There is a book on the table fixed rigidly to the earth. The book is at rest with respect to the earth. The acceleration of this book with respect to the earth is 0. The forces on this book are
a) Gravitational force W exerted by earth.
b) Contact force N exerted by the table.
Is the vector sum of both the forces 0?
A renowned physics book - Concepts of physics by H.C Verma states that "on a very accurate measurement, the above answer is no. The sum of the forces is not 0 although the book is at rest. The earth is not strictly an inertial frame. However, the sum is not too different from 0 and we can say that earth is an inertial frame of reference to a good approximation."
Can someone please elaborate on this? Why the sum is strictly not 0?