Question

A rubber ball is thrown vertically upward with a velocity u from the top of a building. It strikes the ground with a velocity 3u. The time taken by the ball to reach the ground is given by:

• A. 4u/g
• B. 3u/g
• C. 2u/g
• D. u/g

Answer 1

Answer:

A. 4u/g

When the ball will reach the highest point then the final velocity will be zero.

Initial velocity = u m/sec, final velocity = 0 m/sec, acceleration$= - g m /sec^ 2$

$v=u+at \Rightarrow 0 = u - gt_{1}$

$t_{1} = \frac ug$

Initial velocity will be zero as the ball is at the highest point.

$v=u+at \Rightarrow 3u = 0 + gt_{2}$

$t_{2} =\frac {3u}{g}$

Therefore total time is: $t = \frac ug + \frac {3u}{g} = \frac {4u}{g}$