Parallel forces have their lines of action parallel to each other.

If the direction of parallel forces is the same, they are called parallel forces.

Unlike Parallel forces: If two parallel forces are in opposite directions to each other then they are called, unlike parallel forces.

The sum of two or more forces is called the resultant force.

A graphical method used to find the resultant of two or more forces is called the head-to-tail rule.

Splitting up a force into two components perpendicular to each other is called the resolution of that force. These components are:

 $F_{x}=F\cos\theta$            $F_{y}=F\sin\theta$

Torque or moment of the force is the turning effect of the force. The torque of a force is equal to the product of the force and the moment arm of the force.

According to the principle of moments, the sum of clockwise moments acting on a body in equilibrium is equal to the sum of the anticlockwise moments acting on it.

The Centre of the mass of a body is such a point where the net force causes it to move without rotation.

The center of gravity of a body is a point where the whole weight of a body acts vertically downward.

If a body does not return to its previous position when set free after a slight tilt is said to be in unstable equilibrium.

A body that remains in its new position when disturbed from its previous position is said to be in a state of neutral equilibrium.