Though the number of physical quantities is very large yet we do not have as many different units, why?

Can a physical quanitty have different dimensions in different systems of units?

Why do we need to classify the drugs in different ways ?

Suppose we have a large number of identical particles, very small in size. Any two of them at 10 cm separation repel with a force of 3 x 10^-10N.find the value of charge

Suppose we have a large number of identical particles, very small in size. Any two of them at 10 cm separation repel with a force of 3 x 10^-10N. If one of them is at 10 cm from a group of n others, how strongly do you expet it to be repelled?

The atomic mass of an element is the weighted average of the atomic masses of different isotopes of that element .This explains,why atomic masses of many elements show large departures from integer values.However ,even if we consider msases of individual isotpes,they are not strictly integer multiples of the mass of a hydroen atom.How do yo account for this fact?

For a reversible gaseous reaction N_(2)+3H_(2)hArr2NH_(3) at equilibrium , if some moles of H_(2) are replaced by same number of moles of T_(2) (T is tritium , isotope of H and assume isotopes do not have different chemical properties ) without affecting other parameters , then:

Precise measurements of physical quantities are a need of science. For example, to ascertain the speed of an aircraft, one must have an accurate method to find its positions at closely separated instants of time. This was the actual motivation behind the discovery of radar in World War II. Think of different examples in modern science where precise measurements of length, time, mass etc. are needed. Also, wherever you can, give a quantitative idea of the precision needed.