Question

Name the physical quantities which are measured in ‘u’.
A. Mass
B. Energy
C. Volume
D. Length

Answer 1

Hint: There are several units named by different scientists to measure lengths, mass, etc. On a macroscopic level, we even have huge astronomical units to measure the quantities like distance between planets and the diameter of galaxies, for example light years – which means the distance travelled by light in one year. Also on a microscopic or quantum level, we have small units to measure size of electron and radius of nucleus, etc.

Complete answer:
 In order to express the masses of microscopic particles like electrons and nuclei, scientists suggested a unit to measure the mass of such particles. This is called ‘u’. This is also called ‘amu’, an abbreviation for unified atomic mass. This is also called Dalton and is defined as mass of 1/12 of the mass of an unbound neutral atom of carbon-12 in the nuclear and electronic ground state and at rest.
In short, ‘amu’ is the unit and ‘u’ is the symbol for measuring mass in molecular scale.
$1u = 1.67 \times 10^{-27}\ kg$.

So, the correct answer is “Option A”.

Additional Information:
We often see the mass of carbon as 12, oxygen as 16, nitrogen as 14, etc. But what are its units? Its units are 12u, 16u and 14u respectively. If one wants to convert this mass into an easily understandable unit or S.I. units (kilogram), we can do it by multiplying the mass of the element by $1.67 \times 10^{-27}\ kg$.

Note:
 Physics is all about information. Remembering these units will make such information of sizes such as size of nucleus, which is of the order of femtometer, easy to remember. We can easily convert the mass given in terms of ‘amu’ or ‘u’ by multiplying it by $1.67 \times 10^{-27}\ kg$to S.I. units.