A coin is left balanced on its edge. It might stand like that for a long while, but suddenly, without a gust of wind, a loud bang or any external cause, it simply falls over. What is behind this behaviour?
An object’s centre of gravity plays an important role in its stability. The centre of gravity of an object is the imaginary point where its weight is equal in all directions. For something to stand up at all, its centre of gravity must be located over its base. If this point moves off the base, gravity will cause it to lean over and fall.
When an object is stable, it is in an equilibrium position. The shape of an object determines how likely it is to stay balanced. “Systems which have a very small base, like a pin-point, tend to fall over easily,” said Professor Edward J. Davison of the department of electrical & computer engineering at the University of Toronto.
“If you have a tiny base, you have to be very precise so that the centre of gravity is precisely in the [centre] of that base. If it’s not, it’ll topple over.”
An object’s weight is also significant, because it affects how easily the centre of gravity can be influenced. The air is full of microscopic disturbances. If an object is very light, these disturbances can prevent the object from keeping an equilibrium position by pushing the centre of gravity around.
Small disturbances can come from other sources as well. Davison suggests, “It could be that you’re in the room, and you don’t observe anything at all, but indeed this object is in equilibrium for five years and all of a sudden, it topples over. That could be a very low-frequency vibration, like a minor earthquake, which the human body wouldn’t particularly recognize, but which this object would, just enough to cause it to destabilize.”
