Laws of Motion

    An interaction that causes an acceleration of a body is called a force, which can be thought of as a push or pull. The connection between a force and the acceleration it causes was first understood by Isaac Newton. He discovered three primary laws of motion that govern forces and accelerations. Newtonian mechanics uses these laws to study the world.

    Before Newton, people thought a force was needed to keep objects moving at constant velocity. Similarly, objects were supposed to be in their "natural state" when at rest. It was believed objects would naturally stop moving if no force acted on them. After studying Galileo, Newton challenged this belief, showed it was false, then replaced it with his first law of motion. Newton’s first law, called the law of inertia, states: "A body at rest will remain at rest, and a body moving at constant velocity will continue to do so, unless a force acts upon either." So a force is not needed to keep something going at constant velocity. The reason moving objects on earth don’t remain at constant velocity is because forces (like friction or drag) act on them, not because they naturally slow down.

    The exact relationship between forces and acceleration in objects is written in Newton’s second law of motion. "Force equals mass times acceleration" or "F = ma". The mass of an object is the resistance of that object to changes in motion. This law sounds almost too simplistic, but it is a powerful statement. This law governs all of the motion in the human world. Much of physics, specifically mechanics, is based on this principle.

    The third of Newton’s laws of motion might be the most commonly known. The phrase "To every action there is always an equal and opposite reaction" is part of our cultural lingo. But what does this phrase mean? It means forces come in pairs. If a hammer exerts a force on a nail, the nail exerts a force of equal magnitude, but opposite direction of the hammer. If you push on a wall, it pushes back on you. This law can be written F a b = -F b a .

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