Experiment to determine centre of gravity of plane lamina

Center of mass little story

My parents brought me back this wine holder from their most recent holiday.

It doesn’t look much like a wine holder until you insert a bottle; whereupon it magic­ally “stands up”.

An object bal­ances when its centre of mass is above its base. The base of the holder can be any point along the curved edge; the curved shape allows it to pivot freely. The holder bal­ances when the centre of mass is dir­ectly above the pivot.

If the centre of mass is to the left of the pivot the holder rotates to move the the centre of mass right (anti­clock­wise); if it’s to the right then the holder moves the centre of mass to the left (clockwise).

An object free to rotate will rotate around its centre of mass. In the pho­to­graph below (click to enlarge) you can see where the centre of mass is — it hasn’t rotated and there­fore isn’t blurred.

An empty bottle works just as well. A bottle uni­formly full of air and a bottle uni­formly full of water have almost the same centres of mass.

Shifting the neck of the bottle in and out of the holder shows what effect moving the centre of mass has on the balance point. With the neck all the way in the centre of mass is moved to the right and the holder bal­ances further to the right; the opposite is true when the neck is all the way out, moving the centre of mass to the left.

A half-full bottle has a very dif­ferent centre of mass; the ability of the water to move around causes the centre of mass to move around. In the pho­to­graph below the water has settled to the bottom of the bottle, moving the centre of mass towards the left as in the pho­to­graph above right.