PHYSICS [3 Mark Question Answer]

This means “The total MECHANICAL ENERGY (P.E + K.E) of an isolated system at any instant is equal to the sum of kinetic ENERGYand the potential ENERGY.”
Condition : Condition under which the mechanical energy is conserved is “WHEN THERE ARE NO FRICTIONAL FORCES.” In other words the mechanical energy is conserved strictly in vacuum where friction due to air is absent.

The energy is measured in the same unit as work. Therefore the S.I. unit of energy is joule (symbol J).
A body is said to possess an energy of one joule if a force of 1 newton moves the body by a distance of 1 metre in the direction of force.
Another unit of energy is calorie (symbol cal) where 1 cal = 4.2 J. A bigger unit is kilo-calorie (symbol kcal) where 1 kcal = 1000 cal.

The examples that show the conversion of energy from one form to another are :
(1) In a steam engine, the chemical energy of the coal first changes into the heat energy of the steam. Then heat jenergy of steam changes into the mechanical energy which makes the train to move.
(2) In an electric motor (or in fan), the electrical energy changes into the mechanical energy. This energy rotates the axle of motor (or the blades of the fan).

Yes, a body possesses energy even when it is not in motion ;
Consider a body raised to a certain height say h. It its velocity is zero. Kinetic energy will be zero but the body will have.
P.E. = mgh
Thus, a body may possess energy even though it is not in motion.

The energy of a body in motion is called it's kinetic energy. It is defined as follows :
Kinetic energy of a body is the energy possessed by it due to its state of motion.
Actually, it is the work done on the body bringing it to the state of motion. In short form it is written as K.E. or K.
Example: In a swinging pendulum moving to and fro, the bob has the kinetic energy.