# A girl throws a rock horizontally, with a velocity of 10 m/ s, from a bridge. It falls 20 m to the water below. How far does the rock travel horizontally before striking the water?

Nov 7, 2015

$20.2 \text{m}$

#### Explanation:

You use the vertical component of the motion to get the time of flight:

$s = \frac{1}{2} \text{g} {t}^{2}$

$\therefore t = \sqrt{\frac{2 s}{g}}$

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

$t = 2.02 \text{s}$

Now we use the fact that the horizontal component of velocity is constant so:

$s = v \times t$

$\therefore s = 10 \times 2.02 = 20.2 \text{m}$