M-D7: Inertia experiment

PIRA: 1F20.10

Equipment: Steel ball with 2 hooks attached opposite of each other, stand with horizontal rod to support the ball, small short rod (held in hand), piece of soft wallboard to catch ball, several loops made from same piece of fishline (for uniformity) tied with square knot (less danger of high stress at knot).

Procedure: "Point out that if the cord were ideal (massless, flexible, inextensible), the force on the upper loop would at all times be that in the lower loop plus the weight of the ball. Thread the small rod through the lower loop, hold in the hand, and give a quick yank downward. The lower loop breaks.

"Do not try to be glib and say that the lower cord breaks before the ball moves, therefore no force on the upper loop. Point out that the materials are real. When a force is applied to the bottom of the lower loop, that end stretches the loop, which in turn applies a force to the ball. More distortion, more force, till finally the limiting distortion is reached and the loop breaks at the weakest spot. If the force applied at the lower end is large, only a small effect is given to the ball in the way of motion by the time the cord breaks. (ò F dt is small, mv is small, v is small). Now the ball is moving downward stretching the upper loop, but if v is small enough the upper cord will not have reached sufficient distortion to break before the forces (elastic) caused by this distortion have brought the ball to a stop (and the ball starts upward, relieving the distortion).

"I then replace the broken lower loop with another loop as nearly like the first as possible and show that by pulling down slowly so that quasi-equilibrium exists at all times the upper loop does indeed break first. Be sure the wallboard is there below to catch the ball and protect the table from being marred. Also avoid getting the hand in the way of the falling ball by pulling at a slight angle to the vertical. Banged knuckles will be enjoyed by the students but not by the operator." ¾Percy Carr (1970)