Poisel

Meet Poisel, a wooden plank who balances perfectly on one point, even with uneven loads.

Every object has a centre of mass. It is the average position of all its mass, weighted by how much mass sits where. Support an object right under that point and it balances.

For two masses on a line, x_cm = (m₁x₁ + m₂x₂)/(m₁ + m₂). Put a 10 kg weight at 0 m and a 2 kg weight at 3 m, ignoring the plank. Then x_cm = (0 + 6)/12 = 0.5 m. The balance point sits much closer to the heavy end.

When forces act on an object, its centre of mass moves as if all the mass were there. A spinning hammer thrown across a room still has one point that follows a smooth curve.

Tightrope walkers carry long poles mainly because the pole adds a lot of moment of inertia, which makes any tipping much slower and gives the walker time to correct it. Only a pole that droops at both ends also lowers the centre of mass. Racing cars are built low so they resist rolling over in corners.