- Explain what physics studies and name its main branches
- Name the SI units of length, mass, time and temperature
- Convert units using the prefixes kilo-, centi- and milli-
- Find the scale division of a measuring instrument
Why doesn't a steel ship sink, why is the sky blue, and how does a phone battery charge? Physics answers all of these questions. It looks for the causes of natural phenomena and explains them through observation, experiments, measurements and formulas.
What does physics study?
The word physics comes from the Greek physis, which means “nature”. Physics studies the most general laws of matter, energy, motion and forces. These laws are the same everywhere: an apple falling from a tree in Baku and the Moon orbiting the Earth obey the same law.
- Mechanics — motion of objects and forces
- Thermal physics — temperature, heat, states of matter
- Electricity and magnetism — electric charges, current, magnets
- Optics — light and how it travels
- Atomic and nuclear physics — the structure of the atom and radioactivity
A property of an object or phenomenon that can be measured: length, mass, time, temperature and so on. Every physical quantity has a numerical value and a unit, for example 5 m or 20 °C.
To measure a quantity means to compare it with a quantity of the same kind that has been chosen as the unit. If a desk is 1.2 m long, a one-metre stick fits along it 1.2 times.
The International System of Units (SI)
In the past every country had its own measures: cubits, feet, pounds, poods. So that scientists and engineers all over the world could understand each other, the International System of Units (SI) was adopted in 1960. It has seven base units, and all other units are built from them.
| Quantity | Unit | Symbol |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Temperature | kelvin | K |
| Electric current | ampere | A |
| Amount of substance | mole | mol |
| Luminous intensity | candela | cd |
Unit prefixes
To write very large or very small values conveniently, prefixes are added to unit names. For example, it is easier to give the distance from Baku to Ganja in kilometres and the length of a pencil in centimetres.
| Prefix | Symbol | Factor | Example |
|---|---|---|---|
| mega | M | 1 000 000 = 10⁶ | 1 MW = 1 000 000 W |
| kilo | k | 1000 = 10³ | 1 km = 1000 m |
| centi | c | 0.01 = 10⁻² | 1 cm = 0.01 m |
| milli | m | 0.001 = 10⁻³ | 1 mm = 0.001 m |
Aysel walks 1.2 km to school, she is 152 cm tall, and her school bag has a mass of 3500 g. Express these quantities in SI units.
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1 cm = 0.01 m, so 152 cm = 152 · 0.01 m = 1.52 m.
1 g = 0.001 kg, so 3500 g = 3500 · 0.001 kg = 3.5 kg.
Measuring instruments and scale division
We measure length with a ruler or tape measure, mass with scales, time with a stopwatch and temperature with a thermometer. To read an instrument correctly, first find its scale division — the value of one smallest division on its scale.
- athe smaller of two neighbouring numbered marks
- bthe larger of these two marks
- nthe number of divisions between them
On a thermometer there are 5 divisions between the 20 °C and 30 °C marks. The top of the liquid column is 3 divisions above 20 °C. What are the scale division and the temperature?
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Temperature = 20 °C + 3 · 2 °C = 26 °C.
Key points
- Physics studies the most general laws of nature: motion, forces, energy, heat, electricity, light and atoms.
- A physical quantity has a numerical value and a unit: 5 m, 3 kg, 20 °C.
- In SI, length is measured in metres, mass in kilograms, time in seconds and temperature in kelvins.
- Kilo = 1000, centi = 0.01, milli = 0.001.
- Scale division = difference between two neighbouring numbered marks / number of divisions between them.
Check yourself
10 questions. Every correct answer earns XP.