(Q8)A ball is thrown vertically upwards from the top of building of height 29.4 m and with an intial velocity 24.5 m/sec. If the height H of the ball from the ground level is given by H = 29.4 + 24.5t - 4.9t2, then find the time taken by the ball to reach the ground.

Answer:

groundheight

Intial velocity 'U' = 24.5

height of the ball from the ground can be expressed as

H = 29.4 + 24.5t - 4.9t2

4.9t2 - 24.5t - 29.4 = 0

4.9[t2 - 5t - 6] = 0

t2 - 5t - 6 = 0

t2-t + t - 6 = 0

t(t - ) + 1(t - ) = 0

(t - )(t + 1) = 0

t - = 0

t = or t + 1 = 0

t = -1 but 't' cannot be negative so t =

It means in 6 seconds of time the ball reaches ground.