# NCERT Class 11 Physics Solutions: Chapter 2-Unit and Measurement Part 16

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Question: 2.32

It is a well known fact that during a total solar eclipse the disk of the moon almost completely covers the disk of the Sun. from this fact and from the information you can gather from examples 2.3 and 2.4, determine the approximate diameter of the moon.

• The position of the Sun, Moon, and Earth during a lunar eclipse is shown in the given figure.

• Distance of the Moon from the Earth

• Distance of the Sun from the Earth

• Diameter of the Sun

• It can be observed that and are similar. Hence, it can be written as:

• Hence, the diameter of the Moon is

Question: 2.33

A great physicist of this century (P.A.M. Dirac) loved playing with numerical values of Fundamental constants of nature. This led him to an interesting observation. Dirac found that from the basic constants of atomic physics (c, e, mass of electron, mass of proton) and the gravitational constant G, he could arrive at a number with the dimension of time. Further, it was a very large number, its magnitude being close to the present estimate on the age of the universe (~15 billion years). From the table of fundamental constants in this book, try to see if you too can construct this number (or any other interesting number you can think of). If its coincidence with the age of the universe were significant, what would this imply for the constancy of fundamental constants?

• One relation consists of some fundamental constants that give the age of the Universe by:

• Where,

t = Age of universe

e = Charge of electrons

= Absolute permittivity

= Mass of protons

= Mass of electrons

c = Speed of light

G = Universal gravitational constant

• Also,

• Substituting these values in the equation, we get