AS Physics - SI Units Study Notes - New Syllabus-2025-2027
AS Physics – SI Units Study Notes
AS Physics – SI Units Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on AS Physics Study Notes syllabus Candidates should be able to:
- recall the following SI base quantities and their units: mass (kg), length (m), time (s), current (A), temperature (K)
- express derived units as products or quotients of the SI base units and use the derived units for quantities listed in this syllabus as appropriate
- use SI base units to check the homogeneity of physical equations
- recall and use the following prefixes and their symbols to indicate decimal submultiples or multiples of both base and derived units: pico (p), nano (n), micro (μ), milli (m), centi (c), deci (d), kilo (k), mega (M), giga (G), tera (T)
1.2.1-Recall the following SI base quantities and their units: mass (kg), length (m), time (s), current (A), temperature (K)
- All units in physics can be described in terms of seven basic units known as SI units.
- The SI units is also known as the International System of Units.
- The SI units are the base quantities from which every other physical quantity can be described.

1.2.2- Derived Units
- The quantites that can be described in terms of the base quantities are known as derived quaantites.
- Example: Volume = Length * Length * Length = m3

1.2.3- Use SI base units to check the homogeneity of physical equations
- An equation is said to be homogeneous if the units or dimensions are same on both sides of the equation.
- This can be checked by matching the SI units on both sides.
- Example, s = ut+\(\frac{1}{2}at^{2}\).
- In LHS, unit of s = m.
- In RHS, unit of ut = m\(s^{-1}\)s = m and unit of \(at^{2}\) = \(ms^{-2}s^{2}\) = m.
- Since, units are same on both side, this equation is homogeneous.
- NOTE: constants and numbers are dimensionless.
- NON-HOMOGENEOUS EQUATION: when SI units of terms on both sides are different.
- Example, P = ρg\(h^{2}\).
- In LHS, unit of P = kgm\(s^{-2}\)\(m^{-2}\) = kg\(m^{-1}\)\(s^{-2}\).
- In RHS, unit of ρgh = [kg\(m^{-3}\)][m\(s^{-2}\)][\(m^{2}\)].
- Units on LHS ≠ RHS. Therefore, it is non-homogeneous.
1.2.4- recall and use the following prefixes and their symbols to indicate decimal submultiples or multiples of both base and derived units: pico (p), nano (n), micro (μ), milli (m), centi (c), deci (d), kilo (k), mega (M), giga (G), tera (T)
- The magnitudes of physical quantities are often expressed in orders of 10.
- Prefixes can be used to describe this order depending on the order of magnitude.
- Example, 103 is written as kilo, thus 3000g = 3 kg.
AS Physics Physical quantities Worked Out Example
- Work out the derived quantities for:
1. Equation: Pressure =\(\frac{Force}{Area}\).
In terms of base units: Units of pressure = kgm\(s^{-2}\).
2. Equation: Work = Force × Displacement.
In terms of base units: Units of Work = [kgm\(s^{-2}\)][m] = kg\(m^{2}\)\(s^{-2}\).
3. Equation: Power = Work done × Time.
In terms of units: Units of Power = [kg\(m^{2}\)\(s^{-2}\)][s] = [kg\(m^{2}\)\(s^{-1}\)].
Magnitude
- Prefix : magnitudes of physical quantity range from very large to very small.
- E.g. mass of sun is $10^{30} \mathrm{~kg}$ and mass of electron is $10^{-31} \mathbf{k g}$.
- Hence, prefix is used to describe these magnitudes.
Significant number
- Magnitudes of physical quantities are often quoted in terms of significant number.
- Can you tell how many sig. fig. in these numbers?
- 103, 100.0, 0.030, 0.4004, 200
- If you multiply 2.3 and 1.45, how many sf should you quote?
- 3.19, 3.335, 3.48
- 3.312, 3.335, $3 \cdot 358$
SI Units – Fill in…
For you to know…
Key Concept
1. A physical quantity is a quantity that can be measured and consists of a numerical magnitude and a unit.
2. The physical quantities can be classified into base quantities and derived quantities.
3. There are seven base quantities: length, mass, time, current, temperature, amount of substance and luminous intensity.
4. The SI units for length, mass, time, temperature and amount of substance, electric current are metre, kilogram, second, kelvin, mole and ampere respectively.
Homogeneity of an equation
- An equation is homogeneous if quantities on BOTH sides of the equation has the same unit.
- E.g. $s=u t+1 / 2 a^2$
- LHS : unit of $s=m$
- RHS : unit of ut $=\mathbf{m s}^{-1} \mathrm{xS}_{\mathrm{x}}=\mathbf{m}$
- Unit on LHS = unit on RHS
- Hence equation is homogeneous
Non-homogeneous
- $\mathrm{P}=\rho g h^2$
- LHS ; unit of $\mathrm{P}=\mathrm{Nm}^{-2}=\mathrm{kgm}^{-1} \mathrm{~s}^{-2}$
- RHS : unit of $\rho \mathrm{gh}^2=\mathrm{kgm}^{-3}\left(\mathrm{~ms}^{-2}\right)\left(\mathrm{m}^2\right)=\mathrm{kgs}^{-2}$
- Unit on LHS $\neq$ unit on RHS
- Hence equation is not homogeneous
Homogeneity of an equation
- Note: numbers has no unit
- some constants have no unit.
- e.g. $\pi$
- A homogeneous eqn may not be physically correct but a physically correct eqn is definitely homogeneous
- E.g. $s=2 u t+a t^2$ (homogenous but not correct)
- $\quad \mathbf{F}=\mathbf{m a} \quad$ (homogeneous and correct)
Magnitude
- Prefix : magnitudes of physical quantity range from very large to very small.
- E.g. mass of sun is $10^{30} \mathrm{~kg}$ and mass of electron is $10^{-31} \mathbf{k g}$.
- Hence, prefix is used to describe these magnitudes.
Significant number
- Magnitudes of physical quantities are often quoted in terms of significant number.
- Can you tell how many sig. fig. in these numbers?
- 103, 100.0, 0.030, 0.4004, 200
- If you multiply 2.3 and 1.45, how many sf should you quote?
- 3.19, 3.335, 3.48
- 3.312, 3.335, $3 \cdot 358$