Principle of Measurement and History of Observation.
A ration bag says “10 kilograms of rice”. “10” alone would mean nothing, and neither would “kilogram” alone. Measurement lives in the pair. And with measurement in hand, science walked from Thales all the way to Einstein.
1. Why we measure, and the one rule
To know a quantity properly you have to measure it. Without measurement, full knowledge of a quantity is impossible. Knowledge comes in two kinds: qualitative (what sort) and quantitative (how much).
To measure a quantity, a definite and convenient part of it is taken as the standard. That standard is the unit. So the basic principle of measurement fits on one line: measure of a quantity = number × unit.
Swap the unit. The number jumps, yet the rice stays the same.
measurechange the unit
Number × unit
In “10 kilograms of rice”, mass is the quantity, 10 is the number and kilogram is the unit. Now break six measures apart: 10 kg, 5 s, 3 m, 2.5 A, 300 K, 6 mol.
sort themmeasure = number × unit
Split the measure
Pick up a part, then drop it in a box. One question: is this the count, or the unit the count is made in?
Right. Next one.
Not yet. What answers “how many” is the number; what answers “counted in what” is the unit.
All in place. 10 kg means 10 × kilogram, and every measure is cast in this one mould.
2. From Thales to Newton
Thales (622-569 BC) is famous for predicting a solar eclipse. Pythagoras (560-480 BC) worked in astronomy, mathematics and acoustics, and held that everything can be expressed by mathematical formulas. Archimedes (287-212 BC) discovered the principle of the lever and the law of buoyancy.
Galileo (1564-1642) found three laws of falling bodies in 1589 and founded kinematics. In 1610 he invented the microscope and, with the telescope, found moons of Jupiter. Einstein called him the father of modern science.
Newton (1642-1727) published the Principia in 1687. To him we owe the three laws of motion, the law of gravitation, calculus, the spectrum of light, the corpuscular theory of light and the reflecting telescope. The book Opticks, published in 1704, is also his.
3. From Young to Einstein
Light had a rival view too: Huygens proposed the wave theory of light in 1678, and Thomas Young (1773-1829) discovered interference of light in 1801. Michael Faraday (1791-1867) discovered electromagnetic induction in 1831 and gave the laws of electrolysis.
Rutherford discovered the atomic nucleus in 1911 through the alpha-scattering experiment. Max Planck (1858-1947) found the quantum theory in 1900: the energy of each quantum is proportional to the frequency of the radiation.
Einstein (1879-1955): special relativity in 1905, general relativity in 1915. He won the Nobel Prize in 1921 for the photoelectric effect. Brownian motion and E = mc² are also his.
Photoelectric effect: when light of suitable frequency falls on a metal, electrons are emitted from it.
Drag along the timeline. Each name comes with one of their experiments or discoveries drawn.
timedrag the handle
Thales to Einstein
On 11 February 2016 scientists detected gravitational waves directly, a hundred years after Einstein predicted them. Now walk along the timeline and touch all nine in turn.
who came first?look at the birth years
Place on the timeline
Thales. Famous for predicting a solar eclipse.
Pythagoras. Astronomy, mathematics and acoustics; everything can be put in mathematical formulas.
Archimedes. The principle of the lever and the law of buoyancy.
Galileo. Three laws of falling bodies in 1589, the foundation of kinematics.
Newton. The Principia in 1687, three laws of motion, the law of gravitation.
Young. Discovered interference of light in 1801.
Faraday. Electromagnetic induction in 1831.
Planck. The quantum theory in 1900.
Einstein. Special relativity 1905, general 1915, Nobel Prize 1921.
4. Who found what
The commonest question here pairs a discovery with a scientist. Drop ten discoveries into ten boxes.
match themwhich era is it from?
Match discovery to scientist
Pick up a discovery, then drop it in a scientist's box.
Right. Next one.
Not there. Ask: which era is the discovery from, and who lived then?
All ten in place. The wave theory of light is Huygens's, and interference was Young's discovery.
5. Check yourself
Three questions, with the timeline in your head.
check yourselfpick one answer
Question 1
Who is the father of quantum theory?
Thomas Young discovered the interference of light, not quantum theory.
Rutherford discovered the atomic nucleus.
Right. Max Planck found the quantum theory in 1900.
Einstein is known for relativity and the photoelectric effect; he is not the father of quantum theory.
In the second, match the year as well as the name.
check yourselfpick one answer
Question 2
Read these statements about Planck's quantum theory: i. energy is radiated not continuously but in packets ii. the energy of each quantum is proportional to the frequency of the radiation iii. energy flows continuously as a wave
i. Energy is radiated not continuously but in packets ii. The energy of each quantum is proportional to the frequency of the radiation iii. Energy flows continuously as a wave
Right. i and ii are both Planck's theory; iii is not true.
iii is the opposite of Planck's theory, so i and iii cannot both hold.
iii is false, so ii and iii cannot both hold.
iii is false, so all three do not hold.
The last one.
check yourselfpick one answer
Question 3
In '10 kilograms of rice', what are the quantity, the number and the unit respectively?
Rice is the substance, not the quantity; mass is the quantity measured.
The positions of number and unit are swapped.
Right. Measure = number × unit; here the quantity is mass, the number 10 and the unit kilogram.
Unit and quantity are swapped; the kilogram is the unit, not the quantity.
The whole note on eight cards.
cut from the notetap to turn over
Eight cards
QuestionWhat is the basic principle of measurement?
AnswerMeasure of a quantity = number × unit.
QuestionWhat did Galileo find in 1589 and which branch did he found?
AnswerThree laws of falling bodies; kinematics.
QuestionIn which year was Newton's Principia published?
Answer1687.
QuestionWho proposed the wave theory of light and who found interference, and when?
AnswerWave theory Huygens (1678); interference Thomas Young (1801).
QuestionWhat did Faraday discover in 1831?
AnswerElectromagnetic induction.
QuestionIn which year did Planck give the quantum theory?
Answer1900.
QuestionFor which work did Einstein win the Nobel Prize?
AnswerThe photoelectric effect (1921).
QuestionIn which year and by which experiment did Rutherford discover the nucleus?
AnswerIn 1911, by the alpha-scattering experiment.
Source: Physics 1st Paper, Classes 11-12 (NCTB), Chapter 1 “Physical World and Measurement”, section 1.10-1.11.