Air Track - Action-Reaction Pairs


To demonstrate the third law of motion.


Air track with two mass M and one mass 2M gliders, string loops with burner, photogates and timer.

Setup Time

5 min.




Two gliders (either M and 2M or M and M) are tied together with a string loop against the force of a compressed spring. Burning the string releases the gliders with no external force. The photogate timer measures the time it takes for each glider tab to move through its respective gate. A reset switch on a cable clears the timer between measurements without the instructor getting in the line of sight.


Thomas D. Phillips, Finding the External Force, AJP 22, 583-585 (1954). Donald E. Tilley, Dynamical Paradox, AJP 35, 546-547 (1967). W. Klein and G. Nimtz, Inelastic collision and the motion of the center of mass, AJP 57, 182, (1989). Ernie McFarland, Newton's Third Law at a Traffic Intersection, TPT 28, 315 (1990). Kathy Malone and Bob Reiland, Exploring Newton's Third Law, TPT 33, 410-411 (1995). Information Sheet.