Newton's Second Law of Motion
Thus, according to Aristotle there is only a velocity if there is a force, but according to Newton an object with a certain velocity maintains that velocity unless a force acts on it to cause an acceleration (that is, a change in the velocity).
As we have noted earlier in conjunction with the discussion of Galileo, Aristotle's view seems to be more in accord with common sense, but that is because of a failure to appreciate the role played by frictional forces. Once account is taken of all forces acting in a given situation it is the dynamics of Galileo and Newton, not of Aristotle, that are found to be in accord with the observations.
Since acceleration is the rate at which speed changes, it is usually expressed in units of m/s/s (every second, the object which is accelerating will go that much faster). Force is usually expressed in Newtons (N), wich are kgm/s/s (you can see why they changed the name to newtons!)
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