M-S 2: Equilibrium of forces

PIRA: 1J30.25

Equipment: long rope, 5-pound weight with hook on top, stand rods and clamps. The rope is in Set 7, Cabinet 2, Shelf 3 and the weights are in Room 4.

Procedure: With the clamps mounted at the side of the long lecture table, erect in each of them a vertical rod 12 to 18 inches high. Affix to each vertical rod two horizontal rods about 12 inches long projecting to one side of the vertical rod and perpendicular to the long dimension of the table. These horizontal rods should be as close together as the mounting clamps will permit — the space in between is to act as a guide for the rope. Place the 5-pound weight in the center of the table. Lay the rope on the table and between the horizontal rods which act as guides so that when made taut the rope will pick itself up and under the hook on the weight. Adjust the guides so that the angle the taut rope makes with the horizontal is only a few degrees. Select two husky men from the class to pull on each end of the rope with the avowed purpose of lifting the weight. If the angle of the rope has been made flat enough the men will be unable or barely able to lift the 5-pounds, which the lecturer can then easily pick up with one hand.

The lecturer can also pull the husky men at the ends of the rope some distance toward each other by pushing the rope at the midpoint in a direction at right angles to the line joining the men. It should be mentioned that this technique can be used to move a stuck auto by stretching taut rope connecting the stuck car with a tree or heavy auto with brakes set and then pushing at right angles to the rope at its midpoint.