What is Newton's third law: Definition and 250 Discussions

In classical mechanics, Newton's laws of motion are three laws that describe the relationship between the motion of an object and the forces acting on it. The first law states that an object either remains at rest or continues to move at a constant velocity, unless it is acted upon by an external force. The second law states that the rate of change of momentum of an object is directly proportional to the force applied, or, for an object with constant mass, that the net force on an object is equal to the mass of that object multiplied by the acceleration. The third law states that when one object exerts a force on a second object, that second object exerts a force that is equal in magnitude and opposite in direction on the first object.
The three laws of motion were first compiled by Isaac Newton in his Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), first published in 1687. Newton used them to explain and investigate the motion of many physical objects and systems, which laid the foundation for Newtonian mechanics.

View More On Wikipedia.org
Recent contents Top users
  1. F

    Sketches and Forces in an Excavator System

    Homework Statement a sack is in contact with both the base and the vertical back of an excavator scoop. the excavator is moving forward at constant speed in a straight line. make separate sketches showing the forces acting on the sack and the scoop. Homework Equations The Attempt...
  2. B

    Understanding Newton's Third Law

    Homework Statement I'm having trouble thinking about Newton's third law pairs and have been trying to find some help searching the Physics Forums archives. Homework Equations I have found alternative explanations for 3rd law pairs involving thrust in an escaped balloon or in a rocket -...
  3. B

    Environmental Applications of Newton's Third Law

    Ah hi there, I would just like to know the environmental applications of Newton's third law? Also fnet= 0; how does an object have constant velocity, is it 0? that's why its constant since its acceleration is 0?? please, I am new.. to physics.. and this forumz.
  4. K

    Why Does a Pool Ball Stop Dead When Hit at a Straight Angle?

    Hi. I'm having trouble understanding Newton's third law. It states that: To every action there is an equal and opposite reaction. Then howcome if you hit a pool ball with the cue ball, at a straight angle, it'll stop dead. Isn't it supposed to go backwards the same distance as the second ball...
  5. R

    Trouble Understanding Newton's Third Law

    I apologize in advance if this belongs in the Homework section, but my question is not all about one problem and doesn't fit the template. I have some trouble understanding Newton's Third Law. I tried understanding it myself (my teacher is an imbecile who accomplishes almost nothing in every...
  6. R

    Calculating Horizontal Forces in a Tug-of-War Using Newton's Third Law

    Homework Statement In a tug-of-war between two physics types, each pulls on the rope with force of 250N. If both remain motionless, what horizontal force does each exert against the ground? The Attempt at a Solution I can find the tension of the rope, but I am not to sure how to find out...
  7. B

    Newton's Third Law: Solving the Man & Sled Puzzle

    Homework Statement If a man pulls forward on a sled, and the sled pulls backward on the man with an equal amount of force, how can the man and sled move at all? Homework Equations The Attempt at a Solution
  8. H

    Does the Ball Do Work on the Kid? Newton's Third Law Explained

    Homework Statement Newton's 3rd law says that a ball exerts as much force on a kid as the kid exerts force on the ball. Is it correct to say the ball does work on the kid? Homework Equations I don't think the ball does work because the kid is not moved by the ball. The Attempt at a...
  9. T

    Newton's Third Law and pushing against a wall

    My teacher just explained to us Newton's Third Law, and one example she used was her pushing against a wall. She stated that the net force would equal zero because the wall was pushing back, but that doesn't make sense to me... What I was thinking was that the force of her pushing was weaker...
  10. F

    Newton's Third Law, Speed of Light, and Relativity question

    Highschool physics student here with a thought experiment I would like some input on. While learning about Newton's Third Law (are reactions are immediate and there is no delay) I had a thought about the speed of light and relativity. Imagine You have a stick with a length of one light-year...
  11. G

    Exploring the Physics of Newton's Third Law: Acceleration of a Box

    Homework Statement Newtons third law tells that if you push a box with 15N force, it pushes back on you with 15N force. How can you ever accelerate this box if it always pushes back with the same force you exert on it? explain briefly
  12. I

    What orbital pattern occurs when two bodies of equal mass orbit each other?

    Let's say there are two bodies with equivalent mass. The first one orbits the second, and the second one orbits the first. What kind of orbital pattern would result? Assume that the only force considered here is gravity. I know that the centripetal and centrifugal forces are the result of action...
  13. D

    Newton's Third Law and gravity

    This is not really a homework question, just a question I came up with. If the Earth is pulling, for example, a book with a force of 5 N, that means, due to Newton's 3rd law, that the book is also pulling the Earth with a force of 5 N. Does this mean the total force between them is actually...
  14. D

    Newton's Third Law Problem - Tightrope

    Homework Statement A 66.0 kg tightrope walker stands at the center of a rope. The rope supports are 10 m apart and the rope sags 9.00* at each end. The tightrope walker crouches down, then leaps straight up with an acceleration of 8.40 m/s2 to catch a passing trapeze. What is the tension...
  15. J

    How can air displacement balance out forces in Newton's third law?

    Newtons third law states: "If object A acts on object B with force X, then object B will apply force X on object A" in other words "for every action there is an equal and opposite reaction". Here are my few questions 1) If I apply pull a truck with force X and the truck pulls me with force...
  16. S

    Confused about Newton's third law again?(Doc Al and others help)

    Hello everyone, Ok I'm no longer worried about why Newton's third law is occurring. Doc Al advised me in the last thread to look at some action reaction pairs instead, and I think that is what I needed to do. Ok I'm having difficulty understanding Newton's third law when two objects are...
  17. FeDeX_LaTeX

    Understanding Newton's Third Law: The Impact of Forces on Objects in Motion

    Hello; I am a little confused here. A friend asked me that, due to Newton's Third Law of Motion, for every force there is an equal and opposite force. Therefore, why is it, for example, that when we throw a ball up into the air, it moves upwards at all, if the force exerted on the ball...
  18. G

    Exploring Newton's Third Law: Can You Hit a Feather with 200 Newtons?

    Homework Statement My teacher puzzled me in the beginning of the year by saying that one cannot possible hit a falling feather with 200 Newtons because a falling feather doesn't have 200 Newtons to hit you back with. (Newton's Third Law) Homework Equations None The Attempt at a...
  19. T

    Newton's third law is absurd: argue why this isn't true.

    Homework Statement If the forces in Newton's third law are equal and opposite, they cancel, and thus nothing can ever accelerate, so the law is absurd. Homework Equations Argue why this isn't true. This is from my textbook, a very strong argument. The Attempt at a Solution Hey...
  20. D

    Newton's Third Law and Acceleartion

    Right now I am taking a classical physics course and find myself struggling with Newton's Third Law, which states that for every action there is an equal and opposite reaction. I am struggling to interpret this in a way such that I can still accept movement of objects. If I attempt to slide...
  21. G

    Problems with Newton's third Law

    Homework Statement This is more of a concept problem than one of number manipulation. I was wondering, if when a horse pulls forward on a cart with a force of 500 N and the cart pulls back with a force of 500 N by Newton's 3rd Law, how does the cart move if the net force is 0? Homework...
  22. J

    Newton's Third Law: Exploring Impact & Significance

    I can't understand the significance of Newton's third law. What would happen if there were no reaction forces?
  23. S

    Free-Body Diagrams: Several Objects and Newton's Third Law

    Homework Statement A box of mass m2=3.5 kg rests on a frictionless horizontal shelf and is attached by strings to bodes of masses m1 = 1.5 kg and m3 = 2.5kg. Both pulleys are frictionless and massless. The system is released from rest. After it is released, find (A) the acceleration of each...
  24. S

    Who pulls harder in Newton's third law: the sedan or the station wagon?

    1. 2. On a horizontal road, a small sedan is pulling a large station wagon that has run out of gas. While the sedan is speeding up to high way speeds. A. The sedan pulls harder on the station wagon than the station wagon pulls on the sedan; otherwise there would be no acceleration. B...
  25. P

    I don't quite get Newton's third law

    i don't quite get Newton's third law... :( It sounds simple enough, F12 = -F21. However, I'm getting that mixed up with the second law now (ie. if there is a net F on the system, there will be acceleration). For instance, say that there's a box, and the "F" vector is pointing to the right...
  26. J

    1.0kg physics book on slope, and Newton's Third Law?

    Homework Statement A 1.0 kg physics book is on a 20 degree slope. It is connected by a string to a 500 g coffee cup dangling at the bottom side of the incline. The book is given a push up the slope and released with a speed of 3.0 m/s. The coefficients of friction are us = .50 and uk = .20...
  27. E

    Help Explaining Newton's Third Law

    Homework Statement This isn't a homework question, but I thought this would be the appropriate place for the thread. I am having trouble explaining Newton's Laws to my friend. His confusion lies in the fact that the second Law F=ma does not equate to the sum of Forces in The third Law. Ex. A...
  28. I

    How to prove the Newton's third law.

    I can't prove the Newton's third law. Please tell me how to prove the Newton's third law. Thanks.
  29. C

    Newton's Third Law Tire on Road Question

    Homework Statement I was reading a textbook that gave an example of Newton's Third Law using the explanation for every force there is an equal and opposite force. The example given was a car tire on the road being accelerated moving in the negative x direction. In the supplemental diagram (see...
  30. C

    Foundations: Newton's Third Law and time reversal invariance

    Let me propose a list of principles of classical dynamics, specifically designed for education, for introduction to novices: - In the absence of any force: objects in motion move along straight lines, covering equal distances in equal intervals of time - Composition of motion: position...
  31. B

    Stupid question about Newton's third law

    OK, first of all, I wasn't really sure where to post this; it's not really homework because we covered the material weeks ago and moved on, leaving me totally in the dark. My physics teacher has this annoying habit of saying something is "ALWAYS true" or "NEVER true" and then beginning the next...
  32. S

    Newton's Third law problem Astronaut and Force

    The question states: How much force does an 81.0 kg astronaut exert on his chair while sitting at rest on the launch pad? the answer that I got was 794 N using... W=m*g and Fnet=w+fchair=0 But I cannot seem to get the right answer for this second question: How much force does...
  33. N

    How to Use Newton's Laws to Solve a Sliding Ramp Problem

    Homework Statement Two packages at UPS start sliding down a 20 degree ramp. Package A has a mass of 5.0kg and a coefficient of friction of .20. Package B has a mass of 10kg and a coefficient of friction of .15. How long does it take package A to reach the bottom? The package are positioned...
  34. S

    Newton's third law and air resitance?

    Hello, Few questions about these things.This is not homework! 1. Is Newton's 3rd law caused by conservation of momentum? 2. If I drop a stone into a pond. The stone would exert a force on the water and the water would exert a force on the stone. So why doesn't the stone move up! 3. Is...
  35. M

    Newton's third law of motion - help me to explain it to someone

    So there's this guys argument that Newtons 3rd law is wrong: "When I say that for every action there CAN'T be an equal an opposite reaction, it is a FACT. It's mathematically IMPOSSIBLE to have an EQUAL reaction to every action. Because then NO action would succeed. ALL actions would be right...
  36. K

    Newton's Third Law of Motion and car collision

    Jenny and Betty are having a great time at Busch Gardens riding the Ubanga Banga bumper cars. Jenny, who is traveling southward in her bumper car, aims her car toward Betty, who is traveling northward in her bumper car. The cars collide and briefly come to a stop. 1)What is the direction of...
  37. S

    Dynamics - forces in two objects, Newton's third law

    Homework Statement Two buckets of nails are hung one above the other and pulled to a roof by a rope. Each bucket of nails has a mass of 5.0kg. The action-reaction force between the buckets is 60N. Calculate the acceleration and the force applied by the worker lifting them up. Given: (draw a...
  38. S

    Quantum Mechanical analogy of Newton's third law

    Greetings the venerable PF society, I have a question that has been lingering in my mind for a while, so I thought I'd ask it here. Firstly, how far below does the action-reaction (Newton's third law) principle go down fundamentally? How far below can I take it with me so that I don't make...
  39. K

    Newton's third law and free body diagrams

    Homework Statement "Three blocks (m1, m2 and m3) are in contact with each other on a frictionless, horizontal surface. A horizontal force is applied to m1. http://i4.photobucket.com/albums/y111/kathy_felldown/sb-pic0556.png What is the net force on block 1? " When drawing free body...
  40. T

    Newton's Third Law - Using pulleys and slopes

    Homework Statement A 5.9 kg box is on a frictionless 40 degree slope and is connected via a massless string over a massless, frictionless pulley to a hanging 2.1 kg weight. A. What is the tension in the string if the 5.9 kg box is held in place, so that it cannot move? B. If the box is...
  41. C

    Newton's Third Law: Is it True and Why Does it Work?

    I have heard that Newton's third law fails to apply in certain cases. Is it true?? and if it is , is there a reason why it works in the remaining cases? also regarding the second law F=dp/dt is it how force is defined ??or is it a relation between force and change in momentum?? if it is...
  42. R

    Exploring Newton's Third Law: Kicking a Ball

    Well i known this is a kinda fundamental property of mechanics but i always have a weird thinking about this law. Let me illustrate my trouble with an example. Let's say i kick a ball with force F? According to the Law the ball experience F force, and my leg experiences -F. My questions...
  43. B

    How does Newton's third law apply to objects falling at different rates?

    I am so very confused by this third law business. It all started with me thinking about whether it is true that heavy and light objects fall at the same rate (I don't mean approximately). I figured that they don't. While the Earth attracts, say, an elephant and a small rubber ball, at the...
  44. P

    Newton's Third Law and Relativity

    Suppose I have a proton at the origin, and a (stationary) electron on the x-axis at x=r. Taking q to be the elementary charge, the force acting on each of these particles is: F = \frac{q^2}{r^2} Now suppose that the electron is heading towards the proton (in the negative x-direction)...
  45. S

    Newton's Third Law Question

    Homework Statement Two blocks are accelerated across a horizontal frictionless surface as shown. The coefficient of static friction between the two blocks is 0.7, and the coefficient of kinetic friction is 0.5, use M= 1.0 kg. When F=1.2 N, frictional forces keep the two blocks from sliding...
  46. G

    Newton's Third Law with Skimboarding?

    Hello PFers, I've been reading these forums for a little while, and now that I have my own question I thought I would register to the site :). My question is about pumping and Skimboarding. I understand that many of you (if not all) have no idea what skimboarding and pumping even is...
  47. J

    Applying Newton's Third Law to Tetherball

    Homework Statement How does Newton's Third Law of Motion apply to a tetherball? Homework Equations N/A The Attempt at a Solution Forces always occur in pairs (action/reaction force). I know that when you're pushing on a wall, the wall pushes back on you, which is why you don't...
  48. M

    Newton's Third Law hot air balloon

    Homework Statement A physics student in a hot air balloon ascends vertically at a constant speed. Consider the following four forces. that arise in this question. F1 = the weight of the hot air balloon F2 = the weight of the student F3 = the force of the student pulling on the earth F4...
  49. S

    Newton's Third Law - Weight on Scale when catching a ball

    Homework Statement You are standing on scales which read weight in Newtons. A 0.50 kg ball is dropped from a height of 1 m into your hands. Your hands drop from chest level to waist level during the catch, a distance of about 25 cm. Your mass is 62 kg. Assuming that you decelerate the ball...
  50. S

    Newton's third law to explain lift

    Ok so I just read this statement as part of an explanation of list using Newton's 3rd law "The amazing thing about wings is that because they are flying through air which is a fluid, the top of the wing deflects air down as well as the bottom of the wing." What I don't understand is how...
Back
Top