Set two air track gliders, either of equal or unequal mass, on the air track, and place a rubber band over them to compress their spring bumpers and hold them together. Now (with the blower on) cut the rubber band at a point between the bumpers (poking the scissors through the opening in one of the springs works well, if you clear the spring as it flies away). The gliders will move in opposite directions, at equal velocities if they are of equal mass, and at unequal velocities if they are of different mass, the larger one having the smaller velocity.
You can perform this demonstration either on the (3.8-meter-long) Ealing air track, or on the (2-meter-long) PASCO air track. Though it is possible to use the Ealing track in both lecture halls 1610 and 1640, in 1640 the smaller PASCO unit is probably more convenient.
This demonstration provides an illustration of Newton’s third law of motion, which states (as quoted in Resnick, Robert and Halliday, David. Physics, Part 1, Third Edition (New York: John Wiley and Sons, 1977), p. 79) To every action there is always opposed an equal reaction; or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts. At the start, the tension of the rubber band is compressing the springs between the two gliders. These two forces are balanced (equal and opposite), so that the gliders stay put. When you cut the rubber band, the only (unbalanced) forces acting on the gliders are those of the springs between them, which, in expanding, push the gliders apart. Each cart with its spring is pushing against the other cart, with its spring. Absent any external forces, whatever force one cart with spring exerts on the second cart, the second cart must simultaneously exert an equal and opposite force on the first. Hence, when we remove the tension of the rubber band, both gliders go flying off in opposite directions.
You can illustrate what happens, both when the gliders have equal mass, and when they have unequal mass. For a given force, the acceleration is inversely proportional to the mass of the object being accelerated (F = ma). This is why, as noted above, if the masses of the gliders are equal, then they go off with equal and opposite velocities, but if one glider has greater mass then the other, it goes off with a smaller velocity than that of the lighter one. Total momentum is conserved, and the center of mass of the system stays at rest, unless you set the gliders in motion and then cut the rubber bands (a tricky task), in which case it continues moving after the separation as it had been before. For the case in which the gliders are initially at rest (which is most likely how you would perform this demonstration), both the initial and final momenta of the system are zero, so 0 = m1v1 + m2v2, m1v1 = -m2v2, and v1 = -(m2/m1) v2. So, for instance, if one glider has twice the mass of the other glider, when you cut the rubber band, it will fly off at half the velocity of the other glider. If the gliders have unequal mass, because of their difference in velocity, the spring does not do the same work on each of them (it acts on them over different distances), so they go off with different kinetic energies. Since momentum is conserved, though, we can figure out how the kinetic energy is apportioned between them. The kinetic energy of glider one is (1/2) m1v12, and of glider two is (1/2) m2v22. So, if we denote kinetic energy as K, K1/K2 = [(1/2) m1v12]/[(1/2) m2v22]. Substituting -(m2/m1) v2 for v1, and -(m1/m2) v1 for v2, and cancelling the (1/2) terms, we have K1/K2 = [m1(m2/m1)2v22]/[m2(m1/m2)2v12], which gives K1/K2 = [m2(m22v22)]/[m1(m12v12)], or K1/K2 = [m2(m2v2)2]/[m1(m1v1)2]. The terms in parentheses are the squares of the momenta of the two gliders, which we know are equal. Therefore, these cancel, and we have K1/K2 = (m2/m1). That is, the gliders go off with kinetic energy in inverse proportion to their masses. If they are of equal mass, of course, they go off with equal kinetic energy and equal and opposite momentum.
References:
1) Resnick, Robert and Halliday, David. Physics, Part One, Third Edition (New York: John Wiley and Sons, 1977), pp. 172-3.
2) Sears, Francis Weston and Zemansky, Mark W. College Physics, Third Edition (Reading, Massachussetts: Addison-Wesley Publishing Company, Inc., 1960), pp. 160-1.