Beam supported at ends with platform scales and toy truck as load to demonstrate moment arms.
What it shows:
The concept of moment arms is exemplified by this model of a truck on a bridge.
How it works:
A beam (board) supported at its two ends by platform scales is loaded with a toy truck. The scales read different forces depending on the position of the truck along the beam. The loaded toy truck is moved so that it's directly over one scale. The class observes its weight. Then the dials of both scales are concealed (a small black cloth will do), and the truck is pushed a quarter or a third of the distance along the "bridge". The class is asked to compute the readings of the scales before the experimental values are revealed.
Setting it up:
Two kitchen-type platform scales 1 with large dials reading to 120 N are placed 1.5 to 2 meters apart. Across them is laid a light board, which rests on two wooden fulcrums placed on the flat tops of the scales, and adjusted so that the scales read the same. A toy truck loaded with weights (a lead brick is convenient) is placed on the board.
1 Metro Equipment 25 lb scale which we have converted to the metric system by adding our own face dial with appropriate markings and numbers. Sometimes we use a Chatillon 10 lb pan spring scale.