Ch 1Nature of Physical World and Measurement — Class 11 Physics, concept-first.

Before physics can be introduced as a subject in its own right, it helps to ask a broader question: what is science itself, and where did the impulse to study nature systematically come from?

Physics comes from the Greek phusis, meaning nature -- and true to its name, physics is the attempt to understand nature and natural phenomena in the most basic possible terms.

Physics does not proceed by guesswork; it follows a disciplined scientific method -- a repeatable, step-by-step way of turning a raw observation into a tested law. The method has five ingredients:

Physics is not one narrow subject but an umbrella over many branches, each opened up at a different point in history and each with its own characteristic scale, method and mathematics.

Physics stays exciting for two reasons that reinforce each other: the astonishing range it covers, and the unpredictable way its biggest discoveries arrive.

Technology is physics put to practical use -- applying physical principles to invent products or solve problems. Physics and technology together shape society, directly and indirectly:

Measurement is the comparison of a physical quantity with its standard unit, and it underlies every scientific study.

A physical quantity is anything that can be measured, and in terms of which the laws of physics are stated -- length, mass, time, force, energy, temperature, electric current, and so on.

Physical quantities split into exactly two categories:

Measurement is fundamentally an act of comparison. To say a rope is '10 metres long' is to say it is 10 times as long as an object whose length has been defined as 1 metre -- and that reference length…

A system of units is a complete set of units used to measure all fundamental and derived quantities. Historically, mechanics used three different systems, distinguished by which three base units they…

Since 1960, the internationally agreed system of units has been the International System of Units, or SI (from the French Système International) -- what scientists and engineers everywhere now call si…

Defining a unit is only half the story -- physics also needs practical methods for measuring the three most basic mechanical quantities: length, mass and time.

Length is the distance between any two points in space; its SI unit is the metre. Objects of interest range from the macrocosm (galaxies, stars, planets -- a large world of large objects and large dis…

Mass is a property of matter -- the quantity of matter contained in a body -- and, unlike weight, it does not depend on temperature, pressure or the body's location in space.

A clock measures a time interval. Historically many kinds of clocks have been used -- electric oscillators, electronic oscillators, solar clocks, quartz-crystal clocks -- but today's atomic standard r…

No measurement is ever perfectly exact -- the result obtained always carries some uncertainty, called error, and any calculation built from that measured value inherits the error too.

Two words are often used loosely in everyday speech but mean two different things in measurement theory:

The uncertainty in a measurement is called an error; there are three fundamentally different kinds.

Once repeated readings of a quantity have been taken, the arithmetic mean is treated as the best available estimate of the true value, and four related error measures are built from it.

A final experimental result is rarely built from a single measured quantity -- it usually combines several quantities, each measured (possibly with a different instrument, and hence a different error)…

The digits reported in a measured (or calculated) value are not all equally trustworthy -- some are known reliably, and exactly one more is a reasonable estimate; the rest are meaningless noise a calc…

The significant figures (or significant digits) of a measured value are the digits known reliably, plus exactly one more digit that is uncertain.

A calculator's raw output almost always has more digits than the measured data justify -- the result must never be quoted with more significant figures than the least-precise input carried.

Significant figures must be tracked correctly through arithmetic, or a calculation can manufacture precision the original data never had. Two different rules apply depending on the operation:

Dimensional analysis is the technique of tracking, algebraically, which of the fundamental quantities (and to what power) go into building a derived quantity -- without worrying about the actual numer…

Every derived physical quantity can be written as some combination of the seven fundamental (base) quantities, called its dimensions, denoted with square brackets: for length, for mass, for time (the…

Classifying by dimension. Every physical quantity falls into exactly one of four categories:

Dimensional analysis, built on the principle of homogeneity (1.8.2), has three practical applications -- and five genuine limits on what it can do.

Physics is an experimental science, and every measurement must be expressed in units -- a bare number, without a unit, says nothing physical. Every physical quantity has both a magnitude and a unit.

Sample papers and previous-year board questions for this subject.