Ch 1Units and Measurements — Class 11 Physics, concept-first.

Physics is fundamentally a quantitative science — its laws are stated as precise numerical relationships between measured quantities, not vague qualitative descriptions.

Over the years, physics has used several different systems of units, and it helps to know all of them because older instruments, textbooks and regional habits still sometimes use them:

A physical quantity that does not depend on any other physical quantity for its own measurement is called a fundamental (or base) quantity.

Beyond the seven fundamental quantities, physics deals with a huge number of other quantities — speed, momentum, resistance, electrical conductivity, and so on — each of which depends on some combinat…

SI lays down precise conventions for how unit symbols must be written, so that scientific writing is unambiguous everywhere in the world:

One of the seven fundamental quantities, length, is measured in the SI unit metre (m). The definition of the metre has itself evolved over time as measurement technology improved.

Large astronomical distances — such as the distance of a planet or star from the Earth — cannot be measured directly with anything like a metre scale, so astronomers instead use the parallax method.

The Sun is the star closest to Earth; the next-closest star is roughly light years away. At such enormous distances, even the parallax measured using the widest baseline available on the Earth's surfa…

Once the distance to a planet or star has already been found (for instance, by the parallax method of section 1.3.1), the same basic idea of an angle subtended at a distance can be turned around to me…

The astronomical techniques of the previous sections work for very large distances, but at the opposite end of the scale — measuring the size of atoms and molecules — conventional length-measuring ins…

Since 1889, the standard kilogram was defined as the mass of a specific, carefully preserved cylinder of platinum-iridium alloy, kept under controlled conditions in a special glass case at the Interna…

The SI unit of time is the second (s). For a long time, the definition of the second was tied to the Earth's own rotation: one mean solar day — the average time interval from one solar noon to the nex…

As introduced in section 1.2.2, a derived physical quantity can always be expressed as some combination of the seven fundamental (basic) quantities.

Dimensional analysis — comparing the dimensional formulae of the different terms in a physical relationship — turns out to be useful in three distinct ways:

Powerful as it is, dimensional analysis has real limitations, and it is important to know when it cannot be trusted to give a complete answer:

Physics is fundamentally a science built on observation and experiment: whenever we study a physical system — for example, during an atmospheric study, where we might measure atmospheric pressure, win…

Any real measurement of a physical quantity can be affected by faults in the measurement process, which lead to errors.

Suppose a physical quantity is measured repeatedly, giving a set of readings . The arithmetic mean of these readings is and this arithmetic mean is taken as the most probable value of the quantity bei…

When an experiment involves several measured quantities that must be combined (added, multiplied, divided, or raised to a power) to compute a final result, it is essential to know how the individual e…

Our ability to measure a physical quantity accurately is fundamentally limited by the least count of the instrument being used — the least count is simply the smallest measurement that a given instrum…