Question

A ball is thrown vertically upwards with a velocity of 20 metre per second from the top of a multi storey building .The height of the point from where the ball is thrown is 25 m from the ground .

How high will the ball rise and how long will it be before the ball hits the ground ?

Take g = 10 m/s

I got X - X_{​​​​​​0} as 20 m . Is this the distance ball rised or is that X ?

Thank 😺 You

Answer 1

I think you took X as the distance that the ball reaches from ground. And X_{​​​​​​0} as the buildings height. Here we want to find out the distance that the ball reached from the ground. For that
H = u²/(2g)
H = 20²/(2×10)
H = 20 m

The ball will reach 20 m high from the point of projection. That is from the building , the ball reaches a height of 20m. That is X-X_{​​​​​​0
​​​}
The height that ball reaches from ground is 20+25=45m.
Now the second question is to find out the time of journey. t1 be the time to reach the height where v=0
t1 = 2u/g
t1 = 2×20/10
t1 = 4 seconds

If v is the velocity with which it hits the ground then,
v = √(u² + 2aS)
v = √(20² + 2×10×25)
v = 30 m/s

v = u + at
t2 = (v - u)/g
t2 = (30 - 20)/10
t2 = 1 second

Time taken for it to hit the ground = t1 + t2
= 4 + 1
= 5 seconds


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