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. T

    Newton's Third Law, Universe Imploding?

    If everyone believes that there's an equal and opposite reaction for everything, should the universe go out to a point and stop; or will it end up coming back to its origins?
  2. S

    Newton's Third Law and Pulley system

    The lower block in the figure is pulled on by a rope with a tension force of 20 N. The coefficient of kinetic friction between the lower block and the surface is 0.30. The coefficient of kinetic friction between the lower block and the upper block is also 0.30. What is the acceleration of the...
  3. S

    Blocks connected by rope, Newton's Third Law

    The figure shows two 1.0 kg blocks connected by a rope. A second rope hangs beneath the lower block. Both ropes have a mass of 250 g. The entire assembly is accelerated upward at by force. What is F? What is the tension at the top end of rope 1? What is the tension at the bottom end of rope 1...
  4. E

    Do all cases of Newton's third law follow both the strong and weak forms?

    [SOLVED] Newton's third law Homework Statement My book gives two forms of Newton's Third Law: Weak Form: The forces exerted by two particles \alpha and \beta on each other are equal in magnitude and opposite in direction Strong Form: The forces exerted by two particles \alpha and \beta on each...
  5. J

    What Are The Magnitudes of Forces in a Tractor Pulling Contest?

    Homework Statement In a tractor pulling contest, a 2300-kg tractor pulls a 4900-kg sledge with an acceleration of 0.61 m/s^2. If the tractor exerts a horizontal force of 7700 N on the ground, determine the magnitudes of (a) the force of the tractor on the sledge; (b) the force of the sledge...
  6. S

    Newton's Third Law of pushing a box

    Ok, I'm getting a lot of mixed messages about the reactionary forces involved with the 3rd law. if i push a box at a constant speed with 100N of force, is the friction that resists the motion the opposite but equal (100N) reaction force? isn't the friction force LESS than the push force because...
  7. P

    What is the correct method for solving this Newton's Third Law problem?

    [SOLVED] Newton's Third Law problem Homework Statement Three boxes, A, B, and C, are positioned next to each other on a horizontal, frictionless surface. An applied force acting on box A causes all the boxes to accelerate at 1.5m/s^{2}[right]. Calculate the force that box B exerts on box A...
  8. P

    Is the Force on Box A Correctly Calculated?

    [SOLVED] Newton's Third Law problem Homework Statement Three boxes, A, B, and C, are positioned next to each other on a horizontal, frictionless surface. An applied force acting on box A causes all the boxes to accelerate at 1.5m/s^{2}[right]. Calculate the force that box B exerts on box A...
  9. DaTario

    Newton's Third Law - Weak Form Application

    Hi all, I would like to know in which circunstances the third law applies just in the weak form. Thank you DaTario
  10. V

    Conservation of Angular Momentum from Newton's third law

    I am reading Introduction to Mechanics by Kleppner and Kolenkow. I am now reading the articles dealing with conservation of angular momentum. I am not satisfied with the articles dealing with how the third law does not lead to conservation of angular momentum. Could anyone please throw some...
  11. M

    Newton's Third Law - Interacting Systems

    Homework Statement The 1.0 kg block in the figure (attached) is tied to the wall with a rope. It sits on top of the 2.0 kg block. The lower block is pulled to the right with a tension force of 20 N. The coefficient of kinetic friction at both the lower and upper surfaces of the 2.0 kg block...
  12. V

    Newton's Third Law: A Car Pushes A Truck

    Homework Statement A 1000 kg car pushes a 2000 kg truck that has a dead battery. When the driver steps on the accelerator, the drive wheels of the car push against the ground with a force of 4500 N. Rolling friction can be neglected.Homework Equations F=ma acceleration of car = acceleration...
  13. T

    Is Newton's Third Law always true in every scenario?

    Are the forces ALWAYS exactly opposite of each other? Is there a way to accelerate a mass upwards, while having the "recoil" force travel on a horizontal plan or somehow completely eliminated? Any and all information concerning this is much appreciated. Rob
  14. L

    Is Newton's Third Law Violated in Electromagnetism?

    Hi, I was wondering, is Newton's third law true? Let's say I've an electromagnet turned on at a constant intensity for a sufficiently long time for the field to have reached a point some distance away. At that point is another electromagnet, but turned off. Now I turn on the second...
  15. M

    Forces between two current elements and Newton's third law

    Forces between two current elements and Newton's third law Consider two non parallel wires each carrying a current which located at one plane surface. According to electromagnetic theory, the forces between two straight and solid wires mentioned can be estimated as follow: F= i . (L x B)...
  16. L

    Understanding Newton's Third Law

    Hello. I'm in year 12 at the moment doing my AS level. I'm having some troube understanding Newton's Third Law. Would anyone mind fully explaining it to me please? Thanks :)
  17. J

    How Does Newton's Third Law Apply to Stacked Blocks in an Elevator?

    Homework Statement Three blocks are stacked on top of each other inside an elevator as shown in the figure. Answer the following questions with reference to the eight forces defined as follows. the force of the 3kg block on the 2kg block, F of 3 on 2, the force of the 2kg block on...
  18. T

    Newton's Third Law: Understanding Forces Between Compact Cars and Vans

    Homework Statement All small compact car is giving a push to a large van that has broken down and they travel along with equal velocities and accelerations. While the car is speeding up, is the force it exerts on the van larger than, smaller than, or the same magnitude as the force the van...
  19. Y

    SAT Question concerning Newton's Third Law

    I didn't understand the explanation, and think perhaps the book is wrong. A person is standing on a horizontal floor fells two forces: the downward pull of gravity and the upward supporting force from the floor. These two forces are: I put A) have equal magnitudes and form an...
  20. F

    Is Newton's third law disproved

    Hello I am Fusina, I wish to ask you Three questions which bother my mind. Its all about Newton's third Law. Is Newtons third law always obeyed? Is there an example of two moving particles, who's velocities are such that their mutual magnetic force doesn't obey Newton's 3rd law...
  21. G

    Newton's Third Law of women standing

    Homework Statement A woman is standing on a flat section of ground. Her weight is 500N. Newton's third law states that there must be an equal an equal and opposite force to her weight, which is A. The Earth exerting an upward force of 500 N on the woman. B. The woman exerting an upward...
  22. A

    A Question About Newton's Third Law

    Please excuse the newbieness of this question. I recently began studying physics. Does Newton's third law apply in the game of Croquet when one applies force to a static ball between the club and a second ball, thus causing the second ball to continue the force applied by the static ball?
  23. F

    Newton's Third Law: Equal & Opposite Forces?

    Newtons third law states that for every force there is an equal an opposite force. This doesn't mean that these forces are balanced though? Things like cars or people walking can still accelerate so these forces can't be balanced?
  24. G

    Solving an Unexpected Problem: Newton's Third Law

    my professor kind of sprung this problem out of nowhere for the class, and I'm having just a little trouble getting started? The problem goes like this: A current wire pointing in the +j direction has an Idl. A second current wire in the +i direction has a different Idl. The question he...
  25. S

    Identifying Action Reaction Forces: A Newton's Third Law Analysis

    Homework Statement "Identify at least five pairs of action reaction forces in the following diagram" Source: http://www.physicsclassroom.com/Class/newtlaws/U2L4b.html problem 2 Homework Equations Newton's third law The Attempt at a Solution 1) Man and monkey 2)monkey and scale...
  26. N

    Calculating Acceleration and Velocity of a Saturn V Rocket

    A Saturn V rocket has a mass of 2.75 x 106 kg and exerts a force of 3.3 x 107 N on the gases it expels. Determine: a) the initial vertical acceleration of the rocket b) its velocity after 8 s c) how long it takes to reach an altitude of 9500 m. Ignore mass of gas expelled and assume g...
  27. T

    How Long Does It Take Two Accelerating Objects to Meet in Space?

    Here is the question: Astronauts have connected a line between their 3280 kg space tug and a 6100 kg asteroid. Using their ship's engine, they pull on the asteroid with a force of 490 N. Initially the tug and the asteroid are at rest, 490 m apart. How much time does it take for the ship...
  28. K

    Newton's Third Law, Gravity, and the Planck Length

    If you have a object that is about 10^36 less massive than the Earth (about 10^-12 kg) and say it exerts a force on the Earth about 10^-11 Newtons, this would cause the Earth to accelerate about 10^-36 m/s^2. If this object was away from earth, and say if these objects were initial stationary...
  29. B

    Questions about Newton's third law

    I'm not questioning his law or anything, I just need to understand a few things about it. I understand the basic concept of the law, for every force, there is an equal force in the oppostite direction. But what provides the reaction force in the following situations? A car driving, has an...
  30. L

    Current status of Newton's Third Law of Motion

    Hi, I was wondering, what is the status of Newton's Third Law of Mechanics in modern physics? I'm told that it does not always hold. In that case, when and how does it break down? Thanks. Molu
  31. M

    What Happens if Newton's Third Law is Violated?

    What would happen if (if ever) when the action force is greater than or less than the reaction force and were not equal in magnitude and were not equal in size?
  32. H

    Inclined Plane Newton's Third Law

    Can anyone help me with this problem? thank-you A box slides down a 39° ramp with an acceleration of 1.32 m/s2. Determine the coefficient of kinetic friction between the box and the ramp
  33. D

    Violation of Newton's Third Law

    Are there any examples of violation of Newton's third law? The two examples that I know are : 1) Two masses moving relativistically apart from each other. 2) Two charges moving in mutulally perpendicular directions. Can anybody further elaborate on these exmaples with proper explanation and...
  34. F

    Applying Newton's third law

    I hate all of Newton's laws questions. They are so complicated. Can anyone please help me with this question. A boy pushes a lawn mower (m=17.9 kg) starts from rest across a horizontal lawn by applying a force of 32.9 N straight along the handle, which is inclined at an angle of 35.1 degrees...
  35. D

    Newton's Third Law/ Circular Motion

    Question: Mass m1 on the frictionless table is connected by a string through a hole in the table to a hanging mass m2. With what speed must m1 rotate in a circle of radius r if m2 is to remain hanging at rest? My answer: Fnetym2 = T - m2g For m2, a = 0 Therefore, 0 = T - m2g T = m2g...
  36. D

    Newton's Third Law of toboggans

    Three toboggans are attached to each other with ropes. A force of 145 N[forward] is pulling the first toboggan, which has a mass of 42kg. A second toboggan, with a mass of 30kg, is attached to the first second toboggan. A third toboggan with a mass of 24kg, is attached to the second toboggan...
  37. N

    Solving the Pushy Astronaut Puzzle: Newton's Third Law

    Pushy Astronaut -------------------------------------------------------------------------------- a) An astronaut of mass 70 kg pushes off a satellite of mass 420 kg, exerting a force of 120 N for the 0.83 s they are in contact. What is the speed of the astronaut after they have separated...
  38. H

    Understanding Newton's Third Law: Exploring the Forces in a Collision

    So I have a homework question that says: A 100 kg hockey player a 112kg hockey player collide with each other each traveling with a force of 50N. The textbook answer says that the acceleration of the 112kg is 0.89m/s^2 and the acceleration of the 100kg player is -1.0 m/s^2. I'm confused...
  39. P

    Exploring Newton's Third Law: Rope with a Wheel & Work/Potential Energy

    One question related to Newton's third law, v t p u o u o It is a rope with a wheel,v. If 50N is applied from u pulling towards the ground, what's...
  40. D

    Newton's Third Law of a bullet

    Yesterday, one of my cousins asked me about Newton's third law. In this discussion he asked me this question: Why does not a bullet bounce back after striking the surface of glass as it does in the case of steel, considering that in both the cases the force with which it strikes the glass or...
  41. U

    Exploring Newton's Third Law: How We Move Foward

    Why we move foward: The applied force from our foot pushs on the Earth and therefore, according to Newton's 3rd law, the Earth pushes back on me to make me move foward. Yeah, this is like basic material, but just need a verification, is this correct?
  42. T

    Understanding Newton's Third Law: The Case of Box Pushing

    Newton's third law states something along the lines of "For every action, there is an equal and opposite reaction." The problem I have is that I don't understand how it stays true for something as simple as pushing a box. If a push the box (action), how can it be giving an equal opposite...
  43. S

    Normal Force Between Block & Table: Free Body Diagram Explained

    If there was a block on a table what would be the normal force between the block and the table. If you could describe the free body diagram that would be great too.
  44. W

    Frame of reference and Newton's third law

    I can not find the answer for this In a rotating frame of reference, Newton's first and second laws remain valid if we assume that a fictitious force equal to mv^2/r is acting. What effect does this assumption have on the validity of Newton's 3rd law?
  45. benzun_1999

    Do Neutrinos Violate Newton's Third Law?

    hi, Before i begin i am not sure where to put this post wether here or in the general section. Is Newtons 3rd law broken? Neutrinos don't follow this rule. the neutrinos can just pass through particles. so don't they break the 3rd rule? :confused:
  46. P

    Newton's Third Law equal and opposite forces

    Although the equal and opposite forces stated in Newton's third law are sometimes referred to as an action-reaction pair in reality they do not share a cause and effect relationship. They arise simultaneously, abide together and disappear simultaneously. Imagine you are spinning a washer...
  47. M

    What are some examples of Newton's third law in action?

    hi...my teacher has offered an optional project of creating something that used Newton's third law.. I've been trying to think of something but i don't have any ideas... i have to make something .. and umm it can't be a fan on wheel blowing against a wall because that's the example in the book...
  48. B

    Exploring Newton's Third Law of Motion

    can someone explain to me Newton's thrid law of motion?
  49. M

    Newton's Third Law & aerodynamic lift

    I was engaged in a discussion where someone claimed that Newton's Third Law does not apply to the lift created by air flow over a wing. My argument goes as follows: After all the equations, etc. involving fluid dynamics and Bernoulli's principle there is an upward force on the wing...
  50. Z

    Understanding how Newton's Third Law is applied?

    Hello! I've just begun attempting to teach myself physics, and I'm finding it difficult to understand Newton's third law. Would someone be so kind as to shed a little light on the situation? The idea of the equal and opposite reaction for every action makes perfect sense to me in some...
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