Work And Energy

Summary

When a force displaces, work is said to be done. The force should be the net force and there should be a net displacement of the object.

Work done by force acting on an object in the direction of the force is equal to the product of the magnitude of the force and the displacement covered by the object in the direction of the force.

The work done is measured in joule in the SI system after the scientist James Prescott Joule.
Erg is the CGS unit of work.

Conditions for work to be done:
A net force should act on an object.
The object must be displaced in the direction of the net force. Work is said to be done when an object is displaced on applying a certain force.
W = F × s, Where
W = Work done on an object
F = Net force on the object s = Displacement of the object
Joule is defined as the work done when the net force of one newton acts on a body and displaces it in the direction of the force by one metre. Work done could be either positive or negative. When both the force and the displacement are in the same direction, positive work is done. When force acts in a direction opposite to the direction of displacement, the work done is negative.
Summary

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The energy of an object is its ability to do work. Energy is the cause and work is its effect. Therefore both work and energy have the same units, which is joule (J) in the SI system and erg in the CGS system. Energy is also a scalar quantity. Energy exists in many forms.

Examples
Mechanical energy which is either in the form of potential energy or kinetic energy or a combination of the both, electrical energy, light energy, thermal energy, nuclear energy and sound energy etc.
Potential energy is the energy possessed by a body by virtue of its position or state. It is further classified into gravitational potential energy (GPE) and elastic potential energy (EPE). GPE is by virtue of height of a body from a reference level, it can be expressed as GPE = mgh (m being mass of the body, g is acceleration due to gravity and h the height of the body from the reference level) whereas, EPE of a body is by virtue of its stretched state. Kinetic energy (KE) is the energy possessed by a body by virtue of its motion and is given by, K.E = ½mv2.

The law of conservation of energy, which is the fundamental law, says that energy can neither be created nor destroyed, the sum total energy existing in all forms in the universe remains constant. Energy can only be transformed from one form to another.

Electrical energy commercially is measured in the units of kilowatt hour (kW h). Power is defined as the rate of doing work. Power is measured in watt which is equal to joule per second. Power can also be measured as the product of force and velocity of an object. Energy can be expressed in terms of product of power and time.
1 kW h = 3.6 x 106 J
Energy is defined as the capacity to do work.
The unit of energy is the same as that of work, and is measured in joule (J) in SI system.
The energy of an object due to its position or arrangement in a system is called potential energy. The energy possessed by a moving object is called kinetic energy.
The energy of an object due to its position or arrangement in a system is called potential energy.
The gravitational potential energy of an object is the work done in raising it from the ground to a certain point against gravity. Gravitational potential energy, P.E = mgh

If the height ‘H’ of a body is considered from the ground, then the gravitational potential energy of the body, P.E = mgH If the height of the body is considered from a reference level, whose height is ‘h’ from the ground, then the gravitational potential energy of the body is P.E = mg(H-h)
The energy possessed by a moving object is called kinetic energy.
The work done on a body is equal to the change in its kinetic energy.

The kinetic energy of a body is given by K.E = ½mv2.
Energy can be converted from one form into another. According to the law of conservation of energy, energy can neither be created nor destroyed, but can be converted from one form into another.
At the highest position of the body, the total energy possessed by it is in potential form.
While descending, the body possesses both potential and kinetic energies, the total of which is equal to the potential energy at the top position. At the bottom-most position, the total energy possessed by the body is in kinetic form.
Power is the rate of work done.
The commercial unit of energy is kilowatt hour (kW h).
1 kW h = 3.6 × 106 J