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

    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...
  5. DracoMalfoy

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

    Homework Statement [/B] A child sits on a sled (50kg) on a snowy hill with an incline of 5 degrees. The coefficient of static friction is 0.05. What downward force must a parent apply parallel to the normal force, to prevent the child from sliding down the hill? (Answer: 366N)Homework Equations...
  6. T

    I Dropped objects hitting the ground at the same time?

    okay, so I’ve had this random thought. We have all been told that objects fall to the ground at the same speed, even if they have different masses. While it’s true that any two objects, regardless of mass, will accelerate towards Earth at the same speed, that doesn’t mean the Earth is...
  7. 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...
  8. M

    Magnetic levitation with permanent magnets

    so i am literally one step away from producing a floating skateboard powered by permanent magnets, so here's what i wanted to check, to make sure I haven't missed anything obvious vertical lift achieved by using vertical repelling fields rotational motion is controlled by counterbalancing the...
  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

    In 3rd step of Carnot cycle, work done on the piston is pdV?

    Hello. First, look at the figure describing Carnot's cycle. In 1st step (A → B) and 2nd step (B → C), I fully understand that the work done on the pistol (surrounding) by the gas (system) is dW = pdV where dV = Adl since F = pA is the only force on the pistol from the gas (I assumed there...
  11. Surya97

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

    Given a certain situation, how do you figure out the force that will be exerted onto something (you can only use F=ma to find the average net force over a certain time period, and only after you know the acceleration)? For example, if you have a non accelerating object colliding with another non...
  12. M

    Finding number of rotations till arm breaks (circular motion)

    Homework Statement A 400 g steel block rotates on a steel table while attached to a 1.20 m -long hollow tube. Compressed air fed through the tube and ejected from a nozzle on the back of the block exerts a thrust force of 4.91 Nperpendicular to the tube. The maximum tension the tube can...
  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

    Homework Statement Three boxes, A, B, C, are positioned next to each other on a friction-less surface, an applied force acting on box A causes all the boxes to accelerate at 1.5 m/s/s. Calculate the force exerted by box C on box B. ma= 8 kg mb=10 kg mc= 5 kg Homework Equations Fnet=MA The...
  15. D

    Violation of Newtons third Law and conservation of momentum

    Three High energy Photons spontaneously convert into electron positron pairs, The electron on the left marked A is created and and destroyed within a short interval, sending an electric field burst towards electron B shortly after it's created, just enough so that Electrons B's electric field...
  16. Science2Dmax

    What is the opposite action to velocity

    To every action, there is an opposite and equal reaction. This is Newton's third law. What is the opposite reaction of the conversion of kinetic energy to momentum. Don't say stasis because motion is greatly linked to time. If time freezes, it isn't the opposite of it moving forward bot. Ore it...
  17. M

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

    If you fill a balloon with air and let it go, since the initial momentum was 0 and the air in the balloon is pushed in one direction, the balloon must move in the other direction. What is the cause of the force that causes the balloon to move this way?
  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. R

    Calculating Normal Force for Blacksmith's Anvil - Newton's Third Law

    Homework Statement [/B]Find the normal force exerted on a 68.2 kg blacksmith's anvil by the level block of wood that is supporting it.Homework Equations [/B]f=ma Ff=uFnThe Attempt at a Solution [/B]I have tried to find all the forces acting on the anvil while using its mass but I am not...
  20. K

    Conceptual questions on Newtons third law

    Hello! I'm a high school teacher in physics. I will teach Newtons third law this coming week and found the following question (regarding a horse pulling a carriage): "If the force on the carriage is equal and opposite to the force on the horse how can the horse pull the carriage? Is the...
  21. S

    Newtons third law and quantum mechanics

    dear sir, we know that third law of motion says that every action has a equal and opposite reaction. quantum mechanics tells us that it is possible not predict the position as well as the speed of electron. i like to know if electrons ,protons and neutrons form a system called atom...
  22. N

    Newtons third law question- Action reaction pair

    Homework Statement A 2000kg car pushes a 1000kg garbage can at constant speed . The car exerts a force of magnitude CB on the garbage can, and the garbage can exerts a force BC on the car. Which force has greater magnitude, or are they equal? explain Homework Equations Fcb=-Fbc The...
  23. V

    How Do Newton's Third Law and Force Calculations Apply to Hockey Players on Ice?

    Homework Statement two hockey players are standing on ice. Player A is 100kg and player B is 112kg, Player A pushes B with 40N and player B pushes A with 50N. what are the action/reaction forces? what are the accelerations?Homework Equations F = maThe Attempt at a Solution player A has a...
  24. A

    Is Newton's Third Law a Manifestation of Fundamental Laws?

    Sometimes I wonder if it actually possible to prove the action-reaction principle. I do know that Newtons laws are empirical but then if you think about the third law it says that in interaction between two bodies they exert equal and opposite forces on each other. Now what happens in the...
  25. S

    Newton's Third Law Explained: Is it for Cushions?

    Please anybody explain me ... if i punch a soft cushion hardly say "F" force will thw cusion hurt me with the same ... every body says "NO" ok all right everybody says "you, in this case have not applied the "F" force because most part of it is wasted in changing the shape of cusion " so here...
  26. S

    Newtons third law, elevator question (free body diagram)

    Homework Statement An elevator containing 3 passengers(with a mass of 72kg 84 kg and 35 kg) has a mass combined of 1030 kg . The cable attached to the elevator exherts an upward force of 12000 N. But the friction opposing the motion of the elevator is 1400 N. a) Draw a free body diagram...
  27. J

    Newtons third law and conservation of momentum

    Whats more basic - Newtons third law or conservation of momentum You can prove Newtons third law by conservation of momentum but you can also prove conservation of momentum by Newtons third law. What comes first?
  28. C

    Newtons third law and collision of bodies

    Hi friendz I m new to this forum I have a problem in a question which is as follows- If a body of mass m collides with a body of mass 2m it changes its direction of motion buy when it collides with a body of same mass m it stops and when it collides with a body of mass m/2 it does not change...
  29. A

    Coloumbs law and Newtons third law

    Coloumbs law states that the force exerted by two charged particles on each other is given by f = kq1q2/d^2 Now say two charged particles (charge type irrelevant in question) x and y are d metres from each other then y exerts a force f on x. But since x is also exerting f on y then won't it...
  30. S

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

    Figure shows a block of mass resting on a slope. The block has coefficients of friction and with the surface. It is connected via a massless string over a massless, frictionless pulley to a hanging block of mass 2.0 kg. i got this first one: What is the minimum mass that will stick and not...
  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

    How Does Newton's Third Law Apply to a Particle on Different Slopes?

    Homework Statement A particle of mass 4kg is being towed at a constant speed up a rough plane inclined at 30 degrees to the horizontal by a force 4g N acting parallel to the slope. At the top of the slope the particle moves onto a rough horizontal slope with the same coefficient of friction...
  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

    This is more of a conceptual question, meant to help us understand how tension forces work. Homework Statement Assume that segment R exerts a force of magnitude T on segment L. What is the magnitude FRL of the force exerted on segment R by segment L? Give your answer in terms of T and other...
  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

    Why Do Objects Move Despite Equal and Opposite Reactions?

    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)...