= Air-track test of inertial motion
An actual teaching experiment in Arbor Scientific's \i[The Level Air Track] uses one glider, an <air track>, two <photogates>, and a ruler. Turn on the blower and place the glider near the middle. Adjust the feet until it does not drift when released. Put the <photogates> at least half a track length apart and align them with the glider's flag. Measure the length $L$ that interrupts the beams. Give a gentle push, let go, and record each beam-blocking <time> $\Delta t_i$. The readout is the pair of <speeds> $s_i=L/\Delta t_i$. The worksheet asks how those <speeds> compare, which <forces> remain after the push, and which interaction the <air track> minimizes.<The Level Air Track by Arbor Scientific>[Arbor Scientific, The Level Air Track: Teacher's Notes, p. 1, Required Equipment and Lab Tips; student sheet, p. 2, Procedure 1–9]{ref}
The motivating contrast is between a brief push and the subsequent glide. The air cushion reduces contact <friction>; it does not remove the downward <gravitational weight> or the upward support <force>. Similar successive <speeds> after release are consistent with a small horizontal net <external force>, rather than with a continuing push. The experiment approximates the zero net <external force> model over a finite track and observation interval.
To strengthen the documented procedure, use independent <measurement calibration>: check the flag length and each timer against a known length and <time> reference; measuring $L$ sets the length scale but does not calibrate the timer. Repeat launches in both directions without contact during the timed passage, and check for drift at several track locations. These are additional controls, not reported steps or measured results in the worksheet. A residual <inclined plane> produces a downhill component of <gravitational weight> with the same laboratory direction on both runs; <friction> and <drag force> oppose the direction of travel. Reversing the launch can therefore help distinguish these effects.
A <photogate> directly measures a beam-blocking <time>; $L/\Delta t_i$ is an average <speed> during that passage, not an exact instantaneous <velocity>. Assign the direction separately. Equal readings at two gates do not prove zero <acceleration> everywhere between them: opposing changes could cancel. <Measurement uncertainty> from timer resolution, an incorrectly measured flag length, residual slope, airflow disturbances, and remaining <friction> or <drag force> limit the inference. No measured glider <speeds> are assumed here.
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