Newtons Third Law of Motion - Definition, Examples, FAQs

Edited By Vishal kumar | Updated on Jul 02, 2025 04:32 PM IST

What is newton’s third law of motion?

Newton’s third law of motion definition: This law states that every action has a matching and opposing reaction. If body A applies force Fa to body B, then B applies force Fb to body A at the same time. Furthermore, the forces exerted on each body are equal in size and opposite in direction: Fa = - Fb.

Newtons Third Law of Motion - Definition, Examples, FAQs

Furthermore, in some cases, one body between these two determines the direction and magnitude fully. Consider the case where item A is exerting force on object B. The force acting on object B is called "action," while the force acting on object A is called "reaction." As previously stated, this law is also known as the action-reaction pair law, with Fa and Fb standing for action and reaction, respectively.

In some circumstances, however, both bodies work together to determine magnitude and direction. It is irrelevant to say which force is "activity" and which “reaction” in this circumstance is. Furthermore, action and reaction happen at the same time belong to the same interaction, and neither happens without the other.

Newtons third law of motion

Two skaters push against each other to demonstrate Newton’s third law of motion. The first skater to the left exerts a normal force of N12 on the second skater to the right, while the second skater to the left exerts a normal force of N21 on the first skater.

Both forces have identical magnitudes but opposite directions, as specified by Newton’s third law of motion class 9.

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Newton’s third law examples

The action-reaction pair law can also be observed in daily life. Consider the following applications of Newton’s third law of motion class 9:

An individual On the Road Walking, When a person goes on a walk, the person pushes against the ground in a reverse motion. The ground, on the other hand, applies a forward-directed force to the man or woman, causing them to go forward.

Swimming: When a person swims, he or she pushes the pool water backward, which is an illustration of Newton’s third law of motion. The water, in turn, propels the swimmer ahead.
Bird in the Air: A bird, in accordance with Newton’s third law of motion, uses its wings to push air downward. The air, on the other hand, aids the bird's upward movement.
Rockets, for example, propel themselves forward by ejecting high-velocity gas backward. This means that the rocket applies a significant backward force to the gas inside the rocket combustion chamber, and the gas, in turn, applies a large forward reaction force to the rocket. Thrust is the name for this reactive force. Rockets are commonly misunderstood to propel themselves by pushing on the ground or air behind them. They perform better in a vacuum because the exhaust gases can be expelled more easily.
Helicopters create lift in a similar way by pushing air down and feeling an upward response force. Birds and airplanes fly by exerting force on air in the opposite direction of the force they require. To get the lift and forward motion, a bird's wings, for example, force air downward and backward.

Related Topics,

Newton’s third law of motion says that when someone walks, they push on the floor, and the floor pushes back. This is Newton’s third law of motion in action. Newton’s third law of motion can be observed in action as someone walks: they push on the floor, and the floor pushes back.

Similarly, a car's tires press against the road, which pushes back on the tires—the tires and road push against each other at the same time. These forces are affected by friction; for example, a person or car on ice may be unable to exert the necessary action force to produce the required response force.

The law of conservation of momentum has been derived from Newton’s third law of motion, conservation of momentum is a more fundamental idea (derived from Galilean invariance via Noether's theorem). In circumstances where Newton’s third law of motion appears to break, such as when force fields and particles both have momentum and in quantum mechanics, this holds true.

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NCERT Physics Notes:

Frequently Asked Questions (FAQs)

1. What is Newton's third law of motion?

If object A exerts a force on object B, object B must reply with a force of equal size and in the opposite direction, according to Newton’s third law of motion.

This law illustrates nature's symmetry: forces always exist in pairs, and one body cannot exert a force on another without also being subjected to one. This law is sometimes referred to as action-reaction, with the action being the force exerted and the reaction being the force received as a result.

2. What is Newton's third law of motion?

Newton's Third Law states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on another object, the second object exerts an equal force back on the first object, but in the opposite direction. These forces always occur in pairs and act on different objects.

3. What five examples do you have of Newton’s third law of motion?

Examples of Newton’s third law of motion are:-

An elastic band is being pulled.
Swimming or rowing a boat are two options.
When pushing an object, there is static friction.
Walking.
Standing on the ground or sitting in a chair are both acceptable options.
The rocket's upward thrust.
Resting against a tree or a wall.
Slingshot.

4. Newton’s 3rd law of motion is concerned with which two factors?

In collisions between two objects, Newton’s third law of motion is naturally applied. When two things collide, forces of equal size and opposite direction are experienced by both objects. As a result of such forces, one object often accelerates (gains momentum) while the other slows down (lose momentum).

5. What is the third law of motion?

When two bodies contact, Newton’s third law of motion states that they apply forces to each other that are equal in magnitude and opposing in direction. The law of action and reaction is another name for the third law.

6. Why don't action-reaction force pairs cancel each other out?

Action-reaction force pairs don't cancel each other out because they act on different objects. While the forces are equal in magnitude and opposite in direction, they affect separate bodies. For example, when you push against a wall, the wall pushes back on you with equal force, but these forces act on you and the wall respectively, not on the same object.

7. How does Newton's Third Law apply to rocket propulsion?

Rocket propulsion is a perfect example of Newton's Third Law. As the rocket expels hot gases downward (action), the gases push back on the rocket with an equal force upward (reaction). This upward force propels the rocket into space. The more forcefully the gases are expelled, the greater the upward thrust on the rocket.

8. Can you explain Newton's Third Law in terms of walking?

When you walk, you push backward against the ground with your foot (action). According to Newton's Third Law, the ground pushes forward on your foot with an equal force (reaction). This forward force on your foot is what propels you forward. Without this reaction force from the ground, you wouldn't be able to move forward.

9. How does Newton's Third Law relate to conservation of momentum?

Newton's Third Law is closely related to the conservation of momentum. When two objects interact, the forces they exert on each other are equal and opposite. This means that the change in momentum of one object is equal and opposite to the change in momentum of the other object, ensuring that the total momentum of the system remains constant.

10. Why do you feel a recoil when firing a gun?

The recoil you feel when firing a gun is a direct result of Newton's Third Law. As the gun exerts a forward force to propel the bullet (action), the bullet exerts an equal and opposite force back on the gun (reaction). This backward force is what you feel as recoil. The faster and more massive the bullet, the stronger the recoil.

11. How does Newton's Third Law explain the motion of a boat being rowed?

When rowing a boat, the oars push water backward (action). According to Newton's Third Law, the water pushes the oars and the boat forward with an equal force (reaction). This forward force on the boat is what propels it through the water. The harder you push the water back, the faster the boat moves forward.

12. Can you give an example of Newton's Third Law in sports?

A great example of Newton's Third Law in sports is a swimmer pushing off the wall of a pool. As the swimmer pushes against the wall (action), the wall pushes back with an equal force (reaction). This reaction force propels the swimmer in the opposite direction, giving them a boost of speed. The harder the swimmer pushes, the greater the reaction force and the faster they'll move.

13. How does Newton's Third Law apply to air resistance?

When an object moves through air, it pushes air molecules out of the way (action). According to Newton's Third Law, the air molecules push back on the object with an equal force (reaction). This reaction force is what we call air resistance or drag. The faster an object moves or the larger its surface area, the greater the air resistance it experiences.

14. Why do you feel a force pushing you back into your seat when a car accelerates?

When a car accelerates forward, you feel pushed back into your seat due to Newton's Third Law. The car seat exerts a forward force on you to accelerate you along with the car (action). Your body, resisting this change in motion due to inertia, pushes back on the seat with an equal force (reaction). This backward force is what you feel as being "pushed" into your seat.

15. How does Newton's Third Law explain the flight of birds?

Birds fly by pushing air downward and backward with their wings (action). According to Newton's Third Law, the air pushes back on the wings with an equal force upward and forward (reaction). This upward force provides lift to counteract gravity, while the forward component propels the bird through the air. The stronger the downward push, the greater the lift and forward motion.

16. Can Newton's Third Law explain why it's harder to walk on ice than on concrete?

Yes, Newton's Third Law helps explain this difference. When you walk, you push backward on the surface (action). On concrete, the rough surface provides enough friction to push back strongly (reaction), propelling you forward. On ice, the slippery surface provides less friction, resulting in a weaker reaction force. This weaker force makes it harder to move forward and easier to slip.

17. How does Newton's Third Law apply to the interaction between Earth and the Moon?

Newton's Third Law governs the gravitational interaction between Earth and the Moon. Earth exerts a gravitational force on the Moon (action), and the Moon exerts an equal gravitational force on Earth (reaction). These mutual forces keep the Moon in orbit around Earth and also cause ocean tides on Earth.

18. Why do lighter objects experience more acceleration than heavier objects when the same force is applied?

This is related to Newton's Second Law (F = ma) rather than the Third Law, but it's a common misconception. When the same force is applied to objects of different masses, the lighter object experiences more acceleration because acceleration is inversely proportional to mass (a = F/m). Newton's Third Law ensures that the force applied to both objects is the same, regardless of their mass.

19. How does Newton's Third Law explain the concept of normal force?

Normal force is a direct application of Newton's Third Law. When an object rests on a surface, gravity pulls it downward (action). The surface pushes back on the object with an equal upward force (reaction) to prevent it from sinking through. This upward reaction force is called the normal force. It's always perpendicular to the surface and equal in magnitude to the weight of the object when on a horizontal surface.

20. Can you explain how Newton's Third Law applies to magnetic forces?

Magnetic forces also obey Newton's Third Law. When one magnet attracts another (action), the second magnet exerts an equal attractive force on the first (reaction). Similarly, if two magnets repel each other, the repulsive force each magnet experiences is equal in magnitude but opposite in direction. This is why magnets can appear to "float" when properly arranged - the forces are balanced.

21. How does Newton's Third Law relate to the concept of tension in ropes or cables?

Tension in a rope or cable is a great example of Newton's Third Law in action. When a rope is under tension, it pulls on the objects at both ends with equal force. If you're pulling a rope, the rope pulls back on you with the same force. This is why a tug-of-war game can remain static if both teams pull with equal force - the action and reaction forces balance out.

22. Why don't action-reaction pairs violate the law of conservation of energy?

Action-reaction pairs don't violate the law of conservation of energy because they don't create or destroy energy; they merely transfer it. When two objects interact, energy may be transferred between them, but the total energy of the system remains constant. For example, when a ball bounces off a wall, kinetic energy is transferred between the ball and the wall, but no energy is created or lost in the process.

23. How does Newton's Third Law apply to buoyancy?

Buoyancy is a result of Newton's Third Law. When an object is submerged in a fluid, the fluid exerts an upward force on the object (buoyant force). This is a reaction to the weight of the fluid displaced by the object (action). The magnitude of this buoyant force is equal to the weight of the displaced fluid, which is why objects float when their density is less than that of the fluid.

24. Can you explain how Newton's Third Law applies to the sensation of weight?

The sensation of weight is a direct application of Newton's Third Law. Your body exerts a downward force on the ground due to gravity (action). The ground pushes back on you with an equal upward force (reaction). This upward force is what you feel as your weight. If you stand on a scale, the scale measures this reaction force, which is equal to your weight.

25. How does Newton's Third Law explain the recoil of a spring?

When you compress a spring, you exert a force on it (action). According to Newton's Third Law, the spring exerts an equal and opposite force back on you (reaction). When you release the spring, it pushes against whatever was compressing it with a force equal to the force that compressed it. This is why a compressed spring can launch an object - the stored energy is released as a force pair.

26. Why do astronauts need to push off surfaces to move in space?

In the microgravity environment of space, astronauts rely on Newton's Third Law to move around. When an astronaut pushes off a surface (action), the surface pushes back with an equal force (reaction), propelling the astronaut in the opposite direction. Without this action-reaction pair, and in the absence of gravity or air to push against, astronauts would have no way to generate motion.

27. How does Newton's Third Law apply to the concept of thrust in jet engines?

Jet engines work on the principle of Newton's Third Law. The engine expels hot gases backward at high speed (action). In response, the gases exert an equal force forward on the engine (reaction). This forward force is what we call thrust, and it propels the aircraft forward. The greater the mass and velocity of the expelled gases, the greater the thrust produced.

28. Can you explain how Newton's Third Law applies to the collision of two objects?

In a collision between two objects, Newton's Third Law ensures that the forces experienced by both objects are equal and opposite. When object A exerts a force on object B (action), object B exerts an equal and opposite force on object A (reaction). These forces act for the same amount of time, which is why the change in momentum for both objects is equal and opposite, conserving the total momentum of the system.

29. How does Newton's Third Law explain why heavier objects don't fall faster than lighter objects in a vacuum?

This is actually explained by Newton's Second Law, but it's a common misconception related to the Third Law. In a vacuum, all objects fall at the same rate regardless of mass because the gravitational force (weight) is proportional to mass, and acceleration due to gravity is mass divided by weight (which cancels out). Newton's Third Law ensures that for every object, regardless of mass, the Earth pulls on it with a force equal to the object's pull on Earth.

30. Why do you feel a forward force when a car suddenly stops?

When a car suddenly stops, you feel a forward force due to Newton's First Law of Motion (inertia), but the interaction between you and the car seat is governed by the Third Law. As the car decelerates, the seat pushes you backward (action). Your body, wanting to continue moving forward due to inertia, pushes back on the seat with an equal force (reaction). This forward push is what you feel as a force throwing you forward.

31. How does Newton's Third Law apply to the concept of pressure?

Pressure is force per unit area, and it always acts perpendicular to a surface. When a fluid exerts pressure on a surface (action), the surface exerts an equal and opposite pressure back on the fluid (reaction). This is why fluids in enclosed containers exert pressure in all directions - the container walls push back with equal force, creating a balanced system.

32. Can you explain how Newton's Third Law applies to the concept of lift in airplanes?

Lift in airplanes is a complex phenomenon, but Newton's Third Law plays a crucial role. As an airplane's wings deflect air downward (action), the air pushes back on the wings with an equal upward force (reaction). This upward force is a major component of lift. The faster the plane moves and the more air it deflects downward, the greater the lift force generated.

33. How does Newton's Third Law explain why it's easier to float in saltwater than in freshwater?

While buoyancy is the main principle here, Newton's Third Law helps explain the difference. In both cases, your body displaces water, and the water pushes back up (reaction) to your weight (action). Saltwater is denser than freshwater, so for the same volume displaced, it pushes back with a greater force. This greater upward force makes it easier to float in saltwater.

34. Why do you feel a kick when using a fire extinguisher?

When using a fire extinguisher, you feel a kick due to Newton's Third Law. As the extinguisher expels its contents forward with high pressure (action), it experiences an equal force pushing it backward (reaction). This backward force is what you feel as a kick. The more forcefully the contents are expelled, the stronger the kick you'll feel.

35. How does Newton's Third Law apply to the concept of friction?

Friction is a force that resists the relative motion of surfaces in contact. When one surface tries to move along another, it pushes on the second surface (action). According to Newton's Third Law, the second surface pushes back with an equal force (reaction). This reaction force is what we call friction. The rougher the surfaces or the harder they're pressed together, the stronger the frictional force.

36. Can you explain how Newton's Third Law applies to the sensation of g-forces in a roller coaster?

G-forces in a roller coaster are a result of Newton's Laws, including the Third Law. When the coaster changes direction or speed, your body wants to continue its previous motion due to inertia. The seat exerts a force on you to change your motion (action), and your body pushes back on the seat with an equal force (reaction). These forces, particularly when they're stronger than usual due to rapid changes in motion, are what you feel as g-forces.

37. How does Newton's Third Law explain why it's harder to jump on sand than on a hard surface?

When you jump, you push down on the surface (action). According to Newton's Third Law, the surface pushes back up on you with an equal force (reaction). On a hard surface, this reaction force is immediate and concentrated, providing a strong push-off. On sand, some of your push is absorbed by the sand moving, resulting in a weaker and less concentrated reaction force. This makes it harder to jump as high on sand.

38. Why do you feel weightless during free fall, even though gravity is still acting on you?

The sensation of weightlessness during free fall is related to Newton's Third Law. Normally, you feel weight because the ground pushes up on you (reaction) in response to gravity pulling you down (action). In free fall, there's no surface to push back on you, so you don't feel this reaction force. You and everything around you are accelerating at the same rate due to gravity, so there's no relative force to create the sensation of weight.

39. How does Newton's Third Law apply to the concept of centripetal force in circular motion?

In circular motion, centripetal force is the force that keeps an object moving in a circular path. When an object moves in a circle, it constantly changes direction, requiring a force pointing towards the center. According to Newton's Third Law, the object exerts an equal and opposite force outward. This outward force is often mistakenly called "centrifugal force," but it's actually the reaction to the centripetal force.

40. Can you explain how Newton's Third Law applies to the concept of drag in fluid dynamics?

Drag is a force that opposes the motion of an object through a fluid (liquid or gas). As an object moves through a fluid, it pushes fluid particles out of the way (action). According to Newton's Third Law, these fluid particles push back on the object with an equal force (reaction). This reaction force is what we call drag. The faster the object moves or the more dense the fluid, the greater the drag force.

41. How does Newton's Third Law explain why it's easier to push a shopping cart than to push a car?

Newton's Third Law states that the force you exert on the cart or car (action) is equal to the force it exerts back on you (reaction). The difference in difficulty comes from friction and inertia, not the Third Law. A car has much more mass and thus more inertia to overcome, and its tires have more friction with the ground. The shopping cart, being lighter and on wheels, has less inertia and friction to overcome, making it easier to push.

42. Why do you feel a jolt when catching a fast-moving ball?

When catching a fast-moving ball, you feel a jolt due to Newton's Third Law. As your hands exert a force to stop the ball (action), the ball exerts an equal force back on your hands (reaction). This reaction force is what you feel as a jolt. The faster the ball is moving, the more force is required to stop it, and thus the stronger the jolt you feel.

43. How does Newton's Third Law apply to the concept of torque?

Torque is a rotational force, but it still obeys Newton's Thir