What is Newton's third law: Definition and 251 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. D

    B How's it possible to push on systems with 2 parts? (Newton's third law)

    An alternative title could have been "how do forces propagate" but google searches bring up things related to waves only. Initially my problems started with a mass on a spring but I was able to boil it down to any general system comprising at least two 'parts' in succession. Suppose you have...
  2. rosedog09

    Blowing Into Your Own Sail

    C. There is no net change in force, momentum, or velocity of the sail craft because the fan exerts a forward force on the air; however due to Newton's third law the air exerts an equal and opposite force on the fan. This air then exerts a forward force on the sail which cancels out the rearward...
  3. String theory guy

    Newton's Third Law for book on a table

    This problem is from MIT's module again. I have a doubt about Newton's Third Law. How is the normal and weight of the book not Newton's 3rd Law pairs? When the book is placed on the table, I imagine that the book's weight exerts a force on the table and the table exerts and equal and...
  4. C

    B Newton's third law (equal and opposite force)

    Something about Newton's third law confuses me, when I hold my phone and I move it around it's velocity is changing, therefore because Newton's first law it's acted on by a force, and because Newton's second law, the force is directly proportional to the mass and acceleration of that object...
  5. P

    I Exploring Newton's Third Law in an Imaginary Magnetic Field

    Hi, here's a theoretical problem that I am trying to find a satisfactory answer for. Imagine a coil that is temporarily switched on an off and generates a magnetic field that permeates through space. Now imagine a charged particle passing through this field, at time that the coil is already...
  6. J

    Newton's third law -- I have trouble with some real-life examples

    I have gathered that forces always occur in pairs and are equal in magnitude but opposite in direction and that according to the second law a force is the product of the mas and acceleration of the object exerting the force. My problem is with getting this make sense with real life examples. If...
  7. A

    Unbalanced force to find altitude of airplane

    Since the airplane is flying at a constant speed, then the lift force equals the weight of the airplane. This means that the lift force prior to acceleration is the mass * gravity constant = 29400 N. When the airplane increases horizontal speed, the lift force increases to 30 kN. This means...
  8. P

    How do you identify the systems in which Newton's Third Law is obeyed?

    eg. For a ball in free fall (ignoring air resistance) there is only one force acting if the system is just considered to be the ball. It seems necessary to go to the system which includes the Earth for identification of the reaction force to be made. Is there any rule for how big a system must...
  9. I

    Grade 11 Question: Newton's Third Law -- Calculating acceleration of an airboat

    for c) i need help d) Given: Let [forward] be positive. F 1 = — 250 N F 2 = 360 N Unknown: a = ? Equation: F net= F1+ F2 F net= m a Solve: F net= (— 250 N) + 360 N = 110 N F net= m a a = F net/ m = 110 N / 220 kg= 0.5 m/s [forward] Statement: The acceleration of the boat is 0.5 m/s [forward].
  10. A

    Does Newton's Third law apply to torque/rotation?

    For example, let's say i have force acting on an object about some pivot. That object will apply an equal force about the same pivot, at equal lever arm, back. So there it seems like if the forces are in line, acting on the same pivot point, there is an equal and opposite torque. However, if i...
  11. jaumzaum

    Is Newton's third law valid for rotation / Torque?

    Is third Newton law valid for rotation / Torque? I mean, can we say that for every torque there must be another torque with equal magnitude and opposite direction? This can only be true for contact forces or radial forces, as these forces will create a reaction that will cancel the torque...
  12. R

    The violation of Newton's third Law

    Let us consider two point charges: one of which moves (in the simplest case) rectilinearly with a constant speed and the other is at rest. The electric field of the first charge is renormalized due to the effects of retardation and generation of a solenoidal electric field (see Am. J. Phys...
  13. LCSphysicist

    Where does Newton's third law fail?

    I have read in some books and articles of classic mechanics that the Newton's third law does not always hold, could you say an example to me?
  14. Baran

    Newton's third law -- Calculate the recoil of a paintball gun

    I've been stuck on this for sometime now, and am unsure how to approach this question.
  15. R

    Working with Newton's Third Law and friction

    I know that the normal force and Fg are unbalanced in this case, but I don't get how to set up the equilibrium equations with that. I struggle with determining what you set the equations equal to when its either balanced or unbalanced forces you're dealing with.
  16. Petronius

    Newton's third law problem/kinematics (Airboat problem/no friction)

    Thank you very much your time! I first found the force of the air on the boat using the principle of Newton's third law and the fact that no friction is involved. Faction= -Freaction 150 N backwards = -150 N backwards 150 N backwards = 150 N forward I then sought to determine the...
  17. MrDickinson

    Question about Newton's Third Law and and the Force between 2 objects

    m1 is the large cruise ship m2 is the tug boat The question doesn't state it explicitly, but I assume that both masses are undergoing acceleration because the tug boat is experiencing a change in velocity. I assume the system accelerates uniformly, and I assume that m1 and m2 accelerate...
  18. J

    Trivial question regarding Newton's Third Law and Elasticity

    1. Drawing Free Body Diagrams for all components we get : 2. Following this we can find total elongation using ##\Delta L = \frac {1}{AY}(F_1*L_1 + F_2 *L_2+ F_3 *L_3) ## My questions : a) I am assuming that the internal forces (3t) are neglected in the FBD because of Newton's third law whereby...
  19. Callista

    Conceptual check: Newton's third law

    Homework Statement A spherical rubber balloon inflated with air is held stationary, with its opening, on the west side, pinched shut. (a) Describe the forces exerted by the air inside and outside the balloon on sections of the rubber. Homework Equations ? The Attempt at a Solution The air...
  20. Zeke Bevan

    Newton's Third Law Problem: Two masses, a rope and a pulley

    Homework Statement Two blocks are attached to opposite ends of a massless rope that goes over a massless, frictionless, stationary pulley. One of the blocks w/ a mass of 1.5 kg accelerates downward at (3/4)g. A. What is the mass of the other block? Homework Equations There were no given...
  21. A

    Trouble understanding Newton's Third Law in Pulleys

    Hi everyone. I found this image online that made me question a couple of things that might be a bit humiliating to ask: I always thought that tension is simply a pulling force, meaning that the mass is suspended because the tension force pulling the mass is equal to the weight force of the...
  22. M

    Newton's Third Law: Acceleration of box and worker

    Homework Statement On a horizontal frictionless floor, a worker of weight 0.900 kN pushes horizontally with a force of 0.200 kN on a box weighing 1.80 kN. As a result of this push, which statement could be true? A ) The box will not move because the push is less than its weight. B ) The worker...
  23. R

    Elastic collisions and Newton's third law

    Hi everyone, I am new to this forum, and I'm having a hard time understanding Newton's third law and collisions, first of all I want to say that it is not homework and that I do know the basics of physics, vectors, energy, work, and momentum I also know and understand Newton's first and second...
  24. S

    How Does Newton's Third Law Apply When a Bat Hits a Ball?

    The following reasoning leads to an apparent paradox; explain what’s wrong with the logic. A baseball player hits a ball. The ball and the bat spend a fraction of a second in contact. During that time they’re moving together, so their accelerations must be equal. Newton’s third law says that...
  25. L

    Newton's third law - normal force

    Homework Statement A book is placed on a chair which is standing on the floor. An iPad is placed on the book. The floor exerts a significant force: A) only on the book B) Upwards on chair and downwards on book C) ONLY upwards on chair D) upwards on chair book and iPad E) downwards on...
  26. E

    Understanding Newton's Third Law in Action: A Truck Pushing an SUV

    Homework Statement A 2,300 kg truck pushes a 2,500 kg SUV, and they both move at 3.75 m/s2. I am to find the force that the SUV exerts on the truck. Homework Equations F=ma The Attempt at a Solution I solved this problem already, it was just taking the acceleration I had found (3.75) and...
  27. PhysicsKid0123

    Center of mass, Newton's third law, and rocket thrust

    Can someone clarify this for me-- how is the motion of the center of mass unaffected by internal forces, as is stated towards the end of the paragraph in the picture I've provided. It seems to me that motion of the center of mass directly affected by these internal forces. Specifically, the...
  28. A

    Some calculations in Newton's Third Law

    Hello, 1- IF we have an object has 200 kg and we also have a machine has 5 kg and pushes that object with force of 600 n , in Newton third law it must be another force =-600n then : the acceleration of the object must be 3 m\s^2 and the acceleration of the machine is 120 m\s^2 , Am I wrong...
  29. M

    New Explanation to How Newton's Third Law is Satisfied in Magnetism

    <Moderators note. Moved from the quoted thread to avoid detracting> Newton third law does apply in magnetism, and this is proved by applying the conservation of momentum principle. This will be clearer when the causes/origin of the magnetic force is understood as described in the work "Two new...
  30. Haniah

    Calculating Toolbox Mass in Horizontal Pulley System

    Homework Statement A group of construction workers are building a house and want to lower down an excess bundle of 1400 kg bricks to ground. They tie one end of a rope to the bundle of bricks, loop it through a pulley, and tie the other end of the rope to a toolbox. If the coefficient of...
  31. Varun Narayanan

    Shouldn't force pairs equal in magnitude cancel each other?

    Homework Statement If every action has an equivalent and opposite reaction, how is that anything ever happens ?. Shouldn't force pairs equal in magnitude cancel each other? Homework EquationsThe Attempt at a Solution I tried drawing a free body diagram, but it didn't help much. This is a...
  32. parshyaa

    Newton's third law is frame independent?

    how can you say(prove) that Newton's third law is frame independent. you will say that as real forces are frame independent , therefore Newton's third law is also frame independent, so tell me how real forces are frame independent?
  33. Awais gul

    Why Does a Ball Bounce Back from a Wall But Not from Sand?

    Hello, I am Awais gul, a student of science subjects and learning the basic of Physics , chemistry and computer science.I have a Question about Newton's third law of Motion. Question : If we Throw a ball on a wall it goes back but if we throw a ball on sand why it does not go back? Newton's...
  34. Mr Davis 97

    Newton's Third Law textbook -- question about the sign of a force

    Imagine that I am pushing on a wall. Then my textbook says that by Newton's third law, ##F_{AB} = F_{BA}##, where I am B and the wall is A. Isn't this wrong? Shouldn't it be that ##F_{AB} = -F_{BA}##?
  35. Akshat

    Exceptions to the Laws of Physics: Newton's Third Law

    I've been noticing that there are exceptions to every scientific law. For Example, with Newton's First and Second Laws are totally invalid at the quantum level, but I'm failing to find such an example for Newton's Third Law. Is anyone able to help me?
  36. Evangeline101

    Newton's third law: Rower makes boat move forward

    Homework Statement Use Newton's third law to explain how a rower makes his rowing boat move forward through the water. Homework Equations no equations required The Attempt at a Solution According to Newton's third law, there is a reaction force for every action force. In this case, the...
  37. E

    Understanding Newton's Third Law: Exploring Confusion and Misconceptions

    I am not the best at drawing, I know. If A exerts a force of 200N on B, then by Newton's third law, 200N would be exerted on it. Similarly, if B exerts 100N on A, by Newton's third law, 100N would be exerted on it. Then why wouldn't both accelerate in opposite directions with accelerations equal...
  38. F

    I Sum of internal forces equals zero

    This is probably a very trivial question, but my brain isn't "playing ball" today so I'm hoping someone can help me with this. Suppose I have a system of ##N## mutually interacting particles, then the force on the ##i##-th particle due to the other ##N-1## particles is given by...
  39. A

    Is Newton's third law of motion consistent with GTR?

    If an apple hanging in the tree has only reaction upwards, then what will happen to a Newtons third law? how is it that there is no "force " downwards, but have spacetime curvature which "mimics" a force. why can't spacetime curvature be itselt a force?
  40. B

    Newton's Third Law and Car Crashes

    Homework Statement In terms of Newton's Third Law, why is it better for 2 cars of the same mass, both going 50km/h to crash into each other than for 1 car going 100km/h to crash into a wall? Homework Equations Newton's Third Law F=ma The Attempt at a Solution I have no idea. I'm really confused
  41. M

    Identifying Newtonian force pairs

    Homework Statement Hello, Regarding the idea of Newtonian force pairs, imagine a bottle resting on a table: Weight force exerted on bottle by Earth and normal contact force on bottle by table are not Newtonian force pairs because they are different types and act on the same object. Weight force...
  42. B

    Newton's Third Law and a canoe

    Homework Statement Explain why when a person in a canoe throws a package onto the shore, the canoe moves away from the shore? Homework Equations No equations, just Newton's third law. The Attempt at a Solution I think it's because the person applies a force onto the package and the package...
  43. faiziqb12

    An atomic explanation of Newton's Third Law of Motion

    let's look at force at the atomic level to understand the Newtons third law of motion. I'll use Helium atoms as an example. Now imagine we start with one atom HE2 stationary, and throw another atom HE1 at it.It is the velocity of HE1 that affects the motion of HE2 , because the system of these...
  44. D

    Newton's third law force homework

    Homework Statement Emma and Caleb are playing a game. I don't know what the name is, but they put their fingers together like a hook and then pull. Caleb won. does that mean that Caleb pulls with greater force than Emma pulls him? Caleb wins. Does it mean that he pulls with greater force than...
  45. ecoo

    Trouble with Newton's Third Law and motion

    Hey guys, I have some confusion with Newton's Third Law. I understand that, for example, if I push against a wall the wall feels my force and the wall puts an equal force onto me. One confusion comes from the horse cart problem, or any motion problem that uncludes one object pushing another...
  46. L

    Newton's Third Law: Equal Forces & Stationary Object?

    The Newton's third law states that the force exerted by body A on body B is equal to the force exerted by Body B on Body A . In this case, isn't the object supposed to be stationary because equal forces are acting on both sides? Can someone please explain to me . Thanks
  47. astrololo

    Newton's Third Law: Explaining Why You Can't Lift Yourself

    Homework Statement I need to explain why I can't lift myself by pulling my shoes' laces. Homework Equations Newton's third law The Attempt at a Solution My explanation is that by pulling on your laces, there is always going to be a force that goes against your force, so this is why you...