Friday, 13 June 2014 16:34

## D1-55: ROTATING ELASTIC RINGS

• ID Code: D1-55
• Purpose: Demonstrate "centrifugal reaction" and to indicate why the earth is oblate.
• Description:
We have a pair of thin steel rings mounted on a rotating base. The top of the rings is free to slide along its axis, while the bottom is fixed to the rotating base.

Turning the crank causes the elastic rings to rotate about the vertical axis. The rotation mechanism here uses the mechanical advantage of a large cranked wheel driving a smaller pulley to give the rotating rings a very high angular velocity.

##### Engagement Suggestion
Before rotating at high speed, invite students to predict what will happen to the rings when you get it spinning as fast as you can. Will they:
• a) keep their circular shape
• b) flatten at the top and bottom and bulge in the middle
• or c) extend upwards and grow narrower in the middle?
Afterwards, encourage students to relate this to other physical phenomena.
##### Background
As the rings rotate, their form distorts, growing wider around the center and flattening at the top and bottom. Interestingly, this is not due to a true outward force acting on the metal at this point, but is an artifact of its rotating reference frame and the forces acting to keep it moving in a circle. This is often termed a centrifugal reaction or centrifugal force, though it is technically a pseudo-force arising from the reference frame.

This effect is seen in astronomy and geography, as rotating planets, stars, and other bodies take on similarly oblate spheroidal forms.

• Availability: Available
• Loc codes: D1