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.

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  1. A

    Newton's third law in terms of inertial position vectors for n-body system

    Assuming $$\vec{r_{a}}$$ and $$\vec{r_{b}}$$ is calculated from an inertial frame of reference. then for any two objects (named a and b) in a system of more than two objects, Is this the Newton's third law, $$\frac{d^{2}}{dt^{2}}m_{a}\vec{r_{a}}=-\frac{d^{2}}{dt^{2}}m_{b}\vec{r_{b}}$$...
  2. M

    Questions on work done and Newton's third law

    Hello 1- work done to push body forward is greater than to pull body backward what is the explanation for this?can you draw the two FBD in both cases ? 2- why is nail driven in wooden wall but not in the concrete wall ؟ I guess this is somehow related to Newton's third law Thanks
  3. B

    Newton's Third Law Explanation

    For every force exerted by object A on object B, an equal and opposing force is exerted by object B on object A. I need help understanding the principle. I understand it when dealing with a completely stationary object: pushing on a solid wall, it exerts an equal force back to your hand...
  4. S

    Newton's third law question

    Homework Statement Suppose you have a cantaloupe (C) on a table (T). Let FTC be the force on the table by the cantaloupe, and let FCT be the force on the cantaloupe by the table. Do the magnitudes of these forces increase, decrease, or stay the same if they are placed in an elevator which...
  5. O

    Solving for Force of B on A in a Newton's Third Law Problem

    Homework Statement Two Blocks, A (10kg) and B (12kg) are placed next to each other on a rough surface. An applied force acting on block A causes both blocks to accelerate at 1.2m/s^2 to the right. If the magnitude of the force of friction on Block B is 18.3N, calculate the force (magnitude and...
  6. P

    Newton's third law and a motor

    hey people,greetings to everyone i'm an freshman in physics so i have some problems understanding Newton's third law. for example how can a motor accelerate an elevator upward,or downward?i know Newton's third law acts on different objects,i understood the house and the cart problem. horse is...
  7. M

    Newton's Third Law and acceleration question

    Two wrestlers are pushing each other. Initially the wrestlers are at rest, however the small wrestler starts to move. Now in either cases, which wrestler exerts more force? My professor said that both exert the same amount of force on each other. This is really confusing as if they are applying...
  8. P

    Making sense of Newton's Third Law

    This is going to be a stupid question, for which I apologize. I am probably over-thinking things but my reasoning is flawed somewhere because when I think about Newton's Third Law and it's application to real-world examples, I can't understand how objects move at all, whether accelerating or...
  9. L

    Exploring the Pain of Board Breaking: A Newton's Third Law Analysis

    Hi all, I have a general question about board breaking. This is not homework. Can anyone explain why it hurts more not breaking the board, compared to breaking the board. I know that there are already threads about this topic. However, I am confused about the different explanations: 1...
  10. P

    The meaning of force and Newton's third law

    I push on a wall and the wall pushes me back (according to Newton's third law), but what i don't understand is that no acceleration is produced hence is it not that i have not applied any force? If i jump then i return to the ground (obviously) then doesn't the Earth accelerate towards me...
  11. cryora

    Does Newton's Third Law apply to magnetism?

    Newton's Third Law states that every action has an equal and opposite reaction. I'm wondering if and how this applies to magnetism. I would assume that it would mean if a moving charge is creating a magnetic field that causes a magnetic force on another charge, then it can be said that the...
  12. S

    Newton's third law when moving our body

    Homework Statement When I push my belly forward, a force is exerted on my belly by my muscles. So there will be a reaction force acting on the muscles right? But eventually it has a net forward force. So if we treat the belly as one object (muscle fat everything in the belly) then where is the...
  13. E

    Hw can we relate conervation of momentum with Newton's third law?

    Hw can we relate conervation of momentum with Newton's third law?? Hw can we relate conervation of momentum with Newton's third law??.. Quantitavely and qualitatevely i want a clarification for the realation between these.. Assume an apple falling towards earth.. Thanks in advance
  14. T

    Symmetric matrices and Newton's third law

    So, I was studying coupled oscillations and came across a statement that I couldn't figure out. It was that a particular matrix was symmetrical by Newton's Third Law. I know what Newton's Third Law is, I know what symmetric matrix is. But, for example, a matrix like this: -2k/m...
  15. A

    Newton's Third law of motion problem.

    Homework Statement An object is free to move on a table, ex- cept that there is a constant friction force f that opposes the motion of the object when it moves. If a force of 10 N pulls the object across the table, the acceleration is 2 m/s2. If a force of 20 N pulls the object across the...
  16. D

    Newton's third law in non-inertial frames

    Is Newton's third law valid in non-inertial frames? For example, in a rotating frame of reference, can Newton's third law still be applied? Or does the non-inertial character of the frame violate it?
  17. S

    Newton's third law and surface area of paddles

    why do paddles/oars have a large surface area? I thought that it does not really matter as i would just apply a force on the water so as to attain a action-reaction force on the boat thus pushing me forward. but if i look at the free body diagram of the paddle 1 force will be me pushing and the...
  18. S

    What happens to free body diagrams when breaking through a wall?

    When I exert a force, I will experience an equal and opposite force on two mutual bodies. However, when I break through a wall how do the free body diagrams of my hand and the wall? When the wall doesn't break, the forces on the hand is normal contact force on my hand from the wall and a force...
  19. S

    Does Newton's third law apply to Atwood's machine?

    Homework Statement Consider Atwood's machine with two masses, m1 and m2 with m1 less than m2. Now, according to application of Newton's third law, the system accelerates in the direction of the heavier mass (here, m2). This, however, seems to contradict Newton's third law, which implies that...
  20. K

    Newton's Third Law: Unpacking the Mystery

    I'm kinda confused with Newton's Third Law. Let me present my problem. Say I kicked the floor with 50N, the floor pushes me back with 50N, there is a reaction pair. But say if I kicked the air with 50N, why aren't I feeling the 50N? By right it should feel the same right?
  21. F

    Newton's Third Law Problems

    Alright, so I have this sheet for homework and I'm not sure where to start. Incase you are unable to see, the questions say: a) What is the acceleration of the system and the tension in each rope if the girl lifts with a force of 100.0N? b) What is the force that she pulls with if T3...
  22. J

    Confusion regarding Newton's Third Law of Motion

    I'm just a humble human being here, so if my question comes across as stupid, please forgive me! :) Newton's Third Law of Motion states that for every action there is an equal (in magnitude) but opposite (in direction) reaction. One of my A-level revision guidebooks state that the attraction...
  23. Z

    Friction and Newton's Third Law

    Hello, I have a question from a conceptual problem in my textbook as follows: A car is moving north and speeding up to pass a truck on a level road. The combined contact force exerted on the road by all four tires has vertical component 11.0 kN downward and horizontal component 3.3 kN...
  24. S

    Newton's Third Law in Inelastic Collisions

    Lets say that I have a cart on a track with silly putty on the end. This first cart is moving with an initial "x" velocity. There is another cart about a foot ahead of it with sillyputty on its bumper also. (Both carts have the same mass) This cart has an initial velocity of zero. Of course...
  25. S

    Newton's Third Law and Forces in an Elevator

    Hi, I am confused on a certain part of Newton's third law. I know that it states that for every action force there is an equal and opposite reaction force that act of different objects. So, let's say you have an elevator that is suspended by a cable. The elevator's mass is 1kg (I know this...
  26. D

    Dynamics problem using Newton's third law.

    Homework Statement Two packages at UPS star sliding down a 20° ramp. Package A has mass of 5.0 kg and coefficient of friction of 0.20. Package B has a mass of 10 kg and a coefficient of friction of 0.15. How long does it take package A to reach the bottom? Homework Equations N/A The Attempt...
  27. S

    Newton's Third Law in an Elevator

    Hi, I was in an elevator the other day and I began to think about the forces acting on me and the elevator during my ascent and I became very confused. Let's say for example that my mass and the elevator's mass combined is 100kg so this weighs 98N. When the elevator is at rest I figured...
  28. D

    Newton's Third Law Questions

    1. Tug Of War: Person A is bigger and stronger than Person B, and chooses to neither pull on the rope, or allows Person B to pull Person A forwards. Assume that Person A's force is in the positive direction, graph both forces on one set of axis. Approach: For Person A to neither pull or...
  29. A

    Can Newton's Third Law Of Motion Be Violated?

    I have read in several books and also in this site that the Newton's Third Law does not hold true for "Action at a distance". Specifically, in the case of Electrodynamics. Can someone explain to me, what actually happens in the case of electromagnetism? I've read that the action-reaction...
  30. S

    Kinetic frictions, pulleys, and Newton's third law

    Homework Statement Block A and B are connected by a string passing over an essentially frictionless pulley. (Block A is on a horizontal plane, while block B is suspended in the air) When the blocks are moving block A experiences a force of kinetic friction of magnitude 5.7N. If mA= 2.7kg and...
  31. A

    Newton's Third Law: Pushing 20kg Box w/140N Force

    If I push a box (mass of 20kg) with a force of 140N, is my acceleration 2m/s^2 in the positive direction? I cause it to accelerate in the same direction as I'm pushing in, and that accelerates at 7m/s^2, but it reacts with 140N also, so shouldn't it have some acceleration in my direction to...
  32. K

    Misinterpretation of Newton's Third Law

    Homework Statement A man standing on the ground is subjected to 2 forces : weight and normal reaction force. Do they form an action-reaction pair of forces? (That's Newton's Third Law). I put yes, but I was wrong. I don't get it. I'm new to Newton's Third Law.
  33. J

    Newton's third law of motion help

    Hello! My question is about Newton's 3rd law: Which one of the following is true, according to Newton's laws of motion? Ignore friction. a. an SUV hits a stationary motorcycle. Since it is stationary, the motorcycle sustains a greater force than the SUV does b. a semitrailer truck crashes all...
  34. L

    Newton's third law of motion - why?

    Greetings everyone, I'm new here! I tried searching the forums for my problem, but most deal more with the common misconceptions, not the question why - if there is such a question. Lately, I've been trying to gain an in-depth look into Newton's work, moreover the third law which has been...
  35. M

    Newton's third law and a box push

    Homework Statement I just don't understand how to describe how, if a person pushes on a box and the box exerts an exactly equal but opposite force on the person that the person is still able to move the box... Homework Equations n/a The Attempt at a Solution n/a
  36. L

    Pushing a cart, using Newton's Third Law

    I am trying to fully understand Newton's Third Law. I am getting there, but there are a few examples that sort of have me scratching my head for a bit. Me and a cart are stationary, then I decide to push the cart. I exert a force on the cart and simultaneously, the cart exerts an equal and...
  37. Z

    Newton's Third Law on a Inclined Plane

    Hello everyone. I think I have almost understood the third one of Newton's motion laws but I have some doubts: Let us think an inclined plane block located on a surface, there is a box on it and assume there is no friction neither between the surface and block nor between the block and the box...
  38. N

    Derivation of Newton's Third Law (?)

    Griffiths says "unlike the first two, Newton's third law does not, in general, extend to the relativistic domain." I don't understand, what is wrong with the following derivation? So we take it as an experimental fact that momentum is conserved in isolated systems (for clarity, the...
  39. N

    Pulley experiment - Newton's third law

    1. Homework Statement TRIAL 1 A. ) Bag A + 6 stones Mass (kg) - 0.037 kg Mass of weight hanger + slotted weights - 210 g/ 0.21kg Tension 1 (N) - ?? Acceleration (m/s2) - ?? B. ) Bag B + 4 stones Mass (kg) - 0.08 Mass of weight hanger + slotted weights - 210 g/ 0.21kg Tension 1 (N) - ...
  40. J

    Newton's third law not universal?

    I found an article on arxiv which basically says that Newton's third law is not verified in certain situations...can anyone please explain more about this in layman terms?? Here's the link:- http://arxiv.org/abs/1104.5011v1
  41. P

    Newton's Third Law: Confused by Box & Ground Forces

    So, I'm a bit confused... As an example, let's use a box resting on the floor: I understand the first pair of forces well: Earth pushes the box down with force W, box pulls the Earth up with a force of magnitude W. Now, the other pair of forces are the normal forces, which my teacher told...
  42. C

    Newton's third law and friction

    Homework Statement Blocks X and Y are attached to each other by a light rope and can slide along a horizontal, frictionless surface. Block X has a mass of 10kg and block Y has a mass of 5.0kg. An applied force of 36 N [right] acts on block X. a) Calculate the action-reaction forces the...
  43. T

    Problem in Newton's third law and rope tension

    Hello hope you're doing well :smile: Homework Statement I'm having a great problem understanding Newton's third law let's say that we have 2 bodies with masses m1 & m2 where m1>m2 the 2 bodies are connected with a rope body1________body2----->F a force F is applied on body 2 as shown...
  44. P

    Question about a Pair of Forces - Newton's Third Law

    I was looking at a picture (attachment) in a physics website and the following doubt arose: I previously thought that the weight of the block (gravitational pull of the Earth on the block) caused a reaction force of the table, but now, from what I understand and, please correct me if I am...
  45. P

    Understanding Newton's Third Law: Forces on a Box

    I'm a bit confused: :grumpy: Let's take a box standing on a table: So there are two pairs of force acting on it? Can anyone help me with the forces on the box? Thanks, Peter G.
  46. R

    Applying Newton's Third Law of Mortion

    Homework Statement After falling from a rest from a height of 30 m, a .50 kg ball rebounds upward, reaching a height of 20 m. If the contact between ball and ground lasted 2.0 ms, what average force was exerted on the ball? Homework Equations 2.0ms=.002s The Attempt at a Solution
  47. J

    Question about Newton's Third Law of Motion

    A brick hits a glass window. The brick breaks the glass, so: is the magnitude of the force of the brick, greater, less than, or equal to the magnitude of the force of the window? Newton's third law of motion states that for every action, there is an equal an opposite reaction. With this...
  48. F

    Newton's third law + forces

    Homework Statement three identical boxes, each of weight W, are to be stacked one on top of another against a vertical wall. The lowermost box is in contact with the wall, and the other two boxes are positioned as shown in the diagram. The middle box is pushed into position by the...
  49. TurtleMeister

    One force or two forces in Newton's third law?

    In Newton's third law, do you consider there to be one force or two forces involved? My thinking is that there is only one force which acts equally in magnitude (conservation of momentum) and opposite in direction for each object. However, many sources that I find on the internet claim that...
  50. F

    Newton's third law, frictional force, acceleration

    Homework Statement a truck driver loads two identical untethered crates stacked one upon the other. no sliding takes place. make separate sketches to show the forces acting on each crate when the truck is traveled on a horizontal straight road while accelerating. Homework Equations...
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