Each ball pictured above has a length of string approximately 18 inches long attached to it. One ball (red) is made of steel, and the other is polyfoam. Hold the red ball suspended at arm’s length in front of you and shoot the dart at it. Since the ball is much more massive than the dart, it barely moves. If you now do the same thing with the polyfoam ball, it will fly around in an arc and hit you on the top of your head. This should be amusing to the students.

The idea, of course, is that the much heavier steel ball has much more inertia than does the polyfoam ball, so it is more difficult to accelerate with the dart (or, in other words, to overcome its tendency to stay at rest). You could also use this demonstration to illustrate exchange of momentum, with the red ball providing an example of what happens when a light object strikes a much heavier stationary one, and the white ball giving an example of what happens when an object strikes a second, stationary object of comparable mass.