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Newtons Third Law of Motion - Definition, Examples, FAQs

Newtons Third Law of Motion - Definition, Examples, FAQs

Vishal kumarUpdated on 02 Jul 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.

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:

  1. 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.
  1. 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.
  2. 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.
  3. 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.
  4. 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.

Also, check-

NCERT Physics Notes:

Frequently Asked Questions (FAQs)

Q: How does Newton's Third Law apply to the concept of torque?
A:
Torque is a rotational force, but it still obeys Newton's Thir
Q: How does Newton's Third Law apply to the concept of pressure?
A:
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.
Q: Can you explain how Newton's Third Law applies to the concept of lift in airplanes?
A:
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.
Q: How does Newton's Third Law explain why it's easier to float in saltwater than in freshwater?
A:
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.
Q: Why do you feel a kick when using a fire extinguisher?
A:
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.
Q: How does Newton's Third Law apply to the concept of friction?
A:
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.
Q: Can you explain how Newton's Third Law applies to the sensation of g-forces in a roller coaster?
A:
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.
Q: How does Newton's Third Law explain why it's harder to jump on sand than on a hard surface?
A:
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.
Q: Why do you feel weightless during free fall, even though gravity is still acting on you?
A:
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.
Q: How does Newton's Third Law apply to the concept of centripetal force in circular motion?
A:
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.
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