M-D4: Acceleration is an inverse function of mass.

PIRA: Unknown

Equipment: 2 carts (1 loaded with a lead brick and the other empty), identical boxes to fit to the top of the carts, spring, twine and matches. The 2 carts and 2 boxes (one of which contains a lead brick) are in Set 9, Cabinet 3, Shelf 3.

Procedure: "To be most effective this experiment should be prepared before lecture out of sight of students. On the underside of the body of the cart is a projecting screw. With the twine tie a loop connecting the screw on one cart with that on the other so that when the carts are set down on the table in a normal position the projecting rods on the end of each cart come in contact. Slip the spring on the rods so that when the carts are placed on the table the spring exerts a force on each cart away from the other. The rods act as guides to prevent folding of the spring. One cart is loaded with the lead, and both are caped with the look-alike boxes and placed on the table near where you expect to use them.

"In lecture prepare the students by drawing a rough sketch on the board (since the details of spring and twine cannot be seen by them) and get them to realize that each cart must have the same force exerted on it; therefore, each should move. Tell them how you are going to carry out the experiment with no mention or hint of possible outcomes. Demonstrate the ease of rolling the connected carts back and forth, showing that they can coast with considerable freedom. Make sure that there is adequate room in front of the low mass cart, light a match, and hold the flame to the twine till it burns in two.

"When the usual gasp of surprise and amazement has quieted, you can say something like the following. ‘I judge from your reaction that some of you, at least, have come to the conclusion that these carts are not alike. That is true; they are not alike. (Remove the box from the light one ¾ it's empty. Remove the box from the other ¾ there's a large black lump. Pick the lead out and set it down on the table with a resounding thump). That is what we mean by mass. For a given force, the acceleration is inversely proportional to the mass.’ The point is made. No more needs to be said.

The same principle can be demonstrated in much the same way on the air track (see M-D5); but since concealment without suspicion is not easy to achieve in an air track set-up, the element of surprise is lacking. Here, the emotional impact of surprise is what fixes the point in the mind." ¾Percy Carr (1970).