Question

A boy standing on a stationary lift (open from above) throws a ball upwards with the maximum initial speed $=49m/s$.
a) How much time does the ball take to return to his hands?
b) If the lift starts moving up with a uniform speed of $5m/s$ and he again throws the ball , how long does it take to return to his hands ?

Answer 1

a)

$S=ut-\frac{1}{2}u{t}^2$

$\Rightarrow S=0$

$\frac{g{t}^2}{2}=ut$

$t=\frac{2u}{g}=\frac{2\times 49}{9.8}$

$t=10s$

b) $LiftmovingwithUniformspeed$ $=$ $5m/s$

$Ballisthrowingfrominsidethelift$

$Relativevelocityofball=49m/s$

$\Rightarrow Relativeacceleration=10$

$Thus,$ $u_{relative}=$ $velocityofbell$

$t=\frac{2u_r}{g_r}=\frac{2\times 49}{9.8}$

$\Rightarrow t=10s$