```    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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