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. M

    Newton's third law a book and table

    Homework Statement Newton's third law of motion is often summarised as 'Every action (force) has an equal and opposite reaction'. A book rests on a table. If the weight of the book is the 'action' force, what is the 'reaction' force? A the pull of the book on the Earth B the pull of...
  2. UseAsDirected

    Why can't I show Newton's Third Law by different means?

    Homework Statement A small mass (1 kg) sits next to a larger mass (3 kg) on a table. A force of 5 Newtons pushes from left to right on the system while a force of 3 Newtons pushes from right to left on the system. Am I justified to conclude that the net force on the larger block has magnitude...
  3. L

    Newton's Third Law Confusion

    Homework Statement Homework Equations principle of moments Newton's 3rd law The Attempt at a Solution Okay. I am not asking how to solve this question. I just have a little misunderstanding of the concept of the Newton's Third Law. I will get straight to the point. For example, in this...
  4. Madhusree Bera

    Violation of Newton's third law

    when a stone is dropped in a beaker containing water,it sinks.But according to Newton's third law of motion the water should also exert an equal force upwards...so the stone should float. This violating the third law but why?
  5. gravityripple

    Newton's Third Law -- Normal vs. Tensile as Opposite of mg?

    Hello, I am returning to college after a ten year hiatus and am taking an online course on edx to try and refresh my knowledge a bit before the fall. I read a few other posts on Newton's Third law, but it seems I am falling short on this one concept. In the case of an object, m_1, which is at...
  6. juviena

    Newton's third law is not correct

    Newton's Third Law concerns the forces of interaction between two bodies. Which of the following statements relating to the third law is not correct ? A. The two forces must be of same type B. The two forces must act on different bodies. C. The two forces are always opposite in direction D. The...
  7. amjad-sh

    Prediction of Newton's third law

    How did Newton predict his third law? If we take the system as the whole universe Newton's third law leads that the net force of the system is zero.Did Newton have an insight about this?
  8. H

    Do weight and normal contact force always have equal and opposite effects?

    Do weight and normal contact force always act in equal and opposite directions?
  9. S

    Normal force: two boxes with different mass?

    I took a practice Ap Physics test today and did well, but there was a basic problem that hung me up: there are two boxes on the ground on top of each other. the top box's mass is 2m, and the bottom's is 1m. question: just looking at the bottoms box's normal force, does it equal the 2m box on...
  10. S

    Physics principle(s) to explain how to stack spherical items

    I need to think of all the physics principles to explain how one can stack spherical items (ex. baseballs) on top of each other. So far I've thought of one. 1. Newton's third law In this case, the reaction is the normal force in each baseball that is stacked and the action is the force of...
  11. H

    Astronaut floats outside spacecraft

    Homework Statement A 91-kg astronaut floats outside a 3,131 kg spacecraft . She's initially stationary with respect to the spacecraft . Then she pushes against the spacecraft , and moves away at 0.195 m/s to the left. Find the SPEED of the recoiling spacecraft . Homework Equations Force =...
  12. V

    Truck collision with a resting car

    So a truck hits a car, which is at rest. Would there be a moment in time when a car stops exerting a force in equal magnitude but in opposite direction (Newton's third law)? To put in some arbitrarily numbers to make the example easier to understand. Truck weighs 1000 kg, and the car weighs...
  13. C

    Newton's Third Law and a rocket in space

    I am the kind of guy that always needs to return to the horse-and-carriage problem to hone in my understanding of Newton's Third Law. Here's my question. Assume a rocket in space is applying a force to another object of equal mass in space. Now, I understand the object in space is experiencing...
  14. J

    Tension in string in system with multiple objects?

    Homework Statement I am stuck on this problem: So the question is asking for F21, F12, F32, and F23. (That's the force by 2 on 1). The mass of m2 is variable. There are four cases where I need to solve for the forces: when m2 has mass of 3kg, 0.3kg, 0.03kg, and 0kg. The mass of m1 = 2.5kg, the...
  15. A

    Assumption on central forces between two particles

    Homework Statement Consider Newton’s force law for two particles interact through a central force F12(r1',r2',u1,u2), where by Newton’s third law F12 = -F21. m1(d^2r1/dt^2) = F12(r1,r2,u1,u2) m2(d^2r2/dt^2) = F21(r1,r2,u1,u2) A. Show that Newtonian mechanics is form invariant with respect...
  16. A

    Proving Newton's third law invariant with Galilean tranfrom

    Homework Statement Consider Newton’s force law for two particles interact through a central force F12(r1',r2',u1,u2), where by Newton’s third law F12 = -F21. m1(d^2r1/dt^2) = F12(r1,r2,u1,u2) m2(d^2r2/dt^2) = F21(r1,r2,u1,u2) A. Show that Newtonian mechanics is form invariant with respect to...
  17. D

    Trouble with Newton's Third Law of Motion

    Please forgive me for my naiveté but I've got to resolve a conceptual problem I'm having. If every action has an equal and opposite reaction then why don't all the forces in the universe cancel each other out. For example if i am in space and I hit a ball with force X then the ball reacts back...
  18. itchybrain

    Newton's third law in relation to field forces

    I need some help understanding how Newton's third law applies to field forces (namely gravitation). The third law in contact forces seems straightforward to me. Billiard ball A, which is moving, hits billiard ball B. The collision exerts a force on Ball B, resulting in its acceleration...
  19. Y

    Newton's Third Law of Motion - Help

    Got it!
  20. B

    Tough question about reaction force Newton's third law -- Please help

    Homework Statement . question is as attached. answer is 2f/5..how do u get this? Homework Equations F=ma F= (5m)a a= F/(5m) The Attempt at a Solution gotten a=F/5m , but why the answer is 2f/5?
  21. oreo

    Newton's third law and a horse

    When a horse pulls tonga, the Tonga exerts an equal force on Tonga then why does it move. How is Newton's third law applied to such case.
  22. S

    Conceptual Newton's Law's Questions - HS Freshman Physics

    NOTE: The following questions are not part of any homework assigned. This is part of finding extra concept-related questions to help me study for my physics unit test. Homework Statement I have conceptual questions, not problems. I'll provide sufficient evidence that I've attempted to think...
  23. Soumalya

    Newton's Third Law and the Horse Cart Problem

    Hi, I am facing some issues while dwelling upon the application of Newton's third law of motion to the famous "Horse-Cart" Problem.Anybody who is clueless about what I am talking about please follow the link: https://www.lhup.edu/~dsimanek/physics/horsecart.htm As one can see the whole...
  24. T

    Newton's third law and attraction

    Is the force of attraction between an electron and a proton in a hydrogen atom a example of action and reaction to which Newton's 3rd law applies?
  25. A

    Question about Third Newton Law

    If I jump, I accelerate away from Earth. However, if I drop a human on the ground, he doesn't accelerate away from the Earth. In both cases the force used on the Earth is about the same, but acceleration away from the Earth exists in one case and doesn't exist in another case. Why so? (not a...
  26. M

    Opposing forces and Newton's third law

    According to Newton's third law, every force has an equal and opposite reaction force. Which force is equal and opposite to both friction and tension?
  27. A

    Is Newton's third law applicable everywhere?

    1. If Big Bang really happened, then what are the action and reaction forces acting? 2. A quantity like Entropy is always increasing. If average temperature is made constant then universe is getting heated continuously..then what is that being cooled? 3. Total of action and reaction force =0, if...
  28. R

    Explain using Newton's third law of motion

    1. no matter how strong a car engine is a car cannot accelerate on an icy surface 2. A vehicle pushes a car of lesser mass from rest, causing the car to accelerate on rough dirt.
  29. U

    Why is a negative spring constant unrealistic in this system?

    Homework Statement A spring system consists of two parallel springs on top and a single spring connected to them on the bottom as shown: http://i.imgur.com/hgMr12U.png If the system is extended by 7 cm (i.e. x = 0.07m), F = 5N and k2 = 10 N/m, explain why the value of k1 is unrealistic...
  30. C

    Newton's third Law - action reaction pair

    Hi, I am a little confused with the concept of action reaction pair of forces. Does this depend on mass? For example, if a train engine is pulling a buggy with force F, what would be the force applied by the buggy on the engine? The masses of the two are different and there is force of...
  31. D

    Newton's Third Law doesn't make sense

    Newton's third law states that for every action, there is an equal and opposite reaction. This doesn't make sense to me, because it seems as though everything would cancel out and so nothing would ever happen. For example, imagine a person attempting to push a large box; not just a quick push...
  32. M

    Why don't forces cancel each other out when they act on two different bodies?

    Why forces acts on two different bodies with same magnitude and opposite in direction doesn't cancel each other?
  33. swayne221b

    Relation between force couple and Newton's third law

    After being through with Newton's 3rd law of action reaction pairs, there arise a doubt regarding the categorization of force couple (related to torque) of being or NOT being an example of action reaction pairs.
  34. davidbenari

    Conservation of momentum without Newton's third law

    So I've recently read Newton's third law violates the principles of relativity. I only know how to prove conservation of momentum if Newton's third law holds. I was hoping someone could explain to me this (proving conservation of momentum when Newton's third law is false) without using extremely...
  35. davidbenari

    Newton's third law in ropes and tension

    For some reason I couldn't answer this, and I hope you can help me: a) Two people pull on a rope, one with 200 N , the other 100 N. Why is the tension on the rope the shared magnitude of both forces? b) If two people pull on a rope with equal force (say, 100 N), each person pulling feels...
  36. P

    Newton's Third Law and Linear Momentum Conservation

    I have always been under the impression that Newton's third law and the law of conservation of linear momentum are really the same thing; synonymous, so to speak. But then I was wondering if Newton's Third Law holds for a non-isolated system. I mean, I know for sure that momentum conservation is...
  37. R

    Newton's Third Law and the Conservation of Momentum

    Newton's third law confusion? If A exerts some force on B then it experiences a force of same magnitude and in opposite direction. This didn't seem intuitive to me so I thought of it this way, Let A and B be a single system. Now there is no net external force acting on the system so the...
  38. Conservation

    Conceptual problem on Newton's Third Law

    Newton's Third Law states that for every action force, there is a reaction force. So take a classical problem with a basketball player. The Earth has a gravitational force on the player, and thus player exerts an equal and opposite force on the earth. The player is stationary. My first...
  39. H

    Chain Link Problem? [Tension Forces; Newton's Third Law]

    Homework Statement A chain of three links, each with a mass 0.2 kg, is being pulled up by a person lifting the top link with 8.88 N of force and the chain accelerates upward. Calculate three forces that are acting on the middle link while the chain is accelerating. Homework Equations ƩF = ma...
  40. O

    Newton's Third Law Problem [Checking Solution]

    Homework Statement Two hockey players are standing stationary on the ice facing one another. Player A has a mass of 85kg and Player B is 110kg. They shove each other and player A ends up with a velocity of 1.5m/s [W]. Find the final velocity of Player B. Solution Fa=Maaa Fa=(85)(1.5-0/t)...
  41. S

    Newton's Third Law of Motion and wrecking ball

    "For every action there is an equal and opposite reaction." I'm having a difficult time wrapping my head around this. I'm having a hard time applying it to certain situations. For example, a hammer hitting a nail, but more presently a problem in my homework. A building is being torn down...
  42. O

    Calculating Acceleration and Velocity in Newton's Third Law Scenario

    Homework Statement A paintball with a mass of 0.15 kg is fired from a paintball gun that has a mass of 5.5kg. The paintball leaves the gun with a velocity of 45 m/s [N] having accelerated for 0.10s. Calculate the acceleration and the final velocity of the paintball gun.
  43. 0

    Newton's Third Law of a car and truck

    I have a question for which I am intensely confused. Lets say a car was pushing a big truck from behind. And the car (and truck) are accelerating, then there is a breakdown because the car applies all the force it has to the truck, which pushes back with equal force. So the car shouldn't be...
  44. N

    Newton's Third Law and Unbalanced Forces

    Hello, I have seen several explanations on the Internet about how to resolve Newton's Third Law and Unbalanced Forces, but none made sense to me in a way that I really understood it. Scenario 1: There is a block on the ground, the ground has friction with say static friction force 10 N...
  45. B

    Newton's third law and energy transfer

    If Newtons third law says there is an equal and opposite reaction how can there be energy transfer. For example if I push a box and give it 1000 joules of energy wouldn't it "give back" the energy. Also does an object have to be moved for it to gain energy, if I were to punch a book many times...
  46. S

    Does SR and GR agree on Newton's Third Law of Motion ?

    Sorry guys not a physicist!Only wish to know if there is agreement between Newton laws of motion and SR/GR in particular?I know these two don't match in some cases? What are they in simple words please!? Cheers
  47. G

    Radiation reaction force and Newton's third law

    There seems to be a problem with radiation reaction force. Let's look at charged sphere, attached on a rod to an object with larger mass, and rotating around its centre together with it. The rotating charged sphere emits radiation with power P. The sphere's speed is V. Assuming energy is...
  48. D

    Quick question on Newton's third law & energy conservation

    I have a quick question on Newton's third law. When a 100kg astronaut in space is holding a 100 kg rock and then pushes it away from him with 50 joules of energy (the kinetic energy put into the system) and a second astronaut (observer) is watching, sitting still relatively to the location...
  49. M

    Newton's third law, bullet through wood question

    1. A 0.0048kg pullet traveling with a speed of 200m/s penetrates a large wooden fence post. If the average resisting force exerted on the bullet was 4500N how far did the bullet penetrate. 2. F=ma? 3. F = ma F/m = a 4500/0.0048 = 937500? So there's a reverse acceleration of...
  50. B

    Calculating Take-Off Speed Using Newton's Third Law

    question is 29./ about bob and throwing his rock, I have found a./ (225N) easily enough it is b./ which i cannot figure out. I am assuming that it is a Newtonian action-reaction pair and that the rock pushes back on bob at 225N as well, which would mean Bob's acceleration would be 3.0ms/s^2. But...
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