Question

Problem 1. Explain the definition of the unit of quantity, the definition of SI, and the basic unit of SI. Also, select one of the SI basic units and explain the definition of the unit in detail. However, write it down so that high school students can understand it. Summarize the above in A4.

Answer 1

The magnitude of any thing is represented or measured by the unit of quantity. For example; if a students weighs 60kg, then kg is the unit of weight (or mass) of that student. Just saying 60 will not help to determine what it is. But when we say 60kg, it tells that it is the weight of student.

Hence, unit is the measurement of any quantity. Example: Length has unit in meters; mass has unit in kg; time has unit in second. All these units help us understand what property are we taking about.

(Just saying 60 does not imply anything. But if we say 60seconds, this means we are taking of time. If we say 60m this means we are taking of Length. Hence, units are essential to measure any quantity.)

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SI is the International system of Units. It is a standard unit system that is followed in general by every one around the world. This is so, because of every has its own unit of any quantity then it would be impossible to interact with such infinite number of units for same quantity. Hence, SI units are set of standard units which are follwed by all.

There are 7 basic SI units.....

1). Kg(for mass)

2). Second, s (for time)

3). Meter, m (for length)

4). Kelvin, K (for temperature)

5). Mole, mol (amount of substance)

6). Ampere, A (for current)

7). Candela, Cd (for luminous intensity)

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Length has SI unit of meter(m).

Generally we Define any unit in terms of some of the natural constants which are present in nature : example; speed of light(c), boltzman constant, Avogadro number, etc.

We Define the Basic units in terms of these constants.

So, lets Define a METER:

We know that speed of light is a universal constant. So we Define meter as the length of the path travelled by light in a vacuum in 1299 792 458 of a second.

We use the speed of light to determine how much seconds it take to travel 1 meter . This distance is hence called a meter. Thus, light takes 1299792458 of a second to travel 1 meter. Hence this is the definition of a METER.