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Elementary Laws of Motion

Sir Isaac Newton, an English scientist who taught at Cambridge University, made monumental contributions to mathematics and physics. One of his most important contributions was formulating the elementary laws of motion. Newton's "Philosophiae Naturalis Principia Mathematica," published in 1687, describes these laws, now referred to as Newton's three laws of motion.

Force and Inertia
- Newton's elementary laws of motion center on force and inertia. Without force, neither motion nor changes in motion can occur. Forces result when objects affect one another, whether from physical contact (e.g., friction and tension) or from a distance (e.g., gravity and magnetism). If the forces acting on an object are balanced, or equal to each other, the object's state of motion remains unchanged. For a change in motion to occur, an unbalanced force must act on the object. In addition, objects resist changes to their state of motion, a tendency known as inertia. The greater an object's mass, the greater its inertia.
First Law of Motion
- Newton's first law of motion describes the effects of inertia. Because of inertia, a stationary object will not move unless a force causes it to move. Likewise, an object travelling in a straight line at a constant speed will continue moving in the same direction and at the same speed unless a force, such as friction, acts upon it.
Second Law of Motion
- According to Newton's second law of motion, an object accelerates in the same direction as the force applied to it, and its acceleration is directly proportional to the strength of that force. The equation that expresses this law is F = ma (force equals mass times acceleration). In addition, the acceleration is inversely related to the object's mass, which is expressed mathematically as a = F/m (acceleration equals force divided by mass). That is, a force's ability to accelerate an object depends on the object's mass.
Third Law of Motion
- Newton's third law of motion holds that forces happen in pairs. In other words, for every action there is an equal and opposite reaction. When one object acts on another, two different forces occur: an action force and a reaction force. These two forces are equal but act in opposite directions. This law is seen when artillery or firearms fire a projectile. The action force propels the projectile forward, while the reaction force causes the firearm or artillery to recoil.

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