Used to describe the fact that when one variable increases, another increases similarly. Put another way, the quotient of the two values remains constant. An example of this phenomenon is that the acceleration of an object is directly proportional to the net external force in the direction of the acceleration.
A and B can be said to be directly proportional if
A = k * B,
where k is an arbitrary constant. Note that it is not enough for A and B to increase similarly. In addition the following must be true:
A = 0 <=> B = 0
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