Question

A ball is thrown vertically upward with a velocity of 20 m/s from top of the building.the height of the point from where the ball is thrown is 25 m from ground.how high will the ball rise ? how long will it be before the ball hits the ground?

Answer 1

a) At heighest poing velocity will be 0

So by formula v^2 = u^2 - 2gh        
(here - is because you are moving against gravity)

So h = 20*20/2*10 = 20 m

So ball will rise up to 25+20 m from the ground.

b) In second part we have to calculate the time of journey from top of 25 m high building to ground.

For this we should use  

-h = ut - gt^2/2                  
[in this case we are using upward direction as + ve and downward as - ve]

So -25 = 20t - 10t^2/2

or 5t^2 - 20t - 25 = 0

or t^2- 4t - 5 = 0

or (t+1)(t-5) = 0  

or t = 5 , -1

So t = 5 sec.