# Physics Free Fall?

There is a 250-m-high cliff at Half Dome in Yosemite National Park in California. Suppose a boulder breaks loose from the top of this cliff. (a) How fast will it be going when it strikes the ground? (b)how long a time will a tourist at the bottom have to get out of the way after hearing the sound of the rock breaking loose (neglecting reaction time and the height of the tourist, which would become negligible anyway if hit)?

Please Explain

### 2 Answers

- derframLv 72 years agoFavorite Answer
Assuming the boulder does not hit anything on its way down.

d = at²/2

2d = at²

t² = 2d/a

t = √(2d/a)

t = √(2*250/9.81)

t = √50.968

t = 7.14 seconds

v = at

v = 9.8 * 7.14

v = 69.97 m/s

Speed of sound at STP = 343 m/s

The tourist will not hear the sound of the boulder breaking loose until

250/343 = 0.73 seconds after it happens.

So the tourist has 7.14 - 0.73 = 6.41 seconds to get out of the way.

- Anonymous2 years ago
70 m/s

7.1 s