx.)

i hope it will help