Question

A ball is thrown vertically upwards from the ground. It crosses a point at the height of 25 m twice at an interval of 4s. The ball was thrown with the velocity of:

• A. 20 m/s
• B. 25 m/s
• C. 30 m/s
• D. 35 m/s

Answer 1

The correct option is C 30 m/s

After reaching the maximum height, ball falls for 2 seconds to reach the 25m height.

Distance it has fallen is $h=\frac{1}{2}g{t}^2=\frac{1}{2}\times 10\times 2^2=20m$

Maximum height$=25+20=45m$

Velocity at the bottom or the initial velocity is found as,

$v^2-u^2=2(-g)h$

Since final velocity $v=0$ and acceleration due to gravity will be negative because ball is thrown vertically upward)

$u^2=2\times 10\times 45=900$

$u=\sqrt{900}$

$u=30m/s$