Projects in High-Speed Photography
A Rubber Band Puzzle
The following puzzle was first posed on Dr. Donald E. Simanek's web site. Look for many more interesting demonstrations and puzzles there.
Problem: Two nails are hammered into a block of wood about 12 cm apart. A heavy rubber band with approximately a 20-cm relaxed length, is stretched between the nails. One end of the band is then pulled back an additional distance of about 10 cm. After this end of the band is released, when does the other end of the band break contact with the nail?
Here's an overhead view of the situation:
(If you doubt whether the front end of the band actually does break contact, hold your finger 1-2 cm in front of the nail and then release the band.)
The forward end of the band breaks contact ...
a) at the instant the released end of the band is stopped by the rear nail.
b) before the released end stops.
c) after the released end stops.
A series of stop-action photographs of the collapsing rubber band provides an answer and a new question. The method of capturing these photos is described on the next page.