The Formula Triangle Method for IGCSE Physics

This page teaches you how to use the formula triangle method to rearrange physics equations quickly and accurately.

What is a formula triangle?

A formula triangle is a visual tool for rearranging equations that have three variables. The variable that is alone on one side of the equation goes on top. The two variables that are multiplied together go on the bottom.

To find any variable: cover it with your finger. What remains tells you the calculation.

How to build a triangle

Take the equation: speed = distance / time (v = s / t)

1. The variable on its own (numerator) goes on top: s (distance).
2. The two variables on the bottom: v (speed) and t (time), side by side.

    s
  -----
  v | t

• Cover s: you see v x t, so s = v x t
• Cover v: you see s over t, so v = s / t
• Cover t: you see s over v, so t = s / v

Major IGCSE Physics formula triangles

Motion: s = v x t

    s
  -----
  v | t

Force: F = m x a

    F
  -----
  m | a

Density: rho = m / V

    m
  -------
  rho | V

Pressure: P = F / A

    F
  -----
  P | A

Ohm’s Law: V = I x R

    V
  -----
  I | R

Electrical Power: P = I x V

    P
  -----
  I | V

Wave speed: v = f x lambda

    v
  ---------
  f | lambda

Weight: W = m x g

    W
  -----
  m | g

Work done: W = F x d

    W
  -----
  F | d

Charge: Q = I x t

    Q
  -----
  I | t

Worked example 1: Finding time

Question: A car travels 150 m at a speed of 25 m/s. Calculate the time taken.

Using the triangle for s = v x t:

• Cover t: t = s / v
• t = 150 / 25
• t = 6 s

Worked example 2: Finding current

Question: A resistor has a resistance of 50 ohm and a potential difference of 12 V across it. Calculate the current.

Using the triangle for V = I x R:

• Cover I: I = V / R
• I = 12 / 50
• I = 0.24 A

Worked example 3: Finding mass from density

Question: A block of aluminium has a density of 2700 kg/m^3 and a volume of 0.05 m^3. Calculate the mass.

Using the triangle for m = rho x V:

• Cover m: m = rho x V
• m = 2700 x 0.05
• m = 135 kg

When triangles do not work

Formula triangles do not work for equations with more than three variables or equations that involve squaring, such as:

• Kinetic energy: KE = 0.5 x m x v^2
• Gravitational potential energy: GPE = m x g x h (four quantities, though g is often constant)
• Power: P = I^2 x R

For these, use standard algebraic rearrangement. Write the formula, substitute, then solve.

Exam advice

Always write the rearranged formula as part of your working. The examiner awards a mark for the correct formula, not for the triangle. The triangle is your private tool; the written formula is what earns the mark.