What is a force? A force is a push or a pull. The movement experienced by the man above can be explained as a combination of forces. Though gravitational force pulls him downward, the water shooting out of his jet pack provides a force pushing him up. Sir Isaac Newton (1642–1727) was the first to study forces as the cause for acceleration. The connections he understood are known as Newton’s Laws of Motions, and are explained in this chapter.
To understand physics, it is necessary to understand how objects move. Acted on by forces, objects are subject to Newton’s Three Laws of Motion. The most widespread force is gravity, which has the same effect on all objects all over the world; it is the force of gravity that gives an object of some mass its weight. Finally, a significant force often overlooked in elementary physics courses is friction, but understanding friction equations and friction’s influence on an object’s motion is necessary to properly calculate its trajectories.