Systems of Units
1. In what three types of units can all physical experiments encountered in mechanics can be expressed?
Solution: All physical measurements encountered in mechanics can be expressed in the units of mass, length and time.
2. Why meter is now defined in terms of speed of light?
Solution: The speed of light is one of the most accurately determined quantities and its value in vacuum is 299792458 m/s.
3. Why is the unit of mass least satisfactory?
Solution: Unlike the unit of length and time, which are based on measurement procedures that can be repeated by scientists anywhere, the unit of mass is still defined in terms of a particular object – the international prototype kilogram kept at the International Bureau of Weights and Measures in France. Although kept under controlled condition, it could conceivably change in value. Further, comparison with such a standard is less convenient than a procedural standard that can be checked in a laboratory.
System of Units
4. Why is the unit of mass not based on atomic standards?
Solution: The reason that unit of mass is not yet based on atomic standards is that the measurement of atomic masses and their comparison with large scale masses is not yet as precise as the measurement that can be made on large scale objects such as standard kilogram.
5. What is the current thinking of scientists about the redefinition of unit of mass?
Solution: The scientists are now working on techniques to measure the spacing of atoms in a crystal of silicone, essentially counting the atoms in a given volume and therefore scale from the mass of a single silicon atom to a new definition of the kilogram.
6. Why the units of mass, length and time are called fundamental units?
Solution: The fundamental quantities mass, length and time are not definable in terms of other quantities. For this reason, the units of these quantities are called fundamental quantities.
7. What is the importance of base units?
Solution: There are only seven base units in SI. The unit of every physical quantity encountered in science and engineering can be expressed in terms of these base units. This eliminates the need for assigning a separate unit for each physical quantity.
8. In Physics, the quantities used to describe nature are defined very precisely. Why?
Solution: In physics, the quantities used to describe nature are defined so that they have the same meaning for every person using them. These quantities differ from the descriptive words used in everyday language. For example, you might ask “I will force him to work for my election”. Here the words “force” and “work” are used to create and impression. In physics, the precise scientific meanings of force and work have nothing to do with their meanings in the quoted sentence.
9. Why scientists and engineers all over the world agreed to use SI units?
Solution: Because the results of experiments conducted in different countries are comparable.
10. Why is sometimes a special name given to Derived SI Units?
Solution: Often a special name is given to some derived SI units, usually to honor a noteworthy physicist. For example, the derived SI unit of force is kg-m/s2 is called Newton in honor of Sir Isaac Newton.
11. In defining the standard of length, we have to specify the temperature at which the measurement should be made. Are we justified in calling length a fundamental quantity if another physical quantity has to be specified in choosing a standard?
Solution: The length of an object changes with temperature. But the unit of length i.e. meter, is now defined in term of wavelengths. This is not affected by temperature. Therefore we are justified in calling length a fundamental quantity.
12. What is the advantage in choosing the wavelength of light radiation as a standard of length?
Solution: The advantages are : -
i) The wavelength is accurately defined
ii) The wavelength of light is not affected by time and environment.
iii) It can be easily reproduced in any standard laboratory.
Follow us @ Facebook : Advanced Tech World
