Two quantities a and b are said to be directly proportional, if the quotient is constant. This is written as .
Constant of Proportionality
The constant is called constant of proportionality. Therefore a proportionality can be written as:
- Here, the constant is
- Here, the constant is c², the square of the speed of light.
The role of the quantities and of the constant are not interchangeable. It would be wrong to say in the examples above that or .