What is the second law of motion?

Newton's second law of motion can be stated as follows: The acceleration of an object produced by a force is directly proportional to the magnitude of the force, in the same direction as the force, and inversely proportional to the mass of the object.

F ∝ a

F ∝ m

Combining these two we get F ∝ ma or F = kma, where k is the constant of proportionality.

If k =1 then F = ma

The second law of motion allows us to define and measure force.