Formula Centre
Formula guides and key subject rules for every grade and subject, with worked examples and practice.
Concentration of a Solution
Physical Sciences • Chemistry: Mole and Stoichiometry • Grade 11
When to use it
Use it for solutions and titration calculations.
Worked example
0,5 mol in 250 cm³ = 0,25 dm³ → c = 0,5/0,25 = 2 mol·dm⁻³.
Common mistakes
- Leaving the volume in cm³
- Using mass instead of moles
Practice question
What is the concentration when 0,2 mol of NaOH is dissolved in 500 cm³ of solution?
Grade 10 Physical Sciences essentials
Physical Sciences • Essential reference • Grade 10
When to use it
Use when force, mass and acceleration are related.
Worked example
For 2 kg accelerating at 3 m/s², F = 2 × 3 = 6 N.
Common mistakes
- Using grams instead of kilograms.
Practice question
Find the force on 5 kg accelerating at 4 m/s².
Impulse-Momentum Theorem
Physical Sciences • Momentum and Impulse • Grade 12
When to use it
Use it for collisions and impacts where a force acts for a short time.
Worked example
0,5 kg ball stopped from 4 m·s⁻¹ in 0,2 s → F = (0,5 × −4)/0,2 = −10 N.
Common mistakes
- Ignoring the direction of Δv
- Using speed instead of change in velocity
Practice question
A 0,15 kg cricket ball is struck from 30 m·s⁻¹ to −40 m·s⁻¹ in 0,05 s. Find the average force.
Kinetic Energy
Physical Sciences • Work, Energy and Power • Grade 12
When to use it
Use it for the energy of a moving object and in the work-energy theorem.
Worked example
m = 2 kg, v = 3 m·s⁻¹ → E_k = ½(2)(9) = 9 J.
Common mistakes
- Squaring the mass instead of the speed
- Forgetting the ½
Practice question
Calculate the kinetic energy of a 60 kg athlete running at 8 m·s⁻¹.
Mole Calculation
Physical Sciences • Chemistry: Mole and Stoichiometry • Grade 11
When to use it
Use it to convert between mass and moles before applying a reaction ratio.
Worked example
36 g of water, M = 18 g·mol⁻¹ → n = 2 mol.
Common mistakes
- Using kg instead of g
- Reading molar mass off the wrong element
Practice question
Calculate the number of moles in 88 g of CO₂ (M = 44 g·mol⁻¹).
Newton's Second Law
Physical Sciences • Mechanics: Motion and Forces • Grade 12
When to use it
Use it whenever a resultant force acts on an object and you need its acceleration (or vice versa).
Worked example
F_net = 20 N on a 5 kg box → a = 20/5 = 4 m·s⁻² in the direction of the force.
Common mistakes
- Using weight (N) where mass (kg) is required
- Adding forces without regard to direction
Practice question
A 1 200 kg car accelerates at 2,5 m·s⁻². Calculate the resultant force.
Ohm's Law
Physical Sciences • Electricity and Electric Circuits • Grade 11
When to use it
Use it for a conductor at constant temperature to relate potential difference, current and resistance.
Worked example
V = 12 V, I = 2 A → R = 12/2 = 6 Ω.
Common mistakes
- Applying it across the wrong component
- Adding parallel resistances directly
Practice question
A resistor of 15 Ω carries a current of 0,4 A. Calculate the potential difference.
Wave Equation
Physical Sciences • Waves, Sound and Light • Grade 12
When to use it
Use it for any wave when two of speed, frequency and wavelength are known.
Worked example
f = 50 Hz, λ = 4 m → v = 200 m·s⁻¹.
Common mistakes
- Using wavelength in cm without converting
- Confusing frequency with period
Practice question
A sound wave travels at 340 m·s⁻¹ with a frequency of 425 Hz. Find its wavelength.