Safety Net

Sand has a peculiar physical property known as hysteresis. The dictionary defines hysteresis as a condition in which a reaction is delayed or lags its cause. In sand, the reaction is not only delayed, it is absorbed and dispersed.

Part of the reason it hurts so much to crash into a brick building has to due with Newton's Third Law of Motion, "For every action there is an equal an opposite reaction." You hit the bricks, the bricks rebound and hit you back.  In sand, however, the force of the impact is absorbed. There is a reaction; the laws of physics are not violated. But the energy is dispersed throughout the sand particles. There is no rebound.

The Wrights were well aware of the dangers of exploring manned flight; many investigators had been killed or injured before them.  They were also aware that sand's unique properties offered a kind of "safety net" for people falling out of the sky. So they insisted on a sandy location to learn to fly. And a good thing, too, for their own records show they crashed many times.


The sands dunes at Jockey's Ridge State Park (just south of Kitty Hawk) are still an awesome place to learn to hang glide.

Sand and Hysteresis