Undergraduate
  1. Classical mechanics  1.1. dynamics  1.1.1. Motion in one dimension  1.1.2. Motion in two dimensions  1.1.3. projectile motion  1.1.4. Relative speed  1.1.5. Uniform circular motion  1.1.6. Non-uniform circular motion  1.1.7. Reference frames and transformations  1.2. Newton's Laws of Motion  1.2.1. First law of motion  1.2.2. Second law of motion  1.2.3. Third Law of Motion  1.2.4. Applications of Newton's Laws  1.2.5. Friction and its types  1.2.6. Free Body Diagram  1.2.7. Constraints and pseudo-forces  1.3. Work and Energy  1.3.1. work done by the force  1.3.2. Work–energy theorem  1.3.3. Kinetic energy  1.3.4. Potential energy in classical mechanics  1.3.5. Conservative and non-conservative forces  1.3.6. Energy Conservation  1.3.7. Power and efficiency  1.4. Speed and collisions  1.4.1. Linear momentum  1.4.2. Conservation of momentum  1.4.3. Impulse and impact force  1.4.4. Elastic and inelastic collision  1.4.5. Center of mass and speed  1.4.6. Rocket Propulsion  1.5. Rotational motion  1.5.1. Torque and Angular Momentum  1.5.2. Moment of inertia  1.5.3. Rotational Kinematics  1.5.4. Rotational Energy  1.5.5. Rolling Motion  1.5.6. Precession and gyroscopic motion  1.6. Gravitational force  1.6.1. Newton's law of universal gravitation  1.6.2. Gravitational potential energy  1.6.3. Orbital mechanics  1.6.4. Kepler's laws of planetary motion  1.6.5. Gravitational field and potential  1.7. Fluid mechanics  1.7.1. Pressure and Pascal's Principle  1.7.2. Buoyancy and Archimedes' principle  1.7.3. Fluid Dynamics and Bernoulli's Principle  1.7.4. Viscosity and Poiseuille's law  1.7.5. Surface tension and capillarity  1.8. Oscillations and waves  1.8.1. Simple Harmonic Motion  1.8.2. Damped and driven oscillations  1.8.3. Coupled oscillations and common modes  1.8.4. Wave properties and types  1.8.5. Sound Waves and the Doppler Effect  1.8.6. Wave interference and superposition
  2. Electromagnetism  2.1. Electrostatics  2.1.1. Electric charge and properties  2.1.2. Coulomb's law  2.1.3. Electric field and electric potential  2.1.4. Gauss's Law  2.1.5. Capacitance and Dielectric  2.1.6. Electric dipole and potential energy  2.2. Electric circuits  2.2.1. Current and Resistance  2.2.2. Ohm's law  2.2.3. Kirchhoff's Laws  2.2.4. RC Circuit  2.2.5. AC Circuits and Reactance  2.2.6. Electric power and energy  2.3. Magnetism  2.3.1. Magnetic Fields and Forces  2.3.2. Ampere's Law  2.3.3. Magnetic dipole moment  2.3.4. Biot-Savart law  2.3.5. Magnetic Materials and Hysteresis  2.4. Electromagnetic induction  2.4.1. Faraday's Law  2.4.2. Lenz's law  2.4.3. Self and mutual induction  2.4.4. Transformers and Inductive Circuits  2.5. Maxwell's equations  2.5.1. Gauss's law for electricity  2.5.2. Gauss's law for magnetism  2.5.3. Faraday's law of induction  2.5.4. Ampere-Maxwell law  2.5.5. Electromagnetic waves
  3. Thermodynamics  3.1. Laws of Thermodynamics  3.1.1. Zeroth law of thermodynamics  3.1.2. First law of thermodynamics  3.1.3. Second Law  3.1.4. Third Law  3.2. Heat and work  3.2.1. Heat transfer (conduction, convection, radiation)  3.2.2. Thermal expansion  3.2.3. Heat engines and refrigerators  3.2.4. Carnot cycle and efficiency  3.3. Statistical mechanics  3.3.1. Maxwell–Boltzmann distribution  3.3.2. Entropy and Probability  3.3.3. Partition function  3.3.4. Fermi–Dirac and Bose–Einstein statistics
  4. Optics  4.1. Geometrical Optics  4.1.1. Reflection and Refraction  4.1.2. Mirrors and lenses  4.1.3. Optical Instruments  4.1.4. Fermat's principle  4.2. Wave optics  4.2.1. Interference in wave optics  4.2.2. Diffraction  4.2.3. Polarization  4.2.4. Coherence and holography
  5. Quantum mechanics  5.1. Wave–particle duality  5.1.1. Blackbody radiation in wave–particle duality in quantum mechanics  5.1.2. Photoelectric effect  5.1.3. Compton scattering  5.1.4. De Broglie wavelength  5.2. Schrödinger Equation  5.2.1. Time-independent Schrödinger equation  5.2.2. Particle in a box  5.2.3. Quantum Tunneling  5.2.4. Potential wells and obstacles  5.3. Quantum States  5.3.1. Wave function  5.3.2. Quantum operators  5.3.3. Heisenberg uncertainty principle  5.3.4. Angular momentum and spin
  6. Relativity  6.1. Special relativity  6.1.1. Lorentz transformations  6.1.2. Time expansion and length contraction  6.1.3. Relativistic energy and momentum  6.2. General relativity  6.2.1. Principle of Equivalence  6.2.2. Schwarzschild metric  6.2.3. Gravitational waves  6.2.4. Black holes and event horizons
  7. Solid state physics  7.1. Crystal structure  7.1.1. Bravais lattices  7.1.2. X-ray diffraction  7.1.3. Band theory  7.1.4. Phonons and lattice vibrations  7.2. Electrical and Magnetic Properties  7.2.1. Conductors, Semiconductors and Insulators  7.2.2. Superconductivity  7.2.3. Hall effect
  8. Nuclear and particle physics  8.1. Atomic Structure  8.1.1. Atomic Model  8.1.2. Nuclear binding energy  8.2. Radioactivity  8.2.1. Alpha, beta, gamma decay  8.2.2. Half life  8.2.3. Nuclear Fission and Fusion  8.3. Particle physics  8.3.1. Standard Model  8.3.2. Quarks and Leptons  8.3.3. antimatter  8.3.4. Fundamental interactions
  9. Astrophysics and cosmology  9.1. Stellar evolution  9.1.1. Star formation  9.1.2. Black holes and neutron stars  9.1.3. White dwarfs and supernovae  9.2. Cosmology  9.2.1. The Big Bang Theory  9.2.2. Dark matter and dark energy  9.2.3. Cosmic microwave background

Undergraduate → Classical mechanics → Newton's Laws of Motion ↓


Third Law of Motion


In the study of classical mechanics, Newton's third law of motion holds a fundamental place. Formulated by Sir Isaac Newton in 1687, this theory beautifully explains the phenomena we observe in our everyday lives as well as the complex interactions that take place in the universe around us. The third law states:

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

This law is important in understanding how forces work. It is not about a single object, but involves pairs of forces acting on two interacting objects. Understanding this concept not only helps in the tangible aspects of physics but also lays the groundwork for complex physics theories.

Understanding the law of thirds: key concepts

The third law is about interactions. Whenever one object exerts a force on another object, the second object simultaneously exerts a force of equal magnitude in the opposite direction on the first object. It is often summarized with this phrase:

"Action and reaction are equal and opposite."

The main aspects of this law are as follows:

  • Forces always exist in pairs.
  • Both forces are of equal strength.
  • The direction of the forces is opposite.
  • Forces act on two different objects.

Think of it another way: it is impossible to apply a force to an object without the object applying an equal force. These forces do not cancel out because they act on different objects.

Visual example 1: Table and book

Consider a book placed on a table. The book exerts a downward force due to gravity, and in response, the table exerts an upward force. These forces are equal in magnitude and opposite in direction.

[Book]

[table]

Action: The weight of the book falls down on the table.
Reaction: The table exerts an upward force on the book.

Here, the forces are balanced, and the book is stationary. The action and reaction forces do not cancel out because they act on different objects.

Text example: Walking

When you walk, your foot pushes back on the ground. According to the third law, the ground pushes forward on your foot with an equal force in the opposite direction. This reaction of the ground pushes you forward.

Action: The foot pushes back against the ground.
Reaction: The ground pushes the foot forward.

Without the reaction force of the ground, moving forward would be impossible. This is why walking on a slippery surface is challenging because the reaction force required from the ground is reduced.

Visual example 2: Newton's cradle

Newton's cradle is a classic example demonstrating the third law. It consists of a series of metal balls hanging in a line. When you pick up and drop one ball, it collides with the other balls. Here's what happens in terms of forces:

[Ball 1] → Strike → [Ball 2][Ball 3][Ball 4]

Action: Ball 1 exerts a force on ball 2.
Reaction: Ball 2 exerts an equal but opposite force on ball 1.

As a result, the last ball in the series is pushed out, which fully explains the rule.

Lesson example: Rocket propulsion

Rocket engines work on the basis of Newton's third law. When a rocket expels gas from its engine at high speed (action), the gas exerts an equal and opposite force on the rocket (reaction), causing the rocket to move forward.

Action: The exhaust gases are expelled downwards.
Reaction: The rocket moves upward.

This principle enables rockets and jet engines to operate effectively in space, where conventional propulsion methods such as wheels cannot work.

Mathematical representation

The quantitative expression of Newton's third law can be written as follows:

F1 = -F2

Where F1 is the force applied by the first object, and F2 is the force applied by the second object. The negative sign indicates that the direction of F2 is opposite to that of F1.

Key points to remember

  • The third law applies to all interactions where forces are present, including gravitational, mechanical, electrical, and magnetic forces.
  • Although the magnitudes of the action and reaction forces are equal and their directions are opposite, they do not cancel each other because they act on different bodies.
  • This concept applies regardless of whether the objects are stationary or moving.

Historical context and significance

Newton's third law of motion is not only a core concept for beginners but also serves as an important principle in various scientific fields. Before Newton, the understanding of motion and forces lacked coherence. Newton's laws, especially the third law, provided an important foundation for scientific inquiry and subsequent technological advancement.

This law is fundamental in understanding phenomena such as collisions, force transfer, and conservation of momentum. In disciplines ranging from engineering and architecture to robotics and aerospace, Newton's third law is constantly present in design, analysis, and application.

Practical implications and misunderstandings

In practical scenarios, the third law is evident in many situations. When you sit on a chair, you exert a downward force due to your weight, and the chair also exerts an equal upward force. If the chair cannot provide that force, it will fall.

A common misconception is that action and reaction forces cancel each other out. Remember, this doesn't happen because they act on different bodies. If you are considering all the forces on the same object, you apply Newton's second law of motion.

Conclusion

Newton's third law of motion provides profound insight into the nature of force and interaction. It is not only essential for understanding the basics of physics, but also serves as a cornerstone for more advanced concepts and technologies. Understanding this law improves our understanding of how forces work in the universe, from simple everyday phenomena to the motion of celestial bodies.

The simple, yet powerful idea of actions and reactions explains the dynamic relationships between objects and sets the stage for new discoveries and innovations. By understanding the third law, physics students establish a strong foundation for their continuing education in mechanical principles and beyond.


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Third Law of Motion

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