## Saturday, August 18, 2007

### Last Week's Puzzler

Three (out of five) people who answered got this right: what you see is the skydiver accelerating both downward and backwards.

I've met quite a few people who believe that a skydiver is in danger of hitting the elevator (the small, horizontal rear wing on a conventional airplane). They think the wind blast will blow the jumper back – nearly straight back. In fact, many skydivers have tried to touch the elevator and have failed – even when carrying a broomstick or other implement to extend their reach.

The physics here are actually pretty straightforward…

The downward acceleration is from gravity, of course – the jumper accelerates until the drag from his speed reaches equilibrium with the force of gravity. For most people, falling in the natural “frog” position, this occurs between 115 and 135 miles per hour. If the jumper falls head-down (a surprisingly difficult feat), this “terminal velocity” can be as high as 400 miles per hour!

The backwards acceleration (from the perspective of the observer in the airplane) is actually a deceleration – the jumper initially is traveling horizontally at the same speed as the airplane, but the drag of the wind decelerates him to zero horizontal speed, while the airplane is still moving forward at the same speed. Thus, from the observer's perch it appears that the jumper is moving ever faster, backwards.