## Do-It-Yourself Air Table

Air tables provide a nearly frictionless surface that makes them ideal for performing some simple physics experiments.

This video explains how you can make your own air table. Other than a few crucial cuts (which your hardware or lumber store may be able to make for you) this design requires no precision cuts at all. That makes this table easy to make for most amateur scientists. The total cost of the table is less than \$100, and the vast majority of that cost is in the leaf blower. So, if you can get ahold of a used leaf blower for less than \$60 (which is what I paid for my leaf blower) then you could potentially make this table to less than \$30. Note that the leaf blower is very noisy, so you may also want to purchase some hearing protection if you plan on building this.

This video shows a spinning disk floating on the air table. The disk spins for quite a while, even when given just a small push. That's because the air jets support most of the weight of the disk leaving a very small normal force acting at the pivot. This, in turn, means that there is almost no friction acting on the disk. This video shows how low the friction can be. Incidentally, the air flow to the table was set to low for this video: had I increased the air flow setting by one more notch, the disk would have spun for hours, but that would have made this a very boring video.

This video shows that the center of mass of a complicated object moves in a straight line when placed on the air table. Note at the very end, there are gusts of air that cause the object (the map of the United States) to move in a curved path. Thanks to Cornell College student Mackenzie Crow for cutting out this map.

This video shows an inelastic collision between a jet of water and a sponge. Notice that nothing is spinning before the collision, but that the sponge spins after the collision. At first glance, this would appear to violate the conservation of angular momentum, but a point particle that is not aimed at the center of mass of a system will have angular momentum with respect to that system. The angular momentum of the squirt of water is equal to its linear momentum multiplied by the impact parameter (which is the same as the moment arm in this problem). You can measure the impact parameter, determine the momentum of the squirt from the final momentum of the sponge, and from this you can measure the moment of inertia of the sponge.