Non Conservative Forces
The non-conservative forces, by implication, are the forces which do not conserve the mechanical energy, kinetic plus potential, of an object during its motion under these forces. The mechanical energy invariably decreases, the energy loss finally dissipated as heat. Non-conservative forces are, therefore, also referred to as dissipative forces.
In the following discussion, we describe a few important non-conservative forces encountered in mechanics:
(i) Dry friction: Dry friction is the term used for frictional force between two solid surfaces in contact. It arises due to interaction of molecules at the points of contact between the two surfaces and acts tangentially to the surfaces. Experimental observations show that generally dry friction is almost independent of the relative velocity between the two surfaces. In fact, dry friction exists even if the two surfaces are at rest relative to each other; it is then called static friction. Otherwise, it is called sliding friction. Empirically, we find that sliding friction is given by
F fr = 
N
where N is the normal force of action-reaction between the two surfaces. Dry friction always acts opposite to the direction of motion of a given body.
(ii) Fluid friction: When a solid body moves in a medium of gas or liquid, a frictional force tending to inhibit the motion appears between the surface of the solid and fluid in contact. The force of fluid friction depends on the relative velocity v of the body in fluid medium; over a wide range of v, the force of fluid friction is given by,
F (v) = Av + Bv2
where A and B are constants for a given body in a given fluid. The fluid friction thus comprises of two terms: (i) the viscous force Av, and (ii) the drag force Bv2. The drag force, being proportional to v2, is important at large v and is associated with the production of turbulence in fluid. During streamline flow of fluid past the body at low velocities, the viscous force dominates. However, whether viscous; or drag force would be dominant is also decided by the values of empirical constants A and B. In general, for small, tiny bodies (dia ~ 10-6 m), viscous force dominates; for larger bodies (dia ~ cm), drag force dominates except at very low velocities. The fluid friction also always acts opposite to v.
(iii) Forces of inelastic deformations: Why does a ball bouncing on the floor finally stops? Or, why does a ball falling on mud stops even at the first bounce? These are the cases where colliding surfaces are deformed beyond elastic limits and mechanical energy of motion is transformed into internal energy of inelastic deformations.
Similarly, a spring which is compressed beyond elastic limit does not restore its original length when external force is withdrawn. The spring undergoes inelastic deformation. The phenomenon of inelastic deformations and consequent increase of internal energy of the system is called mechanical hysteresis. The forces causing such deformations are non-conservative and are not precisely defined. The work done by these forces can be estimated only indirectly, i.e. by finding the deficit in total mechanical energy.
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