50. Ping Pong Cannon

A ping-pong ball is placed inside the PVC tube and both ends are sealed with pieces of Mylar after which the tube is evacuated to ~ 10 Torr. When the Mylar piece near the ball end is punctured, the ball accelerates due to the expanding air behind it, leaving the tube at speed close to 300 m/s.

The ping pong ball can tear through two aluminum cans. 

It is interesting to note that the Mylar piece used as a barrier at the exit end becomes detached before the ball reaches it. The following photograph is taken from G. Olson, R. Peterson, B. Pulford, M. Seaberg, K. Stein, R. Weber, The Role of Shock Waves in Expansion Tube Accelerators, Am. J. Phys, 74 (12), December 2006, p. 1071-1076.

The explanation requires consideration of nonlinear gas dynamics and shock behavior. Compression waves traveling ahead of the ball quickly develop into a shock wave that reflects off the exit end of the tube and in turn off the ball several times. With each reflection there is a localized pressure and temperature increase so that by the third or fourth one a heated pressure pulse builds up at the exit end and is enough to remove the barrier piece.