Force is a fundamental concept in science, because force is what makes objects change their motion. This notion is summarized in Newton's second law, which states that "the net force on an object equals its change in momentum over time." The mass of an object is usually constant (but not always, such as the case of a rocket launched toward space, whose mass is decreasing as its fuel is burned), Newton's law can be stated in its more familiar form: F=ma. The net force on an object equals it mass multiplied by its acceleration.

Note that both force and acceleration are vectors, typically designated in bold. They have both direction and magnitude.

Many forces can act on an object— gravity, electromagnetic attraction between electrically-charged bodies, outside forces that push or pull, etc. Knowing them, we can determine the acceleration of the object. Using the calculus invented...