    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
                           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
If under a constant force F the object displaced
through a distance S, then work done by the             Energy (E)
                                                           Energy of a body is its capacity of doing
(W)  FScos                                                work.
                                                           It is a scalar quantity.
                                                           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
                                                            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.
                                                        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