Emil is investigating the simple harmonic motion of a floating straw for his EE. The problem is the straw is unstable, it keeps falling to one side. T explain this he had a look at the forces present and where they act. If we first consider a uniform straw sealed at one end. The forces acting are the buoyant force and the weight. The buoyant force acts on the geometric centre of the submerged part and the weight on the centre of mass.

This causes unbalanced torques causing the straw to fall over.

If mass is added to the end of the straw then the centre of mass is lowered but the straw sinks further so the centre of the submerged part also lowers, At some point they will act at the same point and there will be no resultant torque about the centre. Deeper than this and the torque is clockwise correcting the lean of the straw. But what if this happens when the straw is fully immersed? It sinks.

To solve this problem a mass was attached at the end of a thin rigid wire. This lowered the centre of mass but didn't change the centre of the submerged part thus creating a stable floating straw.