Physics Theory : Stability & Moments

Three kinds of equilibrium fundamentally exist. They are :
stable equilibrium (e.g., inverted funnel);
unstable equilibrium (e.g, funnel balanced upright on its narrow tip); and
neutral equilibrium (e.g, funnel lying horizontally on its side).

Two equal and opposite forces acting at a point are said to be in equilibrium because they balance each other.

Three forces in equilibrium, acting at a point, can be represented in magnitude and direction by the sides of a triangle taken in order.
This is so because the three forces being in equilibrium must add up to zero on a vector diagram.

Parallelogram of Forces is the famous theorem that can be used to determine the resultant of two forces acting at a point.

Components are defined as the two forces (acting at right angles) that any singular force can be decomposed into.
It is important to note that the two forces have the same effect as the original singular force.

Moment of a force is defined as the product of the force and the moment arm, where the moment arm is the shortest distance from a point to the line of action of the force.
∴ Moment of a force = Force × Moment arm
The SI unit of moment is N m.
The moment of a force is directly proportional to the turning effect of the force.
Thus, more the turning effect of the force, more is the moment of the force.

Center of gravity of a body is the point where all its weight appears to be concentrated.
It is also the point about which all the clockwise and the anticlockwise moments are equal.
A body with a low center of gravity is more stable than a body with a high center of gravity.