Chapter : Physical World and Measurement
Physics is the study of nature – how matter, energy, space and time behave. It looks for simple laws that can explain many different phenomena.
At school level, we mainly meet mechanics (motion and forces), thermodynamics (heat and temperature), optics (light), electricity, and modern physics (atoms, nuclei, basic particles).
Notice how one subject (physics) connects falling objects, glowing bulbs, hot tea cooling, lenses, and stars in the sky. All are described using a small set of physical quantities and laws.
Key takeaway: Physics looks for common rules behind very different natural phenomena.
Falling objects, moving cars, thrown balls.
Hot to cold flow, heating and cooling.
Rays of light reflecting and refracting.
Charges in circuits, and physics from atomic to cosmic scales.
Any physical quantity is written as number × unit. For example, 3.0 m, 75 kg, 2.0 s.
We choose units so that the numbers stay convenient. Very large or very small lengths are written using prefixes like cm, mm, μm, nm, fm.
- • Fundamental quantities in mechanics: length (L), mass (M), time (T)
- • SI base units: metre (m), kilogram (kg), second (s)
- • Example: speed = 10 m/s → number 10, unit m/s
Move the scale slider and watch how the same physical length can be written using different units. The object we associate with that scale also changes.
Key takeaway: Choice of unit depends on the size of the quantity we want to describe.
Length ≈ 10^-2 m
≈ 1.00 cm
Small objects like pens or coins
A measurement compares a quantity with a chosen standard (unit + instrument).
- • Accuracy – how close the measurement is to the true value.
- • Precision – how finely we can read the instrument (least count) and how repeatable readings are.
- • Absolute error = | measured − true|.
- • % error = (absolute error / true) × 100%.
Significant figures show how many digits in a measurement are considered reliable.
Try changing the least count and the reading. See how a coarse instrument can never reach the exact true value and leads to larger percentage error.
Key takeaway: Every measurement has an uncertainty; better instruments reduce (but never remove) error.
Measured = 12.00 cm
True = 12.00 cm
Absolute error = 0.00 cm
% error ≈ 0.0%
Each physical quantity can be written in terms of fundamental dimensions such as mass [M], length [L], and time [T].
In any correct equation, the dimensions of all terms on the left and right must be the same. This is called the principle of homogeneity.
- • [velocity] = [L T⁻¹]
- • [acceleration] = [L T⁻²]
- • [force] = [M L T⁻²]
Try the common formulas below. For a valid relation, dimensions of both sides match. A dimensionally correct formula may still be numerically wrong, but an incorrect one is certainly wrong.
Key takeaway: Dimensional analysis is a quick test for the possible correctness of an equation.
LHS: [L T⁻¹]
RHS: [L T⁻¹]
Dimensionally correct
All terms represent velocity, so the equation is dimensionally consistent.