The three fundamental physical quantities of mechanics are length, mass, and time, which in the SI system have the units meters (m), kilograms (kg), and seconds (s), respectively. Prefixes indicating various powers of ten are used with these three basic units. The density of a substance is defined as its mass per unit volume. Different substances have different densities mainly because of differences in their atomic masses and atomic arrangements.
The number of particles in one mole of any element or compound, called Avogadro’s number,
The method of dimensional analysis is very powerful in solving physics problems.
Dimensions can be treated as algebraic quantities. By making estimates and making order-of-magnitude calculations, you should be able to approximate the answer to a problem when there is not enough information available to completely specify an exact solution.
When you compute a result from several measured numbers, each of which has a certain accuracy, you should give the result with the correct number of significant figures.