# Force

Force is an external effect in the form of a push or pulls which produces or tries to produce motion in a body at rest, stops or tries to stop a moving body, and changes or tries to change the direction of motion of the body.

- formula is valid only if force is changing the state of rest or motion and the mass of the body is constant and finite.
- If
*m*is not constant, - If force and acceleration have three components along
*x*,*y*, and*z*axis, then - No force is required to move a body uniformly along a straight line.
*a*= 0) - Out of so many natural forces, for distance 10
^{â€“15}m, nuclear force is the strongest while gravitational force is the weakest. - Ratio of electric force and gravitational force between two electron,

**Constant force**If the direction and magnitude of a force is constant. It is said to be a constant force.

**Central force**If a position-dependent force is always directed toward or away from a fixed point it is said to be central otherwise non-central.

**Conservative or non-conservative force**If under the action of a force the work done in a round trip is zero or the work is path independent, the force is said to be conservative otherwise non-conservative.

Example

Conservative force: Gravitational force, electric force, elastic force.

Non-conservative force: Frictional force, viscous force.

# Equilibrium of concurrent force

- If all the forces working on a body are acting on the same point, then they are said to be concurrent.
- A body, under the action of concurrent forces, is said to be in equilibrium when there is no change in the state of rest or of uniform motion along a straight line.
- The necessary condition for the equilibrium of a body under the action of concurrent forces is that the vector sum of all the forces acting on the body must be zero.
- Mathematically, for equilibrium,
- Three concurrent forces will be in equilibrium if they can be represented completely by three sides of a triangle taken in order (Fig. 1).

**Fig. 1** - Lamiâ€™s theorem: For concurrent forces,

**Fig. 2**