What is the Physics Behind Walking on Broken Glass?

For this demonstration the glass bottles should first be soaked in water to remove any paper labels. An alternative is to use Mason jars. It is best to use fairly large bottles so that the pieces formed will have only a gentle curve to them. When breaking the bottles I place them in a canvas sack and use a hammer, being sure to wear gloves and eye protection. The glass should be broken into fairly small pieces. The bed for the glass may be made from half-inch-thick plywood framed by pieces of 2"34” wood. Once the glass has been poured into the bed it should be spread out to a uniform depth. Any piece that has a right-angled bend in it, where the sidewall of the bottle meets the base, is moved to the edges of the bed so that only relatively flat pieces of glass are included in the center of the bed where the walking takes place. As an extra precaution, I cover the glass with a cloth and then use a large cast iron skillet to pound the surface firmly. This ensures no points of glass are sticking up. This is usually done before the audience enters the room. A bed of glass about 8 cm (three inches) deep seems best, as this provides sufficient depth for the glass to be able to shift and settle somewhat as a foot is planted slowly and directly down upon it. When done this way the pieces of glass lay fairly flat and no edge presses perpendicularly against the sole. The bottom of the foot has some give to it and conforms to the shallow curve of the glass pieces. This is similar to a sharp knife being pressed with the flat of the blade against one’s flesh, where considerable force may be used without injury. When walking I place each foot slowly, moving it elsewhere if a point or edge is felt, although that is seldom necessary if the bed has been prepared correctly. Care must be taken to brush off any pieces of glass that stick to the bottom of the feet when stepping off the bed.

To show that the edges of the glass are sharp I use a piece from the bed to cut the string suspending a bowling ball about a half meter (two feet) above the floor.