UNIT 1 JEE MAINS PHYSICS SI UNITS

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What is a unit?

Let us say you were to measure the height of the person. The answer wouldn’t be that the person is tall or short but the answer would be that the person is 160 centimeters tall or 200 centimeters tall or some other value depending on the height of the person.

Now we have compared the height of the person to centimeter and have counted how many times the person is to that of the centimeter. But what if the person height is to be given in Feet and Inches? You would simply compare the person’s height with that of Feet and Inches.

So measurement of any physical quantity with respect to the universally accepted reference is called as unit. Let us simplify this definition with the above given example. Here the physical quantity to be measured is the height. The universally accepted reference is the centimeter. Thus the reference centimeter is called as the unit.

We will return the result of the measurement of a physical quantity with a number followed by the unit.

Why should we use units?

In the above example if you say that the person is tall, then in your point of view the height of the person may be more than 160 centimeters but according to my point of view it may be above 150 centimeters. So if you say tall then I would consider 150 centimeters but according to your point of view it is 160 centimeters and there will be a sort of confusion between the two. But if you specify that the height of the person as 160 centimeters exactly then there will not be any confusion between both of us instead of using the word tall.

Units are further divided into Fundamental and Derived Units which we will be covering in the next tutorial.

There are many universally accepted units among different countries. Some of the well known unit systems are:

FPS: In FPS the measurement of length is done with Foot, mass with Pound, Time with Seconds.

CGS: In CGS systems length is measured in Centimeters, Weight in Grams, Time in Seconds.

MKS: In MKS length is measured in meters, mass in kilograms, time in seconds.

The system which is presently used and accepted is the SI unit. Full form of SI unit is Système Internationale. This is accepted universally and are used in industries and other places worldwide. They have adapted SI units as standard because it is easy to use and also it is easy to convert from this unit to others. It is to be remembered that we shall use SI units in all our tutorials.