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Chapter 4 - Work, Energy and Power


In science, the work is said to be done only when a force produces motion.

The following conditions are required to be satisfied for work to be done on a body:
  • External force acting on the body.
  • Displacement produced on the body.

Work is said to be done when an object covers some distance along the direction of force. On pull and push, force performs some work. Mathematically, work is the product of force applied to an object and displacement produced on it. I.e. work is the scalar product of two vectors and is a scalar quantity. And W= f.d if object moves along the direction of force. The work done is zero if the displacement is zero or the displacement produced is perpendicular to the direction of applied force.

Unit of Work: W = f . d = N.m = joule
SI unit of work is Joule(j).
CGS unit of work id Erg.
1 J = 1 Nm

One joule work is defined as the amount of work done, when one Newton force produces one metre displacement on a body.

If 1 dyne force is applied on an object to displace through the distance of 1 cm in the direction of force applied, the work done is said to be one Erg.

Work is done against two forces:
  1. Work is done against the gravity.
  2. Work is done against the friction.

i. Work done against gravity:

When a body of mass m is lifted up to a height h, force applied to the body is :
F = mass × acceleration due to gravity
F = m g
Work-done = Force × displacement
Work-done = F.h
W = mgh

ii. Work done against Friction:

When a body moves on another body in a horizontal plane, there exists a force in the opposite direction of motion and this force is called friction. Frictional force acts tangential to the surface of contact.
To produce motion on a body, work should be done against friction. It is given by the scalar product of force and displacement.
Workdone = force × displacement
W = f.d


Energy is defined as the capacity of doing work. The SI unit of energy is joule and CGS unit is Erg.

Relationship between Joule and Erg:
1 joule = 1 Nm
1 joule = 1 kg 1 m 1 m/s2
107 dyne cm
107 ergs

Different forms of energy:
Some common forms of energy present around us are:
  1. Mechanical Energy
    • Kinetic Energy
    • Potential Energy
  2. Heat Energy
  3. Light Energy
  4. Sound Energy
  5. Electrical Energy
  6. Chemical Energy
  7. Nuclear Energy (Atomic energy)
Transformation of energy:
The process of conversion of energy from one form to another by means of a device is called transformation of energy.
Classical principle of conservation of energy : It states that "Energy can neither be created nor be destroyed but can be transferred from one form to another".


Power is defined as the rate of doing work or rate of energy transformed.
Formula: Power = work done / time = energy transformed / time

P = W / t
SI unit of Power
Power = W / t = joule / second = J/s = Watt(w)

1 watt = 1 Joule / 1 second
What is one watt ?
One watt is the power of a device that can do 1 joule work in 1 second.

Other Units of Power:
1 HP = 746 watt and is nearly equals to 750 watt
1 KW = 1000 watt
1 MW = 106 watt

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