Bounce the Ping Pong ball between the paddle and the table.  As you make the table to paddle distance smaller, the frequency of the bounce goes up even though the velocity of the ball remains relatively constant.  

Tilt the air track slightly so that the cart bounces well off the bottom stop.  Place a rubber band across the towers to use as the upper stop.  Start the cart at the upper end by the towers and let it slide down and bounce off the bottom stop.  As it bounce back, decrease the distance that the cart can bounce back by sliding the upper towers closer to the bottom stop, thus increasing the frequency of the cart bouncing between the two stops.