Newton's Third Law of Motion: Statement, Formula, Examples and Applications

Newton's third law of motion explains why a balloon flies away when air comes out of it and why a swimmer moves forward by pushing water backward. It tells us that whenever one object applies a force on another object, the second object also applies a force back. This law helps explain many activities around us, such as walking, rowing a boat, and flying rockets. This article gives a detailed explanation of Newton's third law of motion, its formula, examples, and everyday applications.

Table of Contents

• What is Newton's Third Law of Motion
• Mathematical Representation of Newton's Third Law of Motion
• Newton's Third Law of Motion Examples in Everyday Life
• Real-Life Applications of Newton's Third Law of Motion

What is Newton's Third Law of Motion

In 1687, Sir Isaac Newton introduced the three laws of motion in his famous book Principia Mathematica. These laws explain how forces affect the motion of objects. Among them, Newton's third law of motion explains that forces always occur in pairs 

Newton noticed that whenever two objects interact, they both apply forces on each other.

For example, imagine you are sitting on a chair. Your body pushes the chair downward. At the same time, the chair pushes your body upward with an equal force.

 Similarly, the Earth pulls both you and the chair downward due to gravity, and both you and the chair pull the Earth upward with an equal force.

This leads to Newton's third law of motion, which states,

“When one object exerts a force on another object, the second object exerts an equal force in the opposite direction on the first object.”

Or, in simple words,

For every action, there is an equal and opposite reaction.

This means that a single force never exists alone. Whenever two objects interact, both objects experience forces of the same magnitude but in opposite directions.

Mathematical Representation of Newton's Third Law of Motion 

Newton's third law of motion tells us that forces always occur in pairs. Whenever one object exerts a force on another object, the second object exerts an equal force back on the first object in the opposite direction. This relationship can be expressed mathematically. 

FA=−FB

Where, 

• FA = force exerted by the first object on the second object
• FB = force exerted by the second object on the first object

The negative sign shows that the two forces act in opposite directions.

Another useful form of the law is:

m1a1=−m2a2

where,

• m1 = mass of the first object
• a1 = acceleration of the first object
• m2 = mass of the second object
• a2 = acceleration of the second object

This equation shows that when two objects interact, their accelerations depend on their masses, but the forces between them remain equal and opposite.

Newton's Third Law of Motion Examples in Everyday Life

There are many examples of Newton's third law of motion in everyday life.

• Walking: When you walk, your foot pushes the ground backward. In return, the ground pushes you forward, helping you move.
• Swimming: A swimmer pushes water backward with their hands and feet. The water pushes the swimmer forward.
• Rowing a Boat:When the oar pushes water backward, the water pushes the boat forward.
• Flying Birds: Birds push air downward with their wings. The air pushes the birds upward and helps them fly.
• Balloon Releasing Air: When air escapes from a balloon, the balloon moves in the opposite direction.
• Recoil of a Gun: When a bullet moves forward after firing, the gun moves backward. This backward movement is called recoil.
• Rocket Launch:A rocket pushes hot gases downward. The gases push the rocket upward, allowing it to rise into space.
• Fish Swimming: Fish push water backward using their fins. The water then pushes the fish forward.

Real-Life Applications of Newton's Third Law of Motion

The application of Newton's third law of motion can be seen in many places.

• Rocket Propulsion:Rockets move upward because they push gases downward at high speed.
• Jet Aircraft:Jet engines throw gases backward, and the aircraft moves forward.
• Safety While Walking:Walking is possible because the ground provides an equal and opposite reaction force.
• Swimming and Boating: Swimmers and boats move forward by pushing water backward.
• Helicopters:Helicopter blades push air downward, and the reaction force lifts the helicopter upward.
• Sports Activities: Jumping, running, skating, and kicking a ball all involve action and reaction force pairs.

Newton's third law of motion explains that forces always occur in pairs; every action has an equal and opposite reaction. We see this law in many everyday activities, such as walking, swimming, flying, and launching rockets.