Question

Two buildings $A$ and $B$ are $20m$ apart. Building $A$ has a window at a height of $50m$ above the ground and building $B$ has a window at a height of $30.4m$ above the ground. With what speed must a man throw a ball horizontally from the window of building $A$ so that it enters the window of building $B$?

• A. $5m{s}^{-1}$
• B. $10m{s}^{-1}$
• C. $15m{s}^{-1}$
• D. $20m{s}^{-1}$

Answer 1

The correct option is B $10m{s}^{-1}$

To make the ball enter the window time taken to cover the horizontal separation between the two buildings must be equal to the time taken for vertical fall

$t=\frac{x}{u}$

$t=\sqrt{\frac{2y}{g}}$

Comparing the above two equations

$\sqrt{\frac{2y}{g}}=\frac{x}{u}$

$u=x\sqrt{\frac{g}{2y}}$

$=20\sqrt{\frac{9.8}{2\times 19.6}}$

$=10m/s$