1.1.1Physical quantities and kinematics
Physical Quantities
5 min read 9 parts Lesson 3 of 22 1 quiz
Lesson summary
1.1.1 Physical Quantities — AS PHYSICS 9702 notes: Distinguish base and derived quantities, write a quantity as a number times a unit, use SI prefixes and make sensible estimates.Physical quantity = a property that can be measured and is written as a numerical magnitude multiplie…
- Both the number and the unit are needed
- The unit gives the context
- Symbols are italic and units are upright
- A graph axis is labelled quantity / unit
- Base quantities
- The kilogram
Section 1 of 9What a physical quantity is
What a physical quantity is
Physical quantity
Physical quantity a property that can be measured and is written as a numerical magnitude multiplied by a unit.
Both the number and the unit are needed
- 1.8 on its own means nothing, whereas 1.8 m is a complete statement.
The unit gives the context
- The unit gives the context the letter v could mean velocity (m s-1), volume (m3) or, as V, potential difference (V).
Symbols are italic and units are upright
- Symbols are italic and units are upright m for mass, kg for its unit.
A graph axis is labelled quantity / unit
- A graph axis is labelled quantity / unit for example m / kg, because the quantity divided by its unit is a pure number.
Section 2 of 9SI base quantities and units
| Base quantity | SI base unit | Symbol |
|---|---|---|
| Mass | kilogram | kg |
| Length | metre | m |
| Time | second | s |
| Electric current | ampere | A |
| Thermodynamic temperature | kelvin | K |
| Amount of substance | mole | mol |
Section 3 of 9Base and derived quantities
Base and derived quantities
Base quantities
- Base quantities a small set of independent quantities from which all others are built.
The kilogram
- The kilogram is the only base unit that already carries a prefix.
Derived quantity
- Derived quantity defined through an equation linking it to base quantities, for example speed = length ÷ time with unit m s-1.
Force comes from F = ma
- Force comes from F = ma so 1 newton (N) = 1 kg m s-2.
Special names such as newton, joule and watt
- Special names such as newton, joule and watt are for convenience and can always be reduced to base units.
Section 4 of 9SI prefixes
| Prefix | Symbol | Factor |
|---|---|---|
| pico | p | 10-12 |
| nano | n | 10-9 |
| micro | μ | 10-6 |
| milli | m | 10-3 |
| centi | c | 10-2 |
| deci | d | 10-1 |
| kilo | k | 103 |
| mega | M | 106 |
| giga | G | 109 |
| tera | T | 1012 |
Section 5 of 9Using prefixes and converting units
Using prefixes and converting units
A prefix is raised to any power with its unit
- 1 cm3 = (10-2 m)3 = 10-6 m3, not 10-2 m3.
Standard form
- Standard form a × 10n with 1 ≤ a < 10, used for very large or very small numbers.
To convert units
- To convert units multiply by conversion factors equal to one so that each unit cancels.
Always give a final answer with a unit
- Always give a final answer with a unit and with significant figures that match the least precise data.
Keep intermediate values unrounded
- Keep intermediate values unrounded and round only the final answer.
Section 6 of 9Orders of magnitude and estimation
1Order of magnitude
Order of magnitude the power of ten closest to a number, so 3000 m is of order 103 m and 8000 m is closer to 104 m.
2Estimates are made from remembered values
- Estimates are made from remembered values for example mass of a person about 70 kg and speed of a walking person 1 to 1.5 m s-1.
3Estimating energy gained
- Estimating energy gained use Ep = mgh with g ≈ 10 N kg-1, estimated mass and estimated height.
4Mark schemes accept a range of values
- Mark schemes accept a range of values so estimates need only be sensible.
5Check data booklet values first
- Check data booklet values first many constants are already given.
Section 7 of 9Useful sizes to remember
| Quantity | Size |
|---|---|
| Diameter of an atom | 10-10 m |
| Wavelength of UV radiation | 10 nm |
| Wavelength of visible light | 400 to 700 nm |
| Height of an adult human | 2 m |
| Distance Earth to Sun (1 AU) | 1.5 × 1011 m |
| Mass of a hydrogen atom | 10-27 kg |
| Mass of an adult human | 70 kg |
| Mass of a car | 1000 kg |
| Seconds in a day | 90 000 s |
| Seconds in a year | 3 × 107 s |
| Speed of sound in air | 300 m s-1 |
| Power of a light bulb | 60 W |
| Atmospheric pressure | 1 × 105 Pa |
| Density of water | 1000 kg m-3 |
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Physical quantity | A measurable property written as a numerical magnitude times a unit. |
| Base quantity | One of the independent quantities from which other quantities are derived. |
| Derived quantity | A quantity defined through an equation involving base quantities. |
| SI unit | A unit of the international system of measurement. |
| Prefix | A symbol that multiplies a unit by a power of ten. |
| Standard form | A number written as a × 10n with 1 ≤ a < 10. |
| Order of magnitude | The power of ten closest to a number. |
| Estimate | An approximate value found from known sizes and simple calculation. |
Section 9 of 9Exam technique
1
Converting cm3 to m3 with a factor of 10-2 — the prefix factor must be cubed, giving 10-6.
2
Treating the gram as the base unit of mass — the base unit is the kilogram.
3
Leaving off the unit or writing a unit as the symbol — the symbol is the quantity, not its unit.
4
Guessing the unit of force — derive it from the defining equation F = ma.