    Skin friction is a component of drag, the             The friction that exists between two
     force resisting the motion of a solid body             surfaces those are not moving relative to
     through a fluid.                                       each other.
    Internal friction is the force resisting
     motion between the elements making up a            Kinetic friction
     solid material while it undergoes
     deformation.                                          Once the Force of Static Friction is
                                                            overcome, the Force of Kinetic Friction is
Static friction                                             what slows down a moving object!
                                                           The coefficient of kinetic friction is
    The Force of Static Friction keeps a                   typically denoted as μk, and is usually less
     stationary object at rest!                             than the coefficient of static friction for
    The coefficient of kinetic friction is                 the same materials.
     typically denoted as μs                               The friction that exists between two
                                                            surfaces those are moving relative to each
                                                            other.
                           WORK, ENERGY AND POWER
Work (W)                                                (b) body is displaced perpendicular to the
                                                        direction of force, i.e., θ = 90°
    When a force acts on an object and the
     object actually moves in the direction of          Work done by a force is positive if angle
     force, then the work is said to be done by         between F and S is acute angle.
     the force.
    Work done by the force is equal to the             Work done by a force is negative if angle
     product of the force and the displacement          between F and S is obtuse angle.
     of the object in the direction of force.
    The SI unit of work is the newton-metre            Work done by a constant force depends only
     (N m) or joule (J) and CGS unit is erg.            on the initial and final Positions and not on the
    Its dimensional formula is [ML2T-2].               actual path followed between initial and final
                                                        positions.
If under a constant force F the object displaced
through a distance S, then work done by the             Energy (E)
Force
                                                           Energy of a body is its capacity of doing
(W)  FScos                                                work.
                                                           It is a scalar quantity.
          F
                                                           Its SI unit is newton-metre (N m) or joule
                                                           and CGS unit is erg.
                                                           Its dimensional formula is [ML3T-3].
                                                           There are several types of energies, such
            S
                                                            as mechanical energy (kinetic energy and
                                                            potential energy), chemical energy, light
Where W = Work, F = Force, S =
                                                            energy, heat energy, sound energy, nuclear
Displacement, θ = angle between force and
                                                            energy, electric energy etc.
displacement.
                                                        Relativity - When calculating kinetic energy
Work done by a force is zero, if
                                                        (work to accelerate a mass from zero speed to
                                                        some finite speed) relativistically - using
(a) body is not displaced actually, i.e., S = 0
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