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AIR TRACKS

The air track[1] is a laboratory device for producing almost frictionless linear motion over a distance of about one and a half meters. The motion of "gliders", special light aluminum forms that move on a cushion of air only thousandths of an inch thick is observed. The track over which the glider moves is a perforated tube of triangular cross section; air is pumped into the tube at one end and exits upwards through many tiny holes along the two upper surfaces of the track. Ideally, the air does not impart motion to the gliders, but uniformly supports them just off the track surface; the gliders are therefore free to move on the track, to collide with each other and with elastic bumpers at the ends of the track, to be pulled and pushed by string and pulley systems, and so on, with very little friction. Many different experiments are possible, including some in which the entire track is placed at an angle with respect to the horizontal to study forces in two dimensions; the exact experiments are not discussed here.

The air track is described in this manual because students must think about the equipment components before and while doing experiments that treat the track itself as ideal. Because of the extremely thin air layer that is the basis of the track performance, all these components have to be kept at a high performance level; otherwise friction effects can grow very rapidly within a matter of an hour or two and leave you with data that seems to prove very little about the physics of motion.

The Air Supply

The supporting air source is a blower that is no more than a canister type "vacuum cleaner" running backward. One such blower can normally supply air to one, two, or even more air tracks; flexible plastic tubing of four or five centimeter diameter carries the air from the blower to a filter at the air track. Your only concern with this portion of the system is that (1) the air intake of the blower (on its top) is not blocked by debris (paper scraps, books and the like) and that (2) the plastic fittings that connect the tubing to the blower are firmly in place. Listening for air leaks with the blower running is the surest way to test the delivery system. Very slight hissing is of no consequence -- there is ample excess pressure available in the systems as originally set up. But because the fittings are simply force fit into one another, check for tightness initially and avoid moving the flexible tubing around during the experiment. The most troublesome effects on your data come when the supporting air system changes during your measurements.

Air Tube

The effectiveness of the track depends in a noticeable way on the temperature of the air that lifts the glider. You will feel the temperature rise to an operating level over the first five or ten minutes of running a track that is initially at room temperature. The heat comes from the blower system and from the slight compression of the air, and spreads along the metal tube.

After a few minutes of operation, you should have a stable, warmed track. When the blower is turned off, the system will, of course, slowly cool, but it would return to the operating level within a couple of minutes if turned back on. Most people find the blower systems noisy and distracting, and we heartily agree, but do not simply turn the system on and off during your data taking. Run it steadily during actual glider motion observations, and turn it off when your data is complete. Keep warming and cooling times in mind if you want to rerun some parts of your experiment.

Leveling the Track

You will be adjusting only the one single screw at one end of the air track.

The major and most delicate leveling of the air tracks will have been carried out by TAs and laboratory staff, leaving only minor adjustments to the leveling screws located at one ends of the air track. This final leveling of the horizontal axis of the track is done through the use of a glider on the operating track. (See section on gliders below.) Briefly, the glider is given an impulse at one end of the track, and its velocity is then measured at a point near each end of the track. The same measurement is done starting the glider at the other end of the track. For each direction, compute the velocity change as a percent of the initial velocity. Evidence of a tilt would be indicated by the effect of gravity. The leveling screws are then adjusted upwards or downwards, depending on the difference in the two velocity change measurements. This procedure is repeated until no further improvement in the correspondence of the two velocity changes can be made. Sometimes this adjustment must be repeated between data runs.

You can easily tell if your track is approximately level by placing a glider in the center of the track. It should stay centered or drift very slowly to the left or right. If you think there are problems of leveling which are not fixable by small adjustments to the screws, ask your TA for help.

Gliders

One of your main concerns should be the proper handling of the gliders (Fig. 8), which are made of the same aluminum as the track itself. Burrs or scoring of the track result from improper handling of the gliders when placing them on or removing them from the air tube. The sharp corners of the gliders can score the track surface, and a scored or burred surface can in turn damage the glider surfaces. The following rules protect the quality of your air track experiments:

(1) Never place or remove gliders unless the air supply is running, and always gently and vertically.

(2) Never move them on the track unless air supply is running.

(3) Always push gliders along an operating track by light pressure at the sides of the glider (not from the top of the glider).

(4) If the experiment permits, always send a glider down a track by causing it to recoil a spring bumper at the end of the track. There is a self-aligning character to the floating glider that recoil hardware can maintain better than your direct push.

(5) Always add and subtract weights carefully.

(6) If you should wish to make some adjustment to the glider on a running track (such as fixing a string to a pulley system) consider slipping a sheet of creased clean paper under the glider. It will isolate the glider temporarily, while the system as a whole will be stable. Of course, major changes should be done by removing the glider from the track.

(7) Be especially careful about applying tape to the glider - tape is sometimes too useful to avoid, but be sure that traces of its gummy adhesive do not migrate to the lifting surfaces.

(8) Throughout, cleanliness may or may not get you closer to godliness, but it does wonders for your air track experiments.

 


[1] Air tracks/gliders pictured in manual are slightly different than those currently in use in labs.

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