## Thursday, May 17, 2007

### X-revision Work, Energy and Power

Work
Work is said to be done only when the force applied to a body makes the body move (i.e., there is a displacement of the body).

What is the amount of work done by a force?
The amount of work done by a force is equal to the product of the force and the displacement of the pint of application of the force in the direction of force.

If a force acts on a body and the body does not move, no work is done.

The amount of work done by force is zero in the following two situations:

(i) when there no displacement and (ii) when the displacement is normal to the direction of the force.

If the displacement is in a direction opposite to the force, then the work done is negative. This is usually the case when force opposes the motion or it tries to stop a moving body. The example will be friction.

1 joule of work is said to be done when a force of 1 newton displaces a body through 1 metre in its own direction.

1 erg of work is said to be done when a force of 1 dyne displaces a body through 1 cm in its own direction.

What is power?
The rate of doing work is called power.

If 1 joule of work is done in 1 second, the power is said to be 1 watt.

What is energy?
The energy of a body is its capacity to do work. Energy is a scalar quantity. Its S.I. unit of energy is joule. And the CGS unit is erg.

What is kinetic energy?

The energy possessed by a body by virtue of its sate of motion is called the kinetic energy.

The kinetic energy possessed by a moving body can be measured by the amount of work need to be done by an opposing force in bringing the body to rest from its state of motion.

What is potential energy?
The energy possessed by body by virtue of its state or position I called the potential energy.

The gravitational potential energy of a body at a height above the ground is measured by the amount of work done in lifting it up to that height against the force of gravity.

It is interesting to note that whenever mechanical energy changed to other forms, it always in form of kinetic energy and not in form of potential energy i.e., potential energy first changes to kinetic energy and then kinetic energy changes to other form.

Law of conservation of energy
According to the law of conservation of energy, energy can neither be created nor can it be destroyed. It only changes from one form to another.