This week, we’re checking out demonstration D1-55: Rotating Elastic Rings. You can see it in action in this demonstration video featuring physics student John Ball.
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. 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.
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 an important concept in astronomy and geography, too. As a planet rotates, it experiences this same effect. So most planets, including our Earth, are approximately oblate spheroids – that is, like the rings here, they are wider across their equators than they are along their axis of rotation. The exact proportion of this flattening depends on the equilibrium between the rotational effects and the force of gravity pulling the matter inwards.
The difference between the axis of rotation and an axis across the equator of the Earth is very small, about 0.3%. But this can be important to work like satellite navigation and other precision measurements – particularly when carrying out long-baseline interferometry research!