What is Newtons third law: Definition and 53 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

    B Why is KE not conserved when momentum is?

    Its clear in elastic collision that both KE and momentum is conserved. Bodies exchange their velocities. It is seen clearly in this video. There is no decrease in speed. Total KE is constant. But in an inelastic collision momentum is conserved again but not the KE. There is loss in KE (I guess...
  2. O

    B Gravitational force between two masses

    Hello everyone. Probably this question is trivial, but nevertheless I am confused about Newtons law of motion: $$F=G\frac{m_1m_2}{r^2}$$ Now, some sources say, that F is the force between the two masses m1 and m2. Other sources say, that F is the force that m1 exhibits on m2. But isn’t this a...
  3. Callista

    Why doesn't Newtons third law also apply to the frame?

    Homework Statement A locomotive has broken through the wall of a train station. During the collision, what can be said about the force exerted by the locomotive on the wall? a)The force exerted by the locomotive on the wall was less than the force exerted by the wall on the locomotive. b) The...
  4. Zeke Bevan

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

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  5. DracoMalfoy

    Applying Newtons Laws: Incline, static friction, kid on sled

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  6. T

    I Dropped objects hitting the ground at the same time?

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  7. E

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

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  8. M

    Magnetic levitation with permanent magnets

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  9. N

    Problem applying Newtons third law

    Homework Statement Two people each with a mass of 70kg are wearing in line skates and holding opposite ends of a 15m rope. One person pulls forward on the rope by moving hand over hand and gradually reeling in more of the rope. In doing so, he exerts a force of 35N (backwards) on the rope. This...
  10. G

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  11. Surya97

    I How do you find the force that will be exerted by something?

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  12. M

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  13. W

    Problem understanding Newtons third law ...

    Homework Statement [/B] Three blocks move on a frictionless surface and a 42N force acts on the 3kg block. The force exerted by the 1 kg block on the 2 kg block is 14N. What is then the force exerted by the 2 kg block on the 1kg block? If it is 14N, then shouldn't the 2kg block stay in...
  14. I

    Newtons Third Law Involving three objects

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  15. D

    Violation of Newtons third Law and conservation of momentum

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  16. Science2Dmax

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  17. M

    Understanding Newton's Third Law: The Cause of Balloon Movement

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  18. itchybrain

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  19. R

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  20. K

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  21. S

    Newtons third law and quantum mechanics

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  22. N

    Newtons third law question- Action reaction pair

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  23. V

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  24. A

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  25. S

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  26. S

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  27. J

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  28. C

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  29. A

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  30. S

    What is the acceleration of the minimum mass needed to prevent slipping?

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  31. K

    Newtons third law and elevators

    If two objects are stacked in an accelerating elevator what is the force equal and opposite to the normal force on the top object? I was thinking that it has to be either the force of gravity or contact force between the two objects could it be anything else?
  32. I

    Newtons third law regarding spacecraft engines

    Hello PF members, I was wondering whether Newtons third law was exclusive to spacecraft thrust; space being empty and without enough particles for a force to push against. That in order to produce thrust in space, you would need to carry the fuel that you plan to eject with you...
  33. T

    Violation of Newtons Third law

    I've heard many times that Newtons third law is violated in electromagnetism, but I never knew why. I found this link...
  34. P

    Work Done and Force and Newtons Third Law of Motion

    When a car is moving, the engine is transferring Chemical Energy to Kinetic Energy. He is doing work against the force of friction. If the force of friction is 210 N and the engine exerts a pull force for 10 M it transferred 2100J of Chemical Energy from its fuel to Kinetic Energy, considering...
  35. E

    Newtons third law of motion

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  36. 2

    What is the Reaction to the Force of Gravity on a Suspended Lead Block?

    Homework Statement A lead block is suspended from your hand by a string. The reaction to the force of gravity on the block is the force exerted by: (a) the string on the block (b) the block on the string (c) the string on the hand (d) the hand on the string (e) the block on Earth...
  37. Z

    Magnet, wire and Newtons third law

    I have been thinking about a bar magnet and an infinite wire in free space. The wire is positioned below the north pole of magnet and carries a current. 1) The force from the magent on the wire: Due to the magnetic field, the Lorentz force on the electrons in the wire creates a force on the...
  38. Y

    Question about Newtons third law

    in Newtons third law of motion every action has equal and opposite reaction so my question is that when block hits another block the blocks exert equal force on another so why they move they had to stop immediately.
  39. B

    What is the force exerted by the locomotive on the first car?

    Homework Statement A train consisting of two cars pulled by a locomotive experiences an acceleration of 0.33m/s^2 [fwd]. Friction is negligible. Each car has a mass of 3.1 * 10^4 KG. a. ) Determine the force exerted by the first car on the second car. b. ) Determine the force exerted by...
  40. L

    Newton's Third Law: Why Is There Motion?

    I was wondering, if Newton's third law is true why is there motion?
  41. C

    Need help with Newtons third law and friction.

    Homework Statement Hi guys i am trying to do question number 4 but i have not done a Question liek this in a very long time so i am forgetting where to start. http://img405.imageshack.us/img405/6976/101420092845.jpg" Image is to big for the forum so its easier that way. Homework Equations...
  42. J

    What is the force between two masses connected by an external force?

    Homework Statement Two boxes are in contact with each other, both different mases, and are both being pulled by an external force, F. M1M2--------->F I can calculate the acceleration of the system but the Question asks to calculate the force between the mases, ie Newtons third law. If...
  43. S

    Exploring the Relationship Between Rockets and Newton's Third Law

    Why do rockets behave so well in accordance to Newtons third law
  44. D

    Rope, tension and Newtons third law

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  45. S

    Why are N and mg equal in magnitude?

    Hey guys, was just wondering if you could explain the answer to this question. So basically we have a picture of a book resting on a table. On this book there is the weight force downwards and the reaction force upwards, both acting on the center of mass. The question: A class of students is...
  46. S

    Why Doesn't a Windshield Shatter When a Fly Hits It at High Speed?

    Hey guys, I was just wondering if you could help me with a question that has been bugging me for a while now. I've thought of many possibilities to the answer yet i still can't find the right one. The question is that, when a fly strikes the windshield of a truck moving at let's say 60km/h, why...
  47. N

    Understanding Newton's Third Law: Why Does a Rocket Go Up?

    I have spent a ton of time researching my question but i can't find the answer i know that for every action there is an equal and opposite reaction i know if you have a tire on the ground and when it starts spinning it pushes so that the concret will go the other way but the ground it to...
  48. D

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    It says that every reaction there is a opposite and equal reaction. Why do things still move if everything is opposite and equal?
  49. E

    Calculating Acceleration and Tension in a Pulley System with Inclined Planes

    Please help me because my teacher has not explained how to do questions involving angles. Blocks X and Y of masses mx=5.12kg and my=3.22kg are connected by a fishing line passing over a frictionless pulley. Show that block X slides up the incline (35.7 degrees above the horizontal) with...
  50. W

    Solving Newtons Third Law: Mass and Force in Blocks A & B

    Homework Statement Two blocks are lined up in a row. Block A a mass of 1kg, and Block B with a mass of 2kg. If block a is A is pushed forward by 12N, how much force does Block B exert on Block A. Friction can be nelected. Homework Equations Newtons Third law F(A on B)=-F(B on A)...
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