Answer:
Solve it simply,
Vi = 20 m /sec
Vf = 0 ( at max. height)
g = - 9.8 m/ sq. sec. ( -ve sign shows that the motion of the body is in opposite direction to that of gravity)
Height it gains=h = ?
time taken by the body=t=?
Using 3rd equation of kinematics,
2gh=Vf ˄2 – Vi˄2
implies that
h = Vf ˄2 – Vi˄2 / 2a
h = 0˄2 - (20)˄2 / 2(-9.8) = - 400/ -19.6
h = 20.4 m
So the maximum height the body can gain is approximately equal to 20.4 m.
Now , using 1st equation of kinematics:
Vf = Vi + at implies that,
t = Vf - Vi/g = 0 - 20/ - 9.8 = 2.04 sec.
So, it will take 2.04 sec. to get the max. height.
Note: in simple approximation if we take g =- 10 m/ sq.sec then the max. height the body can attain will be 20 m and the time required for it, is 2 sec.