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Newton S Third Law

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The Great Push and Pull: Unpacking Newton's Third Law



Ever wondered why you can walk? Or why rockets launch into space? It's not magic, but a fundamental principle of physics: Newton's Third Law of Motion. Simply stated, for every action, there's an equal and opposite reaction. Sounds straightforward, right? But the implications of this seemingly simple statement are profound, reaching far beyond the classroom and into the very fabric of our universe. Let's delve into the fascinating world of action and reaction.

Understanding the Basics: Action and Reaction



Newton's Third Law isn't about two separate events; it's about a single interaction between two objects. The "action" and "reaction" forces are simultaneous, equal in magnitude, and opposite in direction. Crucially, these forces act on different objects. This is where many misunderstandings arise. It's not that the forces cancel each other out, leading to no movement. Instead, they act on separate bodies, causing them to accelerate according to their respective masses (Newton's Second Law!).

Let's take the simple example of walking. You push backward on the ground (action). The ground, in turn, pushes forward on your feet with an equal and opposite force (reaction). This forward force propels you forward. If the ground were frictionless ice, there would be no reaction force, and you'd simply slip backward.

Beyond Walking: Real-World Applications



Newton's Third Law is far from a mere classroom curiosity. It underpins countless phenomena in the world around us:

Rocket Propulsion: Rockets expel hot gases downward (action). The gases, in turn, exert an upward force on the rocket (reaction), propelling it into space. The greater the force of the expelled gases, the greater the upward thrust.

Swimming: Swimmers push backward against the water (action). The water pushes them forward (reaction), allowing them to move through the water.

Birds in Flight: Birds push air downwards with their wings (action). The air pushes upwards on their wings (reaction), providing the lift needed to stay airborne.

Jumping: When you jump, you push downwards on the Earth (action). The Earth, in turn, pushes upwards on you (reaction), launching you into the air. This might seem counterintuitive, given the Earth's immense mass, but the force is indeed equal and opposite.

Car Driving: A car's engine drives the wheels, which push backward on the road (action). The road pushes forward on the car's wheels (reaction), accelerating the car forward.

Misconceptions and Clarifications



A common misconception is that the action and reaction forces cancel each other out. This is incorrect. They act on different objects, so they don't cancel. Imagine hitting a wall with your hand. You exert a force on the wall (action), and the wall exerts an equal and opposite force on your hand (reaction). Your hand hurts because of the reaction force from the wall, not because the forces cancel out.

Another misconception is that the action force is always greater than the reaction force, or that one force happens before the other. This is false. The forces are always equal in magnitude and simultaneous. This is a fundamental principle of physics, inherent in the laws of motion.

Exploring Deeper: Advanced Concepts



The Third Law also plays a crucial role in more advanced physics concepts:

Conservation of Momentum: Newton's Third Law is intrinsically linked to the conservation of momentum. In any closed system, the total momentum remains constant. The equal and opposite forces acting on interacting objects ensure that the total momentum is conserved.

Collisions: The analysis of collisions, whether elastic or inelastic, relies heavily on understanding the action-reaction pairs involved. The changes in momentum of the colliding objects are directly related to the forces they exert on each other.

Relativity: While Newton's Laws are not entirely accurate at very high speeds (where Einstein's theory of relativity takes over), the principle of action and reaction still holds true, albeit with a more nuanced understanding of forces in the context of spacetime.


Conclusion



Newton's Third Law, though seemingly simple, is a cornerstone of classical mechanics. It's not just a theoretical concept; it's a fundamental principle governing interactions in the physical world, from the smallest particles to the largest celestial bodies. Understanding this law allows us to explain a wide range of phenomena and provides a crucial foundation for further explorations into the field of physics.


Expert-Level FAQs:



1. Can Newton's Third Law be applied to systems involving friction? Yes, but the forces involved become more complex. Friction forces are internal to the system, and you must carefully consider all forces acting on each object.

2. How does Newton's Third Law apply in a non-inertial frame of reference? In non-inertial frames (accelerating frames), fictitious forces need to be considered alongside the real forces, complicating the analysis of action-reaction pairs.

3. Does Newton's Third Law hold true at the quantum level? While the exact interpretation may differ, the principle of equal and opposite interactions remains fundamentally important in quantum mechanics, albeit expressed in a more probabilistic framework.

4. How can we reconcile Newton's Third Law with the seemingly one-sided force of gravity? Gravity is not a one-sided force; it's an interaction between two masses. The Earth exerts a gravitational force on you (action), and you exert an equal and opposite gravitational force on the Earth (reaction), albeit the Earth's enormous mass makes its acceleration negligible.

5. Can Newton's Third Law explain all interactions? No. While a powerful tool for understanding many interactions, it doesn't fully account for interactions involving fields, or those described by more advanced theories like quantum field theory.

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Is Newton's 3rd law valid in Electrodynamics? - Physics Forums 13 Feb 2016 · A google of the topic indicates there are apparently some exceptions to Newton's 3rd Law in Electrodynamics. In many cases, I think Newton's 3rd Law still applies, but it appears the exceptions are numerous enough that electrodynamics calculations can not employ this law, but need to be worked on a case by case basis.

Understanding Newton's Third Law: The Case of Box Pushing 21 Oct 2004 · Newton's third law states something along the lines of "For every action, there is an equal and opposite reaction." The problem I have is that I don't understand how it stays true for something as simple as pushing a box. If a push the box (action), how can …

Tug of war applications using Newton's third law - Physics Forums 3 Oct 2022 · The magnitude of the tension inside the rope must be the same in both directions (third law). Since the total mass of the system abandons its initial state of repose, and starts moving in the direction of the bigger mass, the net force acting on the system of masses and rope can’t be zero (second law).

Is Newton's Third Law Always Obeyed? - Physics Forums 27 Jul 2007 · This is a very good question. Yes, the Biot-Savart magnetic interaction law formally violates the Newton's third law. This situation is a subject of ongoing debate in research literature. See, for example, E. Breitenberger, "Magnetic interactions between charged particles", Am. J. Phys. 36 (1968), 505

Newton's third law a book and table - Physics Forums 16 Oct 2015 · Newton's third law of motion is often summarised as 'Every action (force) has an equal and opposite reaction'. A book rests on a table. If the weight of the book is the 'action' force, what is the 'reaction' force? A the pull of the book on the Earth B the pull of the Earth on the book C the push of the book on the table D the push of the table ...

Equilibrium Applications of Newton's Laws of Motion - Physics … 26 Feb 2011 · equilibrium because the foot also applies a force to it. This force arises in reaction (Newton's third law) to the pulling effect of the forces and . Ignoring the weight of the foot, find the magnitude of . Figure 4.28 (a) A traction device for the foot. (b) The free-body diagram for the pulley on the foot.

Exceptions to the Laws of Physics: Newton's Third Law 5 Sep 2016 · For Example, with Newton's First and Second Laws are totally invalid at the quantum level, but I'm failing to find such an example for Newton's Third Law. Is anyone able to help me? Take two protons moving with velocities at right angles to each other.

Newton's 3rd law & rocket propulsion (detailed explanation) 6 Jul 2016 · Newton's 3rd law then states that the (rigid) walls of the rocket will exert an equal, but opposite force on the gas and everything will remain in equilibrium (since on average there will be an equal amount of force acting on the left and right walls of the rocket, and the top and bottom walls of the rocket) .

Understanding Newton's Third Law: The Impact of Forces on … 29 Mar 2010 · In Newton's 3rd law the equal and opposite forces act on different objects. So, for the rocket, the 10000 N upwards force on the rocket is equal and opposite to the 10000 N downwards force on the exhaust, which is why the exhaust accelerates down so quickly and the rocked accelerates up.

Connecting Newton's 3rd law of motion to circular motion 6 Nov 2013 · Newton's Third law states that when one object exerts a force on another object, then that object exerts an equal (but opposite in direction) force on the first object. Using this law and applying it to the Moon and Earth, there must be a force that is equal to and acting in the opposite direction as the centripetal force.