The
total force upon an object is equal to the rate of change of the object's
momentum. This is Newton's original version of the second law of
mechanics. (Newton referred to momentum as "the quantity of motion.")
This rate of change is defined properly as the time derivative of the
momentum.
The rate of change can be approximated by a ratio of differences. The change in momentum (represented by Delta p) in the numerator of this difference expression is called the "impulse" It may be calculated as in integral of the total force, or as the product of a time interval with the average value of the total force during that time interval.