Newton's Second Law

Newton's First and Second laws, in Latin, from the original 1687 edition of the Principia Mathematica.

Isaac Newton's laws of motion were first published in his work Philosophiae Naturalis Principia Mathematica (1687). Newton used them to prove many results concerning the motion of physical objects. In the third volume (of the text), he showed how, combined with his law of universal gravitation, the laws of motion would explain Kepler's laws of planetary motion.

Newton's second law is a mathematical definition of force, first proposed by Newton himself (thus the name).

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Newton's first law: law of inertia

Lex I: Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.

• "An object at rest or traveling in uniform motion will remain at rest or traveling in uniform motion unless acted upon by a net force."

Note: a net force cannot come from inside. As well, velocity is a vector, therefore a constant velocity is defined as a constant speed in an unchanging direction (i.e. a linear path).

This law is also called the law of inertia or Galileo's principle.

An object may be acted upon by many forces and maintain a constant velocity so long as these forces are balanced for some object you have to apply unbalanced force(the maximum static friction force) for that instant. For example, a rock resting upon the Earth keeps a constant velocity (in this case, zero) because the downward force of its weight balances out the upward force (called the normal force) which the Earth exerts upwardly on the rock. Only unbalanced forces induce acceleration, or a change in the velocity or an object. If you push someone, he or she will accelerate in the direction of the unbalanced force which you have provided (called the applied force). Likewise if you roll a ball along the floor, the imbalanced force of friction will decelerate the ball from some positive velocity to rest.

Before Galileo, people agreed with Aristotle that a body's natural state was at rest, and that movement needed a cause. This is understandable, since in everyday experience, moving objects eventually stop because of friction (except for celestial objects, which were deemed perfect). Moving from Aristotle's "A body's natural state is at rest" to Galileo's discovery was one of the most profound and important discoveries in physics.

It is worth pointing out that, although the 'Law of Inertia' is commonly attributed to Galileo, René Descartes was the first to formulate such a law (although he did not perform any experiments to confirm it). Indeed, Galileo was not completely free of the shackles of the Aristotlean philosophy and he concluded that objects would continue in circular motion unless acted on by a net force.

There are no true examples of the law, as friction is usually present, and even in space, gravity acts upon an object, but it serves as a basic axiom for Newton's mathematical model from which one could derive the motions of bodies from elementary causes: forces. Another way to put it is, "An object in motion tends to stay in motion but decreasing in speed as friction usually present in universe, an object at rest tends to stay at rest until a resultant force acts upon it".

Newton's third law: law of reciprocal actions

Newton's third law. The skaters' forces on each other are equal in magnitude, and in opposite directions

Lex III: Actioni contrariam semper et aequalem esse reactionem: sive corporum duorum actiones in se mutuo semper esse aequales et in partes contrarias dirigi.

• All forces occur in pairs, and these two forces are equal in magnitude and opposite in direction.

Third law follows mathematically from conservation of momentum law.

As shown in the diagram opposite, the skaters' forces on each other are equal in magnitude, and opposite in direction. Although the forces are equal, the accelerations are not: the less massive skater will have a greater acceleration due to Newton's second law. If a basketball hits the ground, the basketball's force on the Earth is the same as Earth's force on the basketball. However, due to the ball's much smaller mass, Newton's second law predicts that its acceleration will be much greater; Not only do planets accelerate toward stars, but stars also accelerate toward planets.

The two forces in Newton's third law are of the same type, e.g., if the road exerts a forward frictional force on an accelerating car's tires, then it is also a frictional force that Newton's third law predicts for the tires pushing backward on the road.

Also see: Physics Study Guide

Importance and range of validity

Newton's laws were verified by experiment and observation for over 200 years, and they are excellent approximations at the scales and speeds of everyday life. In quantum mechanics there is no such concept as force (due to quantization of momentum, time differentiation of momentum is not always possible) - so second and third laws can't be used. At speeds comparable to the speed of light laws hold in original Newtonian notation (F=dp/dt).

Relationship to the conservation laws

The laws of conservation of momentum, energy, and angular momentum are of more general validity than Newton's laws, since they apply to both light and matter, and to both classical and non-classical physics.

Because force is time derivative of momentum, then the concept of force is redunant and subordinate to the conservation of momentum, and is not used in funamental theories (like quantum mechanics, QED, general relativity, etc. Standard model explains in details how fundamental forces originate out of exchange by virtual particles.

However, in macroscopic mechanics instead of use of momentum conservation in numerous interactions we average it over number of interactions and over time and call time derivative of momentum by term "force" for mathematical simplicity. So, force is a macroscopic (statistical) concept.

Newton stated the third law within a world-view that assumed instantaneous action at a distance between material particles. We now know that this is not really the way the universe really works, although it may be a good approximation under certain circumstances. For example, the electrons in the antenna of a radio transmitter do not necessarily act directly on the electrons in the receiver's antenna. According to an everyday timelike observer, momentum is handed off from the transmitter's electrons to the radio wave, and then to the receiver's electrons, and the whole process takes time. If the radio wave itself were to carry a stopwatch and a meterstick and find how long it takes for the momentum to be transferred and whether there is space between the two electrons, then from that perspective the transmitting electron acts directly and instantly on the receiving electron. Conservation of momentum is satisfied at all times and Newton's laws are applicable, for example, the second law does apply to the radio wave (see radiation pressure, radiation reaction force, etc.) It applicability is guaranteed by accounting for radiowave momentum (see momentum of electromagnetic wave).

Conservation of energy was discovered nearly two centuries after Newton's lifetime, the long delay occurring because of the difficulty in understanding the role of microscopic and invisible forms of energy such as heat and infra-red light.

Newton's second law—historical development

In an exact original 1792 translation (from Latin) Newton's Second Law of Motion reads:

"LAW II: The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed. -- If a force generates a motion, a double force will generate double the motion, a triple force triple the motion, whether that force be impressed altogether and at once, or gradually and successively. And this motion (being always directed the same way with the generating force), if the body moved before, is added to or subtracted from the former motion, according as they directly conspire with or are directly contrary to each other; or obliquely joined, when they are oblique, so as to produce a new motion compounded from the determination of both."

Newton here is basically saying that the change in the momentum of an object is proportional to the amount of force exerted upon the object. He also states that the change in direction of momentum is determined by the angle from which the force is applied. Interestingly, Newton is restating in his further explanation another prior idea of Galileo being what we call today the Galilean transformation or the addition of velocities.

An interesting fact when studying Newton's Laws of Motion from the Principia is that Newton himself does not explicitly write formulae for his laws which was common in scientific writings of that time period. In fact, it is today commonly added when stating Newton's second law that Newton has said, "and inversely proportional to the mass of the object." This however is not found in Newton's second law as directly translated above. In fact, the idea of mass is not introduced until the third law. However, it has been a common convention to describe law two of Newton in the mathematical formula F=ma where F is Force, a is acceleration and m is mass. This is actually a combination of laws two and three of Newton expressed in a very useful form. This formula in this form did not even begin to be used until the 18th century, after Newton's death, but it is implicit in his laws.

Newton's Third Law of Motion states: "LAW III: To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts. -- Whatever draws or presses another is as much drawn or pressed by that other. If you press a stone with your finger, the finger is also pressed by the stone. If a horse draws a stone tied to a rope, the horse (if I may so say) will be equally drawn back towards the stone: for the distended rope, by the same endeavour to relax or unbend itself, will draw the horse as much towards the stone, as it does the stone towards the horse, and will obstruct the progress of the one as much as it advances that of the other. If a body impinge upon another, and by its force change the motion of the other, that body also (because of the equality of the mutual pressure) will undergo an equal change, in its own motion, toward the contrary part. The changes made by these actions are equal, not in the velocities but in the motions of the bodies; that is to say, if the bodies are not hindered by any other impediments. For, because the motions are equally changed, the changes of the velocities made toward contrary parts are reciprocally proportional to the bodies. This law takes place also in attractions, as will be proved in the next scholium."

The explanation of mass is expressed here for the first time in the words "reciprocally proportional to the bodies" which have now been traditionally added to Law 2 as "inversely proportional to the mass of the object." This is because Newton in his definition 1 had already stated that when he said "body" he meant "mass". Thus we arrive at F=ma.

See also

• Scientific laws named after people
• Mercury, orbit of
• General relativity

References

• Marion, Jerry and Thornton, Stephen. Classical Dynamics of Particles and Systems. Harcourt College Publishers, 1995. ISBN 0030973023
• Fowles, G. R. and Cassiday, G. L. Analytical Mechanics (6ed). Saunders College Publishing, 1999. ISBN 0030223172

External links

• The Engineering Wiki's shorter page with the same title - engineers may improve it!
• Newtonian Physics - an on-line textbook
• Motion Mountain - an on-line textbook
• Trajectory Video - video clip showing exchange of momentum
• Newtonian attraction for three Planets (Mathcad Application Server)
• Gravity - Newton's Law for Kids