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such variable force is equal to the square under the curve in fig.5.4

5.4 Power

In physics power is the time rate of doing work. The power is ratio of the
elementary work A , which is produced by the force F during the time
dt to the interval dt ,

 A  dr 
N    F    F v  . (5.13)
dt  dt 
For example, F is tractive

force of a car ,v is velocity

of the car (fig.4.5)
In the SI power is
measured in joules per second

or watts (W): 1 W = J/s. One
watt of power is expended
when one joule of work is
Figure 5.5
done per second.

5.5.Energy .Kinetic Energy. Potential Energy.

Energy is defined as the ability or capacity to do work. It occurs in
many forms. In physics the relationship between many forms of energy
and the transformations from one form to another are considered.

Energy is scalar value and is introduced in physics as the general
measure of different forms of matter's motion.
For different forms of motion different types of energy are introduced:
mechanical, internal, electric-magnetic, nuclear etc.

The energy is expressed in joules.
Kinetic energy of an object is the energy which it possesses due to its
motion It is defined as the work needed to accelerate a body of a given

mass from rest to its stated velocity. Therefore consider a common case
of curvilinear motion of material point due to the variable force (fig.5.6).
Result of action of such force is change of velocity from v to v and
2
consequently of kinetic energy. Elementary work done by force on

elementary displacement dS defines elementary change of kinetic energy
. Hence,
dE K    FA  dS  cos  (5.14)

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