Work, Energy, and Power in Physics: Understanding the Basics
Classified in Physics
Written on in 
English with a size of 6.17 KB
Work, Energy, and Power in Physics
Content
Labor Force, Kinetic Energy, Potential Energy, Conservative and Nonconservative Forces, Power.
Development
Labor Force
A constant force produces work when applied to a body, it moves along a certain distance.
While work is done on the body, there is a transfer of energy to it, so it can be said that work is energy in motion. Moreover, if a constant force produces no movement, no work is done. For example, holding a book at arm's length does not involve any work on the book, regardless of effort. Work is expressed in Joules (J).
When the force is in the direction of motion:
L = Fd
L: Work done by force.
When the applied force has an inclination with respect to movement:
L = Fd cos θ
All the forces perpendicular to the motion do no work.
Force can not be mechanical, as in lifting a body or acceleration of a jet plane. It can also be an electrostatic force, electrodynamics, or surface tension.
Energy
The quantity called energy connects all branches of physics. In the field of physics, energy must be supplied for work. Energy is expressed in Joules (J). There are many forms of energy: electric and magnetic potential energy, kinetic energy, stored energy in springs stretched, compressed, or molecular bonds, heat, and even the mass.
Kinetic Energy
When a force increases the speed of a body, it also does work, as for example in the acceleration of a plane by the force of its reactors. When a body moves with variable motion, it develops kinetic energy.
Ec = ½ mv²
L = Fd
L = Ec
Fd = ½ mv²
Ec: Kinetic energy.
The work done by the force acting on a particle is equal to the change of kinetic energy of the particle.
ΔEc = Ec2 - Ec1
L = Ec2 - Ec1
Fd = ½ m(v2² - v1²)
ΔEc: Variation of kinetic energy.
Potential Energy
When an object is lifted from the ground to the surface of a table, for example, work is done by having to overcome the force of gravity, directed downward. The energy imparted to the body for this work increases its potential energy. If work is done to lift an object at a higher height, it stores energy as gravitational potential energy.
When a body changes its height, it develops potential energy.
Ep = mgh
L = Fd ⇒ L = Pd = Ep = mgh
Ep: Potential energy.
The work done by the force equals the weight variation of potential energy.
ΔEp = Ep2 - Ep1
L = Ep2 - Ep1
Pd = mg(h2 - h1)
ΔEp: Variation of potential energy.
In all transformations from one kind of energy to another, total energy is conserved and is known as a theorem of mechanical energy (ΔEM). For example, if work is carried on a rubber ball to lift it, it increases its gravitational potential energy. If you drop the ball, the gravitational potential energy becomes kinetic energy. When the ball hits the ground, it is deformed, and there is friction between the molecules of the material. This friction is transformed into heat or thermal energy.
Conservative Forces
For a body of mass m moves from point 1 to 2 and then from point 2 to 1.
A force is conservative if the work it does on a particle moving in any round trip is 0.
ΔEM = 0
ΔEM: Variation of mechanical energy.
Working conservative forces:
L = ΔEM
ΔEM = ΔEc + ΔEp
L = ΔEc + ΔEp
Nonconservative Forces
For a body of mass m moves from point 1 to 2 and then from point 2 to 1.
A force is nonconservative if the work it does on a particle moving in any round trip is different from 0.
ΔEM ≠ 0
ΔEM = HO
ΔEM: Variation of mechanical energy.
HO: Work of the frictional force.
Working non-conservative forces:
L = ΔEM + HO
L = ΔEc + ΔEp + HO
Where: HO = Frd
Power
The power developed by a force on a body is the work done by it during the time of application. The power is expressed in watts (W).
P = L/t
P = Fd/t
v = d/t
P = Fv
Also:
P = (ΔEc + ΔEp + ΔHO)/t
If there is no friction force:
P = (ΔEc + ΔEp)/t
If there is no change in height:
P = (ΔEc)/t
P: power
Horsepower: Traditional unit for expressing the mechanical power, i.e., the mechanical work that can make an engine per unit of time, usually abbreviated to CV. In the International System of Units, the power unit is the watt, 1 horsepower is equivalent to 736 watts. Its original value was, by definition, 75 kilogram-meters per second.
Author: Ricardo Santiago Netto