Formula Centre
Formula guides and key subject rules for every grade and subject, with worked examples and practice.
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.
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.
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.