Newton's third law of motion

Newton's third law of motion is one of the important principles of physics that explains how forces work. Formulated by Sir Isaac Newton in the 17th century, this law states:

“For every action there is an equal and opposite reaction.”

This means that forces always come in pairs. Whenever one object exerts a force on another object, the second object also exerts a force of equal magnitude and in the opposite direction on the first object.

Understanding action and reaction

The main components of Newton's third law are action and reaction forces. Let's understand these terms:

• Acting force: The force exerted by the first object on a second object.
• Reaction force: The force exerted by a second object on a first object, which is equal to the action force but in the opposite direction.

It is important to understand that these forces do not act on the same object, and therefore, they do not cancel each other out.

Simple examples of Newton's third law

Example 1: Walking

When you walk, your foot pushes back on the ground. This is the action force. According to Newton's third law of motion, the ground pushes forward on your foot with an equal and opposite force, called the reaction force. This reaction force is what moves you forward.

Foot exerts force on ground → Ground exerts an equal and opposite force on foot

Example 2: Jumping

When you jump off a diving board, you push the board down with your feet (action). The diving board pushes you up with an equal and opposite force (reaction). This is why you can jump high into the air.

Legs push down on board → Board pushes up on legs

Example 3: Balloon

A deflated balloon moves in the air because of action and reaction forces. As the air escapes the balloon, it exerts a force backward (action), and the balloon moves forward (reaction) with an equal and opposite force.

Air pushes backward → Balloon moves forward

Visualizing with diagrams

Let's use a graphical representation to better understand these interactions:

Walking diagram:

Balloon diagram:

Text examples to deepen understanding

Space travel

When a rocket launches, it expels gas from its engine at high speed (action force). In response, the rocket is pushed upward with an equal force in the opposite direction (reaction force). This is the principle that makes space shuttles and rockets able to travel in space.

Rocket expels gas downward → Gas pushes rocket upward

Bouncing of the ball

When a ball hits the ground, it exerts a force on the surface (action force). The ground exerts a reaction force on the ball, which causes the ball to bounce back.

Ball pushes on ground → Ground pushes on ball

Mathematical representation

In quantitative terms, if:

F_{action} = - F_{reaction}

This equation shows that the forces are equal in magnitude but opposite in direction. There is no net force on the combined system of two interacting objects.

Applications in real life

Newton's third law is not just a principle with theoretical value; it has practical applications in various fields such as engineering, physics, biomechanics, and even in daily life.

Automobile

In cars, when you speed up, the wheels push back on the road (action) and the road pushes the car forward (reaction). Similarly, brakes work because the brake pads exert a force on the rotating wheels to slow them down, and an equal force is applied back to the brake pads by the wheels.

Wheels push road backward → Road pushes wheels forward

Boating and swimming

When rowing a boat, the oars push the water backward with an action force, and the water pushes the oars forward with an opposite reaction force, causing the boat to move forward. In swimming, the arms and legs push the water backward, and the water pushes the swimmer forward, causing him to move forward.

Oars push water backward → Water pushes oars forward

Summary

Newton's third law of motion is a fundamental principle that states that every force has an equal and opposite counterpart. It helps us understand that forces are not isolated and are always part of the interaction between different objects. This law is evident in everyday activities such as walking, jumping and playing sports, as well as in complex systems such as rockets and industrial machinery.

The universal applicability of this law in countless scenarios demonstrates its importance as a cornerstone in the study and application of physics. By understanding and applying Newton's third law, we can gain a deeper insight into the forces at work in the world around us.

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