Friday, April 29, 2011

Work, Power, Energy & Simple Machines


Work, Power, Energy & Simple Machines

Definition of work

Work is said to be done if due to application of an external force on a body there is actual displacement of the body, except when the displacement is perpendicular to the direction of force.
Work = Force x displacement of the point of application (body on which force is applied) in the direction of force.
If W = work done, F = applied force, d = displacement of the point of application in the direction of force, then from the definition of work, W = F x d.

Units of work

In C.G.S. the unit of work is Erg. In M.K.S. it is Joule.
1 Joule = 107 Erg.

Work done by a force

If the point of application moves in the direction of applied force, work is done by the force.

Work done against a force

If a body has its displacement in the opposite direction of the applied force, work is done against the force.


Rate of work done by an agent is known as its power. If w work is done by a body or a system in time t, then the power P of the body or the system is given by,
P = W/t.

Units of power

C.G.S. unit is Erg / second. In S.I. it is Watt. (Joule / second).

Mechanical energy

The energy that a body or a system acquires by virtue of its motion, position or configuration is known as mechanical energy.
Mechanical energy is of two types - (a) Potential energy and (b) Kinetic energy.
(a) Potential energy: Potential energy of a body is measured by the amount of work done to change its position or configuration.
Potential energy of a body of mass 'm' raised through 'h' above the Earth's surface is mgh.
(b) Kinetic Energy: Kinetic energy of a body generates as a result of motion of the body. It is usually defined as the work done by the body possessing the energy when it is stopped with an opposing force.
It can be proved that a body of mass ‘m’ moving with velocity ‘v’ has the kinetic energy 1/2 mv2.


A machine can be defined as a device with which an external force applied at a point of it in a certain direction is made to react at some other point on it. The reaction force overcomes another force or resistance.
The main advantages of a machine are (i) it enables us to apply a force at a convenient point in an easier manner, (ii) It helps us to overcome large resistance or to raise a heavy weight, (iii) with the help of a machine, a slow motion created at one point may be converted to a rapid motion at another point. Examples: paddle of a bicycle, sewing machine.

Mechanical Advantage

The advantage obtained from a machine in doing some work is called mechanical advantage. It is given by the ratio of the resistance force (W) overcome, to the effort (P) or the force applied to the machine to produce equilibrium.
Mechanical Advantage (M.A) = load / effort; W / P.
Mechanical Advantage has no unit, since it is expressed by the ratio of two forces, so, it is a pure number.


It is the ratio of the work done by a machine to the work done on the machine. The work done by a machine is called the output energy and that done on the machine is known as the input energy. A machine of efficiency 1 is called an Ideal machine.

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