Question

Two buildings $A$ and $B$ are $20\text{m}$ apart. Building $A$ has a window at a height of $50\text{m}$ above the ground, and building $B$ has a window at height $30\text{m}$ from the ground. At what speed must a man throw a ball horizontally from the window of building $A$ so that it enters the window of building $B$. Take $g=10{\text{m/s}}^2$.

• A. $5\text{m/s}$
• B. $15\text{m/s}$
• C. $10\text{m/s}$
• D. $20\text{m/s}$

Answer 1

The correct option is C $10\text{m/s}$

For ball to enter the window of building $B$, time taken to cover the horizontal seperation between the two buildings must equal the time taken to cover vertical seperation between the windows.


For motion in vertical direction (taking vertically downwards as $+ve$):
$S_y=u_{y}t+\frac{1}{2}{a}_y{t}^2$
$+20=0\times t+\frac{1}{2}\left(10\right)t^2$
$\Rightarrow 5t^2=20\Rightarrow t=2\text{s}$

For motion in horizontal direction,
$S_x=u_x\times t$
$20=u\times 2$
$\therefore u=10\text{m/s}$