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# Tiw stone are poject from the top of a tower 100m high each with the velocity of 20m/s. One is pojected vertically upwards and another vertically downwards. Calculate the time each stone takes to reach the ground and the velocity with which it strikes the ground.?

## Class 9 Question

By Gayathri Priya · Jan 26, 2020 ·Class 9
Shrish Raj answered Jan 26, 2020
PLZ FIRST CORRECT THE SPELLINGS

Stone projected vertically upward

height upto which it goes above(h) can be calculated as
v^2=u^2+2aS
v=0{final speed will be zero at the top}, a= negative{against gravity}
u^2=2aS
20ï¿½20= 2ï¿½10ï¿½ h
h=20
total height from which it fall in the end= 100+20
=120
height travelled = ut + 1/2gt^2
120= 0+1/2ï¿½10t^2
t^2=24
t=2âˆš6sec
v^2=u^2 +2a(total height)
v^2=0+ 2ï¿½10ï¿½120
v=20âˆš6

stone projected vertically downward

v^2=u^2+2aH
v^2=20ï¿½20+ 2ï¿½10ï¿½100
=400+2000
=2400
v=20âˆš6
v=u+gt
20âˆš6=20+10t
2âˆš6-2=t

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