NBC Learn's "The Science of NFL Football" episode about Newton's first law of motion explains that, like an offensive line set on the line of scrimmage before the ball is hiked, a body at rest will remain at rest unless acted on by an outside force. Likewise, a body—whether it's a wide receiver streaking downfield to make a catch or a running back charging into the end zone—moving at constant speed and direction will remain in motion unless acted on by an unbalanced force, in most cases a defensive player moving in the opposite direction.
The first law of motion is not a law so much as a scientific generalization based on the study of physical forces (such as gravity) pulling or pushing on an object and its motion due to those forces. It is one of three laws of motion put forth by English scientist and philosopher Sir Isaac Newton more than 300 years ago.
A big part of Newton's first law is inertia, which would be the natural resistance a player at rest experiences as he sets himself in motion, and for a player in motion, to stop moving. A running back is at rest while he waits for the center to snap the ball to the quarterback. Once the play begins, the back pumps his legs to overcome static inertia, takes the handoff, and then tries to maintain moving inertia until a defenseman tackles him, restoring static inertia.
The bigger and heavier a player is, the more mass he has and the greater the force that's required to break his inertia. This is why a 200-pound running back will look to make quick cuts to the left or right to evade a 250-pound tackler—the running back has less mass and can more easily change direction than his more massive opponent. On the other side of the ball, that more massive tackler looks to use his greater mass to generate enough force to stop his opponent's forward inertia.
All of that mass in motion creates momentum, or what Newton called the "quantity of motion," says Tim Gay, a physics professor at the University of Nebraska in Lincoln and author of "The Physics of Football." To illustrate momentum, Gay points to a 215-pound defensive player running at about 33 feet per second toward a running back who has just received a handoff. Under those circumstances the defender would have three times the momentum of the relatively stationary running back and hit him with roughly two thirds of a ton of force.
Defenders running full speed, however, face a risk: the ball carrier could sidestep an overly aggressive player. In that case, as Newton's first law dictates, the defender would have a hard time changing his inertia—that is, stopping and shifting direction. And rather than trying to zero out a ball carrier's momentum with at least an equivalent momentum, for most players, it's more practical to knock the running back off balance so that gravity can do some of the work.