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What is a force? A force is a push or a pull. Looking at the man above, his movement can be explained as a combination of forces. Gravitational force pulls him downwards, but the force of the water shooting down provides a force pushing up. Sir Isaac Newton (1642-1727) first studied forces as the cause for acceleration. The connections he understood are known as Newton's Laws of Motion, 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.