# A hang glider dropped his cell phone from a height of 350 feet. How many seconds did it take for the cell phone to reach the ground?

Jun 17, 2018

$t = 4.68 s$

#### Explanation:

Assuming negligible air drag, it was in free fall (g = 32 ft/s^2) for the entire 350 feet. So we use the formula

$y = u \cdot t + \frac{1}{2} \cdot a \cdot {t}^{2}$

Plugging in the data

$350 f t = 0 + \frac{1}{2} \cdot \frac{32 \text{ft}}{s} ^ 2 \cdot {t}^{2}$

$350 f t = \frac{16 f t}{s} ^ 2 \cdot {t}^{2}$

${t}^{2} = \frac{350 \cancel{f t}}{\frac{16 \cancel{f t}}{s} ^ 2} = 21.88 {s}^{2}$

$t = \sqrt{21.88 {s}^{2}} = 4.68 s$

I hope this helps,
Steve