Explain the reason of using different smaller and larger units for measuring a particular quantity.
To measure a particular quantity we use both smaller and larger units as per requirement. For example. To measure the length of a room , we use metre(m) to measure the distance between two cities we use kilomete(km). For the measurement of the distance of a star from the earth the commonly used unit is light year. Because kilometer is considered to be a very small unit in this case.
To measure the length of a very small quantity we consider smaller unit of lengths as centimetre, millimeter, micron etc. For example to measure the thickness (or diameter) of a wire, we use centimetre or millimeters. In this case unit taken as metre is absurd. If the thickness of a paper measured in millimeter(mm) is 1 mm, then expressed in metre is 0.001 m, which look odd.
Therefore we should carefully examine that what unit we have to take for the measurement of a thing . If we chose a thing to measure we should take its standard unit.
Dimension of a physical quantity
Having defined the units of physical quantities, we now briefly mention the concept of dimensions of other physical quantities.
The dimension of a physical quantity is represents the powers to which the fundamental units have raised in order to obtain the derived units of the physical quantity.
The dimensions of length, mass and time are expressed in L, M and T’ respectively. Dimensions generally represents the nature of the units only. It does not indicate the magnitude of the units.
Discussion : Speed has the derived units –cm/sec., km/hr., mi/hr. etc. The most common feature in them is that they all are derived by dividing a unit of length by a unit of time. So the dimension of speed will be L/T or LT -1 where ‘L’ stands for length and ‘T’ stands for time. Similarly different units of acceleration e.g ., cm/sec2, ft/sec2, mi/hr2 etc. represents a unit of length divided by square of unit of time. Hence its dimension will be L/T2 or L/T -2.
Following the same reasoning ,force which is the product of mass(M) and
aceleration (LT -2), has the dimension MLT -2.
Measuring Instruments
As the measurement of length, volume, mass and time was thoroughly taken into consideration in the previous article, we shall not go through it any more.
In this article we shall rather present a brief description of some appliances for measurement of length which enables greater accuracy to be obtained than is possible by mere eye-estimation.
To measure the different physical quantities, the commonly used measuring devices are the following.
1. Ordinary scale (metre scale) use to measure the length of an object.
2. Common balance - it is used to measure the mass of a body.
3. Spring balance – it is used to measure the weight of a body.
4. Measuring cylinder – it is used to measure the volume of a body a liquid.
It is also used to measure the volume of a solid by displacement method.
5. Clock – it is used to measure the time.
6. Stop watch – it is used to measure a time interval between two events such as race, sports , football etc.
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