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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.
  • 34199.png 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, 34193.png
  • If force and acceleration have three components along xy, and z axis, then
    34187.png and 34181.png(1)
    From (1) it is clear that
  • No force is required to move a body uniformly along a straight line.
    ∴ 34163.png(as a = 0)
  • Out of so many natural forces, for distance 10–15 m, nuclear force is the strongest while gravitational force is the weakest.
  • Ratio of electric force and gravitational force between two electron, 34151.png
    ∴ 34144.png
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.

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, 34138.png
    or 34132.png34126.png34120.png 
  • 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 
  • Lami’s theorem: For concurrent forces,
    Fig. 2

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