Question

A ball is thrown vertically upwards with a velocity of 30 ms$^{-1}$ from the top of a multistorey building of 25 m high. How high will the ball rise from building?

• A. 20 m
• B. 80 m
• C. 45 m
• D. 10 m

Answer 1

Answer: (C)

45 m

Answer 2

The ball is thrown vertically upwards with an initial velocity $u = 30$ m/s. At its highest point, its velocity is $v = 0$. The acceleration due to gravity is $a = -g = -10$ m/s$^2$ (taking upward direction as positive). Using the kinematic equation $v^2 = u^2 + 2as$, we have $0^2 = (30)^2 + 2(-10)s$. Solving for the displacement $s$, we get $0 = 900 - 20s$, which gives $20s = 900$, so $s = 45$ m. This is the height the ball rises above the point of projection (the top of the building).