Stability experiment

I like this very simple experiment to investigate stability. You tilt a pop bottle sideways and record the angle it which it won't snap back to its upright position - the first angle at which it topples. The bigger the angle the more stable an object is. When you tilt an object, part of its weight creates a moment that tries to pull it back to its normal position and the other part of the weight creates an opposite moment trying to knock the bottle over. There is a resultant moment - the biggest moment pulls the bottle in its direction. Provided the line of action of the weight has not passed the edge of the base, the moment pulling back to the original position wins and the bottle remains upright. Objects with low centre of mass and wide base mean that the object can be tilted further before the line of action of weight passes the corner of the base and the bottle topples. The centre of mass of an empty bottle is in the middle. As you add water to the bottle, you lower the centre of mass of the bottle and it becomes harder to topple. When the bottle is full the centre of mass returns to the middle and the bottle is as unstable as it was at the start.