AS Physics - Unit 1 - Section 1.1 - Basic Physics

Sep 5, 2026

AS Physics Unit 1 Section 1.1 introduces core foundations of basic physics, focusing on SI units and how to relate derived units to base SI units. It explains base quantities, demonstrates conversions using worked examples, and shows how to check equations for homogeneity by comparing base units on both sides. The section also covers scalars vs vectors, displacement and its directional nature, and speed vs velocity, including vector addition concepts for both inline and right-angle cases.

LanguageEnglish

CategoryExam

FormatPDF

Pages72

Reading Time3 h 1 min

AccessFree to download

Basic Physics

System Internationale (SI) units

Base quantities and derived units

Homogeneous equations

Scalars and vectors

Displacement

Speed and velocity

Adding vectors

AS Physics - Unit 1 Section 1.1- Basic Physics Systeme Internationale or ‘SI’ units All quantities in physics have an unit which has been agreed on internationally. For example: charge - coulomb, C force - newton, N energy - joule, J In addition, there are about 6 fundamental quantities that can be used to define or derive anyother quantity. These are known as base quantities, and their units are therefore base SI units. Here are 6 base quantities: mass (kg) length (m) time (s) temperature (K) current (A) amount (mol) Since all derived quantities, like ‘energy’ or ‘force’ are based upon base quantities, it must be possible to express their units in terms of base quantity units ( kg , m , s , K , A or mol) . To change SI units into base SI units, you simply need to think of an equation containing base quantities, e.g. changing the ‘newton, N’ into base SI units: In this section the square brackets around a quantity are used to denote Simple equation for ‘force’ 􀃎 Σ F = m x a [h]edunitenotseosf“”tthheaquanitsiftym, eas.sg”.. Hence ‘the units of force’, [F] = kg x ms-2 ∴ If the SI unit for force is the newton, the equivalent unit in terms of base SI units is kgms-2 Example Electrical current, I is defined by the equation, I = Q where Q = charge, t = time. t The SI unit for charge is the coulomb, C. Express this unit in terms of base SI units. Re-arranging the equation to make ‘Q’ the subject: Q = I x t ∴ [Q] = [I] x [t] = A x s ∴ Answer = As (Amps seconds) | Quantity | SI units | BASE SI units | | --- | --- | --- | | Spring constant, k = F / x | Nm-1 | kg s-2 | | Pressure ( = Force/Area) | Pa or N/m2 | kg m-1 s-2 | | Energy or Work done
( Work = Force x distance) | J | kg m2 s-2 | | Voltage (V=Energy/ charge) | V | kg m2 A-1 s-3 | You are NOT expected to remember these BASE SI units, but you may be expected to derive them as shown in the example box above. Why not try to use the equations given in the first column to see if you can derive the BASE SI units shown in the last column? [www.bangor.ac.uk/ASrevision](www.bangor.ac.uk/ASrevision) 1 This information can be used to check equations for homogeneity. An equation is said to be homogenous if both sides have identical base SI units. ( Note : This does not check whether any constants in an equation have the correct values) . Example Show that the following equation is homogenous : E = YA e2 2 L where , E = energy stored in a stretched wire (unit = joule = kg m2 s-2 ) Y = modulus of elasticity (Pascal = unit of pressure, P = F/A) A = Area e = extension L = original length of wire Answer First, we must find the SI units for each expression in the equation : LHS [ E ] = kg m2s-2 (given) RHS [ Y ] = [ Pressure ] = [ F / A ] = [ m a / A ] = kg ms-2 / m2 = kg m-1s-2 [ A ] = m2 [ e ] = m ∴ [ e2 ] = m2 [ L ] = m [ 2 ] = No units ∴ | YA e2 | | --- | | 2 L | = kg s-2 . m2 . m2 m . m = kg m2 s-2 Since the units are identical on both sides of the equation, this equation is homogenous. [www.bangor.ac.uk/ASrevision](www.bangor.ac.uk/ASrevision) 2 Scalars & Vectors Scalar = A scalar is a quantity that has magnitude only. Vector = A vector quantity has magnitude and direction. Examples of scalars are: mass , distance , volume , density , time , speed , pressure Examples of vectors are: displacement , velocity , acceleration , momentum , force , current Displacement Displacement is the vector equivalent of distance. However, displacement is measured in a different way to distance – displacement is akin to the ‘distance as the crow flies’. See the example below: A cyclist travels from Bangor (North Wales) to Betws-y-Coed. A device attached to the wheel measures the distance travelled asthe wheel turns. The reading by the end of the journey was 80km. However, the displacement is shown by the dotted (red) line. The displacement was 27km at a bearing of S 50˚ E. Note : Since disp

FAQs

What are the base quantities in SI and why are they important?

The six base quantities are mass (kg), length (m), time (s), temperature (K), current (A), and amount (mol). They define or derive all other quantities, so derived units must be expressible in terms of these base SI units.

How can you express a derived SI unit in terms of base SI units?

Use an equation that links the derived quantity to base quantities, rearrange it so the target symbol is alone, and then substitute the base units. The example for charge uses Q = I × t to obtain [Q] = A × s.

What is the difference between speed and velocity, and how do vectors affect this?

Speed is based on distance per unit time, while velocity is defined as displacement per unit time. Displacement is a vector, so velocity requires both magnitude and direction.

Sep 5, 2026

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