M-CM 4: Forces in circular motion

PIRA: Unknown

Equipment: 3-speed DC fan motor fitted with wooden disc on shaft, link chain that stretches tightly over outside of disc, glass plate, piece of acoustic wallboard several inches in each dimension, piece of 2 x 4 about the length of the demonstration table's width, small stand rod of small diameter about 6 inches long. The motor and chain are in Set 4, Cabinet 2, Shelf 2.

Procedure: Place the glass plate or the wallboard against the motor base and under the disc on the shaft. Also, lay the 2 x 4 across the table some ten feet away in the direction the chain will roll when forced off the rotating disc. Stretch the chain around the periphery of the disc. Start the motor on low speed while using the smooth end of the short stand rod to prevent the chain from working itself off the disc. At all times keep the face out of the plane of the disc; the chain might break and cost you an eye! As soon as the speed of rotation is great enough to stabilize the position of the chain on the disc, increase the motor speed to the highest setting. Then with the rod push the chain to the outside of the disc till the chain falls on the glass or wallboard underneath. The chain will behave like a rigid hoop. If it falls on glass it will "spin its wheels" before taking off down the table, if on wallboard the take-off is faster. (Do not mar the table by dropping the spinning chain directly on the table surface.) After rolling down the table the chain hoop should strike the 2 x 4 laid out for it and leap far into the air, collapsing into a limp chain upon falling back upon the table or floor.

I think it is well to comment that each link is acted upon by a force at each end (supplied by the adjacent link) and its weight. The weight is negligible compared to the other two forces (which are equal), the resultant of which drives the link into the circular path it is executing. When the link strikes an obstacle the hoop cannot be deformed (assuming perfectly rigid links) until the force supplied by the obstacle on the link is greater than the centripetal force required for the path of the moment. The adjacent links, therefore, "feel relieved" when the middle link is being pushed inward by the obstacle. Elasticity of the links modifies this statement only slightly.