Centre of mass and how to find it. Types of equilibrium
Gravity acts on every part of a body; the point where their resultant is applied is the centre of gravity, i.e. the centre of mass. In uniform symmetrical bodies (a sphere, a cube, a rectangular plate) it lies at the geometric centre. To find the centre of mass of a flat thin plate, hang it by a thread from two or three different points and draw the vertical lines: their intersection is the centre of mass. Types of equilibrium: stable — if the body is displaced it returns (a ball at the bottom of a bowl); unstable — once displaced it moves further away (a ball on top of a dome); indifferent — the body stays in equilibrium in any position (a ball on a flat table). A body on a support is stable if the vertical through its centre of mass falls inside the base area: so a body with a low centre of mass and a wide base is more stable (the Leaning Tower of Pisa, a lorry).
Practical task: with an adult cut an irregular shape from cardboard (round-tipped scissors), make holes at three points (no needle — use a thick pencil tip), hang it from each hole on a thread and draw a vertical line each time; the crossing point is the centre of mass. Safety: use scissors only under adult supervision.