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Newton's laws of motion are three physical laws that describe the relationship between the motion of an object and the forces acting on it. These laws, which provide the basis for Newtonian mechanics, can be paraphrased as follows:
Learn about the three laws of motion that describe how objects behave in response to external forces. Explore the equations, examples, and applications of Newton's first, second, and third laws of motion.
- 5 min
- Overview
- Newton’s first law: the law of inertia
Newton’s laws of motion relate an object’s motion to the forces acting on it. In the first law, an object will not change its motion unless a force acts on it. In the second law, the force on an object is equal to its mass times its acceleration. In the third law, when two objects interact, they apply forces to each other of equal magnitude and opposite direction.
Why are Newton’s laws of motion important?
Newton’s laws of motion are important because they are the foundation of classical mechanics, one of the main branches of physics. Mechanics is the study of how objects move or do not move when forces act upon them.
Newton’s laws of motion, three statements describing the relations between the forces acting on a body and the motion of the body, first formulated by English physicist and mathematician Isaac Newton, which are the foundation of classical mechanics.
Newton’s first law states that if a body is at rest or moving at a constant speed in a straight line, it will remain at rest or keep moving in a straight line at constant speed unless it is acted upon by a force. In fact, in classical Newtonian mechanics, there is no important distinction between rest and uniform motion in a straight line; they may be regarded as the same state of motion seen by different observers, one moving at the same velocity as the particle and the other moving at constant velocity with respect to the particle. This postulate is known as the law of inertia.
The law of inertia was first formulated by Galileo Galilei for horizontal motion on Earth and was later generalized by René Descartes. Although the principle of inertia is the starting point and the fundamental assumption of classical mechanics, it is less than intuitively obvious to the untrained eye. In Aristotelian mechanics and in ordinary experience, objects that are not being pushed tend to come to rest. The law of inertia was deduced by Galileo from his experiments with balls rolling down inclined planes.
For Galileo, the principle of inertia was fundamental to his central scientific task: he had to explain how is it possible that if Earth is really spinning on its axis and orbiting the Sun, we do not sense that motion. The principle of inertia helps to provide the answer: since we are in motion together with Earth and our natural tendency is to retain that motion, Earth appears to us to be at rest. Thus, the principle of inertia, far from being a statement of the obvious, was once a central issue of scientific contention. By the time Newton had sorted out all the details, it was possible to accurately account for the small deviations from this picture caused by the fact that the motion of Earth’s surface is not uniform motion in a straight line (the effects of rotational motion are discussed below). In the Newtonian formulation, the common observation that bodies that are not pushed tend to come to rest is attributed to the fact that they have unbalanced forces acting on them, such as friction and air resistance.
Britannica Quiz
- The Editors of Encyclopaedia Britannica
- Anne Marie Helmenstine, Ph.D.
- Newton's First Law of Motion states that an object in motion tends to stay in motion unless an external force acts upon it.
- Newton's Second Law of Motion states that when a force acts on an object, it will cause the object to accelerate. The larger the mass of the object, the greater the force will need to be to cause it to accelerate.
- Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. What this means is that pushing on an object causes that object to push back against you, the exact same amount, but in the opposite direction.
Apr 6, 2022 · Learn about the three laws of mechanics that describe the relationship between force and motion. Find examples of inertia, force, and action-reaction in everyday life and physics.
Feb 20, 2022 · Isaac Newton discovered the laws of motion that describe these situations. Forces affect every moment of your life. Your body is held to Earth by force and held together by the forces of charged particles. When you open a door, walk down a street, lift your fork, or touch a baby’s face, you are applying forces.
Learn the three physical laws that describe the relationship between force and motion in classical mechanics. See examples, definitions, and explanations of inertia, non-inertial observers, and the third law.
