A special type of equation which links two or more physical variables is called a formula.
An example of a formula is as follows:
P = 2(l + w)
Here, P represents the perimeter of a rectangle and l and w represent its length and width. This formula can be used to calculate the perimeter of any rectangle by substituting the values which are given in the formula.
Point to note:
When using formulas with real-life quantities such as mass, temperature, length etc, the variable will have units attached. These units are not included in the formula but should be provided with the answer.
Substituting into formulae
An example of substituting into formulae is as follows:
The formula of the surface area S of a cuboid is as follows:
S = 2lw + 2lh + 2hw
(l is the length, w is the width and h is the height).
What is the surface area given the following dimensions?
l = 150 cm
w = 32 cm
h = 25 cm (make sure all of the amounts are in the same units)
Substitute the values without the units
S = (2 x 150 x 32) + (2 x 150 x 25) + (2 x 25 x 32)
S = 9600 + 7500 + 1600
= 18,700 cm² (include the units in the answer)
This can also be written as 1.87m²