What is Newton 3rd law: Definition and 51 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. cianfa72

    I Spring balance analysis from a system point of view

    Hi, I was thinking about a simple scenario in the framework of Newton (classic) mechanics. Take a spring scale/balance fixed at one end (wall) with a body attached to the other end's hook. From an analysis point of view we can consider as "system" the spring scale + the wall + the body. Then...
  2. A

    I Star with quadrupole in a binary system violates Newton's 3rd Law?

    Assume that, in a binary system, one (and only one) of the two stars has a non-zero quadrupole moment. Then the other star feels the usual gravity force $F_g$ plus an additional force $F_q$ coming from the quadrupole potential. On the other hand, the first star feels only the usual gravity force...
  3. BurpHa

    A greater force? (A train engine pulling two identical cars)

    My choice is C, which is "equal to." Here is my reasoning: The engine pulls on car 1, so car 1 is having a force acting on it, which pulls car 1 forward. Likewise, the same amount of force is applied on car 2 by car 1 by moving forward with the same amount of force from the beginning. So it is...
  4. Clockclocle

    I Stuck in understanding Newton's 3 laws

    I'm stuck to understand 3 laws of Newton. It doesn't make sense to me. 1. Suppose the case when a person stay in a rest vehicle.When we accelerate the car the person still at rest so the person has to move to the tail of the vehicle cause his intertia of staying rest. If we decrese the...
  5. lela

    Is the force exerted by a pivot always towards the center of mass?

    I thought that the force by the pivot A on the pole AB would be the reaction force to the x-component of the gravitational force on AB. This would mean that the force by the pivot would be parallel to the pole, but in my notes from class the force vector seems to be more along the bisector of...
  6. Rikudo

    A mass collides with an inclined plane

    In my textbook, it is stated that "if an object elastically hit an frictionless inclined surface with angle between the vector of initial velocity and an imaginary line that is perpendicular to the surface ##\alpha##,then the angle between the line and final velocity vector will also be...
  7. Olivia Lam

    What is the reaction force between the ring and the hoop?

    a.)N cos θ=mg N sin θ=mrw^2 sin θ cos θ=g/rw^2 b.) My question is reaction force =N ? or =F=mg tanθ ? If it is N then N=mg cosθ =mg^2/r w^2 or N=mg/cosθ =mrw^2 ? Thank you
  8. TonyCross

    Calculating Theoretical Reaction Force of Ball Traveling in Semi-Circle

    Summary:: Theoretical reaction force due to ball traveling in a semi-circle circum Hi, Can anyone please help me with the following question: Setup... 1 x length of pipe bent 180 degrees with an inner diameter of 22mm, it's radius is 2mtrs, the mass of this object is 100kg. Total length of...
  9. Saptarshi Sarkar

    Calculating Tension on Strings: Results & Confusion

    Attempt: By drawing the Free Body diagrams and calculating the different tensions, I got the following results ##T_1=\frac{(M_1+M_2)}{2}g## ##T_2=\frac{\sqrt 3(M_1+M_2)}{2}g## ##T_3=M_2g## But, I am not sure what the answer is as although ##T_2>T_1## but ##T_3## does not depend on ##M_1##...
  10. Adesh

    I How a rigid body causes a reaction force?

    The definition of rigid body says it cannot be deformed (theoretically). Now, Newton’s third law is caused (I mean the reaction force is caused ) due to the deformation of the body. What I have learned is that every body is like a spring, when we push on it we compress it and hence feel a...
  11. xWaldorf

    Exploring Newton's 3rd Law: Mass & Reaction

    So there's something I'm missing when I think of Newton's 3rd law of motion. If all forces between two objects exist in equal magnitude and opposite direction, how is it that, for example, when I'm driving my car, the car "runs through" all the air molecules, and they seemingly don't exert the...
  12. 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...
  13. 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...
  14. T

    Forces - Space Shuttle Takeoff Calculations

    Homework Statement [/B] The total mass of a space shuttle and its launch vehicle is M=2000t. a) What must be the minimum size of the thrust force, to make the rocket move? b) The actual thrust of the rocket is F=30MN. What is its acceleration in the beginning? c) Assume that a mass of a...
  15. J

    What force propels the runner forward?

    Homework Statement A person is running on a track. Which of the following forces propels the runner forward? a) The normal force exerted by the ground on the person b) the normal force exerted by the person on the ground c) the force of friction exerted by the ground on the person 2. Homework...
  16. AidenFlamel

    Writing: Input Wanted Scientific use of Chi/Qi/not so espiritual energy

    Hi, I wanted you guys to tell me whether you like this idea I had and if you have any ideas on the implication of it, and whether it would work as I describe it or not. So, in this world I created, there is a basic understanding of physics by the scholars, but most people don't even understand...
  17. bornofflame

    Rank forces according to magnitude: Newton's 2nd/3rd laws

    Homework Statement Two crates, A and B, are in an elevator as shown. The mass of crate A is greater than the mass of crate B. a. The elevator moves downward at constant speed. ... iii. Rank the forces on the crate according to magnitude, from largest to smallest. Explain your reasoning...
  18. HydroMarioUSA

    Newton's 3rd Law (Pulleys - 1)

    Hi, I'm new here. Could someone provide some guidance on how to start this problem?
  19. T

    I Object suspended over an empty bucket

    I was recently tutoring a first year student, and a question of her assignment was as follows: Suppose that that you have a bucket of water over a scale. If you then partially submerge an iron rod in the water, while holding the rod so that it does not touches the bucket, will the reading on...
  20. D

    Boyle's law pressure calculation

    Homework Statement An pillow is inflated with an pressure of let's say 20 hPa. Afterwards we place a weight of 10 Kg on the pillow, what is the pressure increase within the pillow? P athmosthere = 1013 hPa P cushion = 1013 + 20 = 1033 hPa m = 10 Kg g = 9.81 m/s^2 Amount of air in the pillow...
  21. K

    Why a book on a table is not an example of Newton's 3rd law?

    In a physics test in a Norwegian website of a school book, it says following: True or false? (1) According to Newton's 3rd Law, the weight of a book that is on a table is just as big, but opposite the force on the book from the table (normal force). FALSE. (2) According to Newton's 1st Law...
  22. A

    Newton's 3rd Law Dynamics Problem

    Homework Statement A 12.0-kg box is pushed along a horizontal surface by a 24-N force as illustrated in the diagram. The frictional force (kinetic) acting on the object is 6.0 N A) What is the acceleration of the object B) Calculate the value of the normal force acting on the object C) If the...
  23. J

    Torque and force on a tire iron

    So I was curious about something. Having a flat tire today and having to change my tire made me wonder. I understand torque and force are 2 different things though related they have separate meanings. So my question is if I have a 2 foot tire iron and I am torquing my nuts. Let's say they...
  24. W

    Newton's 3rd law and an impulse

    If I have a superball and a tomato of the same mass in my hand, and I let it go. The only force acting on my superball and tomato is gravitational force. F=ma, they have same mass so they will hit the bathroom scale with the same force. According to Newton's third law, the bathroom scale will...
  25. 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...
  26. 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...
  27. donaldparida

    Are Net Forces Greater When Both Objects A and B Are in Motion During Collision?

    Suppose there are two objects, denoted by A and B. When they collide with each other with the condition that A is stationary and B is in motion, B exerts an action force on A due to which A exerts a reaction force on B. Thus the net force exerted on A is FactionAB and the net force exerted on B...
  28. C

    I Force to stop a person on a treadmill

    We are doing a study & our calculated force is MUCH higher than our data. I'm wondering if our calculation is right. how much force it would take to stop a person in a wheelchair (83kg) going 6km/hr and stopping in 10 cm or within 2 sec. as an example? This is my calculation..Am I on the right...
  29. Shashank Holla

    Acceleration due to *equal* weights on pulley system

    Homework Statement [/B] Which of the following is correct? 1. The acceleration of pulley B is g/11 downwards 2. The acc. of pulley C is g/11 upwards 3. Tension in string passing over pulley A is 12.46g N 4. Tension in string passing over pulley A is 10g N Homework Equations F=m.a[/B]The...
  30. JustynSC

    Calculating average forces on an Object

    Homework Statement A 0.140kg baseball traveling 35.0 m/s strikes the catcher's mitt, which in bringing the ball to rest, recoils backward 11.0cm. What is the average force applied by the ball on the glove Homework Equations F=ma; FAB=-FBA The Attempt at a Solution Not sure where to start for...
  31. kidgt23

    Forces of motion, Newton 2nd law

    Homework Statement An 8.5 gram slug is fired East from a 0.357 magnum hand gun held 1.5 meters above the ground. The muzzle velocity (as the projectile leaves the gun) is 1400 ft/sec. The bullet is accelerated at a constant rate as it travels through the 10 cm long barrel. After leaving the...
  32. A

    Forces of gravitation of moon and earth + Newtons 3rd law

    In relation to two objects with a gravitational field on them (i.e. a planet and its moon), would there be two pairs of partner forces (the force of gravity exerted from each object and the resulting partner force from each force of gravity)? For example with the Earth and moon: 1. Force of...
  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. 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?
  35. Zack K

    B Action Reaction force on the earth

    "As you sit in your chair and study your physics (presuming that you do), the force of gravity acts downward upon your body. The reaction force to the force of the Earth pulling you downward is the force of your body pulling the Earth upwards". I read this somewhere just a bit ago and it's...
  36. F

    Newton's 3rd law & rocket propulsion (detailed explanation)

    I had someone ask me how rockets are able to accelerate in space and my initial answer was that the rocket fuel combusts and is heated into an energetic gas, the gas is accelerated out of the back of the rocket (i.e. the rocket exerts a force on the gas), then according to Newton's 3rd law, the...
  37. Surya97

    Four things -- Normal force, friction, Pressure, breakage

    I understand that every force has an equal and opposite reaction, and the reason things can move is because the forces acts on different things. Also, F=MA is why the heavier object is accelerated less. The equal and opposite reaction to gravity pulling down towards the Earth's core on a book...
  38. P

    Two blocks on a frictionless surface; find the force

    Homework Statement Two blocks, A and B , are being pushed on a frictionless surface by a froce of 30 N to the right, .Block A has a mass of 2.0kg, Block B, being pushed by Block A is 4.0kg. Calculate the magnitude of the force that block B exerts on Block A. Homework Equations f=ma The...
  39. 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
  40. C

    Problem on Mechanics (Newton's law of motion)

    Homework Statement This question goes like this: In an accident involving a big lorry and small car, the drivers of both vehicles were seriously injured. A student made a remark about the accident as follows: "A big lorry and a small car moved in the opposite directions and made a head-on...
  41. J

    Does a lever/fulcrum reaction demonstrate Newton 3rd Law?

    Say a lever is tilted on a fulcrum with an object on the side in contact with the floor, a mass falls onto the other side, causing the object to go flying. Does this demonstrate Action-reaction ? (action on the lever to action on the object)
  42. C

    How do I draw a free body diagram of a car on a slope?

    Homework Statement A car attempts to accelerate up a hill at an angle θ to the horizontal. The coefficient of static friction between the tires and the hill is µ > tan θ. What is the maximum acceleration the car can achieve (in the direction upwards along the hill)? Neglect the rotational...
  43. Tibriel

    A person and a box of equal mass are on a pulley with v=0....

    Homework Statement person and a box of equal masses are hanging off of the same pulley. The rope is considered massless. The person and the box are separate from each other. (The person is NOT standing on the box.) Homework Equations How far does the box move if the person starts climbing...
  44. G

    Newton's 3rd law and force equilibrium

    Hi. Assume a rock lying on the ground. Furthermore, let's assume both the rock and Earth to be rigid bodies. I'm a bit confused about force equilibrium and Newton's 3rd law here. There's Earth's gravity acting on the rock, and by Newton's 3rd law (or just by looking at the law of gravity) the...
  45. 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...
  46. T

    What are the equal-and-opposite forces of friction forces?

    Homework Statement A block rests on a table, and a second block, connected to the first with a rope, hangs over the side of the table. See the illustration. (Assume the pulley is massless and frictionless.) All objects pictured are stationary. Identify every force involved in the pictured...
  47. pradipta

    Is Newton's 3rd law incorrect?

    is the concept hypthetical
  48. C

    Solving Momentum and Newton's Law: Ike and Jim's Truck Physics Problem

    Homework Statement Ike and Jim are sitting in toy trucks; the masses are 60kg and 40kg respectively. The trucks are moving at 8 metres per second along a track, with Ike's behind Jim's. Ike pushes Jim's truck away with a pole, and Jim moves off 2 metres per second faster than Ike. What is...
  49. S

    What Does Same Type Mean in Newton's Third Law?

    my question is as follows. when an object x exerts a force on object y, then object y exert a force of the SAME TYPE that is equal in magnitude and opposite in direction on object x Explain what is meant by of the same type in this statement?
  50. A

    Centrifugal force and Newton 3rd law

    Ok, I have a general question about centrifugal force. Lets say I have an object who moves in a circular path, we know the object is accelerating because the velocity is constantly changing direction, the acceleration is towards the center. Now, where does the centrifugal force comes to...
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