Newton's third law and applications

In the world of physics, it is important to understand the motion and interaction between objects. One of the fundamental elements of this study is the concept of Newton's third law of motion. Formulated by the famous English scientist Sir Isaac Newton, this law is part of a set of three laws that describe the fundamental principles of motion. Let's learn in detail about this third law, its applications, and related dynamics in mechanics.

Understanding Newton's third law of motion

Newton's third law can best be summarized by this phrase:

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

This simple statement has very deep implications. In short, it means that forces always come in pairs. When one object exerts a force on another object, the second object exerts a force of equal magnitude but in the opposite direction on the first object. It is important to understand that these forces occur simultaneously and cannot exist independently of one another.

Mathematical representation

If we denote the force exerted by object A on object B by F AB and the force exerted by object B on object A by F BA, then Newton's third law can be expressed as:

    F AB = - F BA

Here, the negative sign indicates that the forces are in opposite directions but equal in magnitude.

Simple example

Example 1: Pushing the wall

Consider an example of pushing against a wall. When you apply force to the wall with your hands, the wall "pushes back" with an equal force in the opposite direction. Although the wall doesn't move, it does apply force back to your hands. The forces are applied simultaneously between your hands and the wall.

    F arm = - F wall

Example 2: Walking

When you walk, your foot pushes back on the ground. According to Newton's third law, the ground pushes your foot forward. This forward push from the ground is what pushes you forward. The action and reaction forces are as follows:

    F ft, ground = - F ground, ft

Visual representation of forces

Let us imagine the interaction between the foot and the ground while walking.

_{foot, ground} F _{ground, feet}

In this diagram:

• The green square represents the foot.
• The blue arrow represents the force applied by the foot to the ground (action force).
• The red arrow shows the force (reaction force) applied to the foot by the ground, pushing it forward.

Applications of Newton's third law

Let's explore some real-world applications of Newton's third law in different scenarios:

Jet engines and rockets

The operation of jet engines and rockets is a classic example of Newton's third law. When the rocket expels gas outward, the gas pushes back on the rocket in the opposite direction, causing it to move forward.

For a flying rocket, the forces can be represented as follows:

    F gas, rocket = - F rocket, gas

This principle allows rockets and jet engines to operate effectively even in the vacuum of space, where there is no air to push against.

Float

Swimming involves obeying Newton's third law. When you swim, you push the water backward with your arms and legs. The water in turn pushes you forward.

The forces can be represented as follows:

    F hand, water = - F water, hand

Every movement in swimming is an expression of the reciprocal forces between the swimmer's limbs and the water.

Ship propeller

Ships use propellers to move through water. The propeller blades spin and push the water backward. The water reacts and pushes the ship forward.

The action and reaction forces are expressed as follows:

    F propeller, water = - F water, propeller

Misinterpretations and explanations

Despite its simplicity, Newton's third law often causes confusion. Some common misinterpretations and explanations are as follows:

• Misconception: Action and reaction forces cancel each other out.
  Explanation: The action and reaction forces act on different objects, not the same object, so they do not cancel. For a complete understanding, consider the interaction between the entire system involved (e.g., foot and ground).
• Misconception: Action always refers to the force applied first, followed by the reaction.
  Explanation: Action and reaction forces occur simultaneously; neither one precedes the other.

Complex example: birds flying

Birds use Newton's third law in flight. When the bird flaps its wings downward, it pushes air downward. The air responds by pushing the bird upward, producing lift and helping the bird fly higher.

    f wing, wind = - f wind, wing

This pushing action keeps the birds flying in the air, demonstrating one of nature's adaptations to the physical laws.

Conclusion

Newton's third law of motion provides profound insight into how forces work in pairs, helping us understand the mechanics behind various motions and stresses in our daily lives as well as in the universe. Recognizing the simultaneous nature and coupling of forces described by this law can enhance our understanding of many natural phenomena and technological operations.

By exploring various applications and clearing up common misconceptions, students can appreciate the relevance of Newton's third law, making the study of dynamics in mechanics an inspiring and informative endeavor.

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