What quantity, represented by 'a', appears in Newton's second law?

In Newton's second law, the quantity represented by “a” is acceleration. The familiar equation F = ma states that an object's net force equals its mass multiplied by its acceleration.

Acceleration describes how quickly velocity changes. Because velocity includes both speed and direction, an object accelerates when it speeds up, slows down, or changes direction—even if its speed remains constant during circular motion. The SI unit of acceleration is metres per second squared (m/s²).

The equation also shows how the quantities are connected. For a given mass, a larger net force produces a larger acceleration. For a given force, a more massive object accelerates less. Rearranging the equation gives a = F/m, which makes the role of acceleration especially clear.

Mass, force, and velocity are all important ideas in mechanics, but they do not replace acceleration in this equation. Newton's second law concerns net force and the resulting change in motion. In its more general form, the law relates force to the rate of change of momentum; F = ma applies directly when mass is constant.