Summary of Dynamics: Newton's 3rd Law

🌌 Introduction to Newton's 3rd Law

⚖️ Relevance of the Topic

Newton's 3rd Law, also known as the Law of Action-Reaction, is one of the fundamental pillars of Physics. It plays a crucial role in explaining how objects interact with each other in the universe. Understanding this law is essential to comprehend the world around us, from the simple act of walking to the complex study of planetary movements.

🌍 Contextualization

Newton's 3rd Law is at the heart of the Dynamics module in the Physics curriculum. After introducing the laws of motion and forces, the Law of Action-Reaction emerges as a key element that allows us to understand how forces are related and how they affect the movement of objects. This Law not only applies to Physics studies on Earth but is also fundamental in Astrophysics for the study of celestial movements. Understanding it is the passport to comprehending the most astonishing physical phenomena in the universe.

"For every action, there is an equal and opposite reaction" - Sir Isaac Newton

🚀 Theoretical Development: Newton's 3rd Law

🎯 Components of Newton's 3rd Law

• Action and Reaction: The essence of Newton's 3rd Law lies in the interaction between two bodies. When one body exerts a force on another, the second body reacts with a force of equal magnitude, but in the opposite direction. These forces are known, respectively, as the action force and the reaction force.

• Action Body and Reaction Body: These are the two bodies that interact according to Newton's 3rd Law. The body that initially exerts the force is called the action body, while the body that responds with the reactive force is called the reaction body.

• Understanding of Vectors: Understanding the concepts about vectors is essential to understand Newton's 3rd Law. The direction and magnitude of the action and reaction forces are expressed in terms of vectors, with the reaction being the vector opposite to the action.

📚 Key Terms

• Force: A force is any influence that changes the motion of an object. It is vectorial, has magnitude (size) and direction.

• Force Equilibrium: Occurs when the sum of all forces acting on an object equals zero. That is, the forces are balanced.

• Inertia: It is the resistance of an object to a change in its state of motion. In terms of Newton's 3rd Law, it is the reason why a reaction force occurs.

💡 Examples and Cases

• Walking: When a person walks, they exert a force downward and backward on the ground (action). In response, the ground exerts a force upward and forward on the person (reaction), causing them to move forward.

• Jumping into a Pool: When someone jumps into the water, they exert a force downward on the water (action). The water, in turn, exerts a force upward on the person (reaction) that we perceive as resistance when entering the water.

• Rockets in Space: Rockets do not need anything to "push against" in space. They operate by Newton's 3rd Law. When gases are expelled backward at high speed (action), the rocket moves forward as a reaction to that force (reaction).

📝 Detailed Summary

🎯 Relevant Points

• Definition of Force: Understanding force as an influence capable of changing the motion of an object is the basis for understanding the third law of Newton. Forces are expressed as vectors, having magnitude and direction.

• Duality in Newton's 3rd Law: Newton's 3rd Law always involves two bodies, the action body and the reaction body, and two forces, action and reaction, which are always equal in magnitude and opposite in direction.

• Action and Reaction: The concepts of action force and reaction force are central to the third law. The action force is the force that one object exerts on the other, and the reaction force is the force that the second object exerts back on the first, always with the same magnitude, but in the opposite direction.

• Force Equilibrium and Inertia: Inertia, the resistance that an object has to change its state of motion, and force equilibrium, which occurs when the sum of all forces acting on an object is zero, are important to understand why a reaction force occurs.

• Application in Everyday Examples: Newton's 3rd Law has practical applications ranging from the everyday act of walking to the launch of rockets into space.

🗒️ Conclusions

• Ubiquity of Newton's 3rd Law: Newton's 3rd Law is omnipresent. It operates in all interactions between objects, whether they are on Earth or in space.

• Implications of the Law: The Law of Action-Reaction establishes that for every action there is an equal and opposite reaction. This has fundamental implications not only for the study of Physics but also for other areas such as Engineering and Astronomy.

• Significance of Newton's 3rd Law: Newton's 3rd law is inseparable from the physical reality we live in. It helps to unravel the functioning of the universe, from the simple act of walking to the complex movement of planets.

✏️ Suggested Exercises

1. Walking Through the Room: When we walk, we push the ground back with our feet. Describe the action and reaction involved in this situation.

2. Lifting a Box: What happens when we lift a box from the ground? Explain the action and reaction forces involved.

3. Rocket Launch: How does Newton's 3rd Law apply to the launch of a rocket? Identify the action and reaction in this case.