System of Forces

When several forces of different magnitude and different direction act upon a certain body, they constitute a system of forces.

If all the forces in a system lie in a single plane, it is called a coplanar force system.
If the line of action of all the forces in a system passes through a single point it is called a concurrent force system.
In a system of parallel forces, all the forces are parallel to each other.
If the line of action of all forces lies along a single line, then it is called a collinear force system.
The various system of forces, their characteristics and examples are given in the table below.

Table: System of Various Forces

S.No Force System
Characteristic
Examples
1 Collinear forces Line of action of all the forces acts along the same line. Forces on a rope in a tug of war.
2 Coplanar parallel forces All forces are parallel to each other and lie in a single plane. The system of forces acting on a beam subjected to vertical loads.
3 Coplanar like parallel forces All forces are parallel to each other, lie in a single plane and are acting in the same direction. The weight of a stationary train on a rail when the track is straight.
4 Coplanar Concurrent forces Line of action of all forces pass through a single point and forces lie in the same plane. Forces on a rod resting against a wall.
5 Coplanar Non-concurrent forces All forces do not meet at a point but lie in a single plane. Forces on a ladder resting against a wall when a person stands on a rung which is not at its centre of gravity.
6 Non-coplanar parallel forces All the forces are parallel to each other, but not in the same plane. The weight of benches in a classroom.
7 Non-coplanar concurrent forces All the forces are parallel to each other, but not in the same plane, but their lines of action pass through a single point. A tripod is carrying a camera.
8 Non-coplanar non-concurrent forces All forces do not lie in the same plane, but their lines of action pass through a single point. Forces are acting on a moving bus.
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