Grade 10
  1. Mechanics  1.1. Dynamics  1.1.1. Motion in one dimension  1.1.2. Motion in two dimensions  1.1.3. Displacement and Distance  1.1.4. Speed and Velocity  1.1.5. Acceleration  1.1.6. Graphical representation of motion  1.1.7. Equations of motion  1.1.8. Free fall and acceleration due to gravity  1.1.9. Projectile motion  1.1.10. Relative speed  1.2. Dynamics  1.2.1. Newton's Laws of Motion  1.2.2. Inertia and mass  1.2.3. Force and its types  1.2.4. action and reaction force  1.2.5. Friction and its effects  1.2.6. Coefficient of friction  1.2.7. Circular motion  1.2.8. Centripetal force and centripetal acceleration  1.2.9. Momentum and Impulse  1.2.10. Conservation of momentum  1.2.11. Newton's third law and applications  1.3. Work, Energy and Power  1.3.1. Work done by the force  1.3.2. Work–energy theorem  1.3.3. Kinetic energy  1.3.4. Potential energy  1.3.5. Mechanical energy  1.3.6. Energy Conservation  1.3.7. Power and efficiency  1.3.8. Renewable and non-renewable energy sources  1.4. Gravitational force  1.4.1. Newton's law of universal gravitation  1.4.2. Gravitational field and field strength  1.4.3. Acceleration due to gravity on different planets  1.4.4. Kepler's laws of planetary motion  1.4.5. Orbits and satellites  1.4.6. Weight and apparent weight  1.4.7. Gravitational potential energy
  2. Properties of matter  2.1. States of matter  2.1.1. Solid, liquid and gas  2.1.2. Molecular structure of matter  2.1.3. Intermolecular forces  2.1.4. Plasma and Bose–Einstein condensate  2.2. Pressure  2.2.1. Definition of Pressure  2.2.2. Pressure in liquids  2.2.3. Atmospheric pressure  2.2.4. Pascal's principle  2.2.5. Archimedes' Principle and Buoyancy  2.2.6. Surface tension and capillarity  2.3. Elasticity  2.3.1. Hooke's law  2.3.2. Stress and strain  2.3.3. Elastic Limit and Modulus of Elasticity  2.3.4. Applications of Elasticity
  3. Thermal physics  3.1. heat and temperature  3.1.1. Difference Between Heat and Temperature  3.1.2. Temperature scale  3.1.3. Thermal expansion  3.1.4. Thermal equilibrium  3.2. Heat transfer  3.2.1. Conductivity  3.2.2. Convection  3.2.3. Radiation  3.2.4. Insulation and its applications  3.3. Thermal properties of matter  3.3.1. Specific heat capacity  3.3.2. Latent heat and phase change  3.3.3. Boiling and Melting Point  3.3.4. Heat engines and refrigeration  3.4. Laws of Thermodynamics  3.4.1. Zeroth law of thermodynamics  3.4.2. First law of thermodynamics  3.4.3. Second law of thermodynamics  3.4.4. Carnot cycle
  4. Waves and optics  4.1. Nature and properties of waves  4.1.1. Types of waves in nature and properties of waves  4.1.2. Transverse and longitudinal waves  4.1.3. Wave parameters  4.1.4. Reflection and refraction of waves  4.1.5. Superposition and Interference  4.1.6. Standing Waves and Resonance  4.1.7. Doppler effect  4.2. Sound waves  4.2.1. Characteristics of sound waves  4.2.2. Speed of sound in different mediums  4.2.3. Applications of sound waves  4.2.4. Ultrasound and its uses  4.3. Light Waves and Optics  4.3.1. Nature of light  4.3.2. Reflection of light  4.3.3. Laws of reflection  4.3.4. Mirrors and Image Formation  4.3.5. Refraction of light  4.3.6. Snell's Law  4.3.7. Lenses and Image Formation  4.3.8. Total internal reflection and critical angle  4.3.9. Optical fibre  4.4. Optical Instruments  4.4.1. Microscope  4.4.2. Telescope  4.4.3. Human eye and vision defects  4.4.4. Camera and its working principle
  5. Electricity and Magnetism  5.1. Electrostatics  5.1.1. Electric charge and its properties  5.1.2. Conductors and Insulators  5.1.3. Coulomb's law  5.1.4. Electric field and field lines  5.1.5. Electric potential and potential difference  5.1.6. Capacitors and Capacitance  5.2. Current Electricity  5.2.1. Electric current and its measurement  5.2.2. Ohm's law  5.2.3. Resistance and resistivity  5.2.4. Series and Parallel Circuits  5.2.5. Electric power and energy  5.2.6. Kirchhoff's Laws  5.3. Magnetism and Electromagnetism  5.3.1. Magnetic field and field lines  5.3.2. Magnetic effects of electric current  5.3.3. Electromagnetic induction  5.3.4. Faraday's laws of electromagnetic induction  5.3.5. Transformers and Applications  5.3.6. AC and DC current
  6. Modern Physics  6.1. Nuclear physics  6.1.1. Structure of the atom  6.1.2. Bohr's atomic model  6.1.3. Electron Energy Levels  6.1.4. X-rays and applications  6.2. Radioactivity  6.2.1. Types of radiation  6.2.2. Half-life and radioactive decay  6.2.3. Nuclear reactions and applications  6.2.4. Fission and Fusion  6.3. Quantum physics  6.3.1. Wave–particle duality  6.3.2. Photoelectric effect  6.3.3. Energy quantization  6.3.4. Heisenberg's uncertainty principle
  7. Electronics and Communication  7.1. Semiconductors  7.1.1. Conductors, Insulators and Semiconductors  7.1.2. Diodes and their applications  7.1.3. Transistors and Logic Gates  7.1.4. LEDs and photodiodes  7.2. Communication Systems  7.2.1. Basics of communication  7.2.2. Analog and digital signals  7.2.3. Wireless and optical fibre communication  7.2.4. Radio Waves and Modulation

Grade 10 → Mechanics → Dynamics ↓


action and reaction force


In the study of dynamics in mechanics, a fundamental principle that governs the interactions between objects is Newton's third law of motion. This law is usually stated as, "For every action, there is an equal and opposite reaction." This principle is important for understanding how different bodies exert forces on each other and how motion is affected by these forces.

Newton's third law of motion

Newton's third law helps us understand that forces always come in pairs. These pairs are known as action and reaction forces. If an object A exerts a force on an object B, then object B will simultaneously exert a force of equal magnitude and opposite direction on object A. We call this interaction action and reaction forces.

Understanding the concept

Let us understand this concept with some simple examples:

Example 1: Book and table

Imagine you have a book placed on a table. The book exerts a downward force on the table due to its weight, often called the action force. According to Newton's third law, the table exerts an equal and upward force on the book, known as the reaction force.

Booktablereaction force Action Force

Example 2: Gun recoil

When a bullet is fired from a gun, the bullet exerts a force on the bullet in the forward direction. This is called action force. In response, the bullet exerts an equal and opposite force on the gun in the backward direction. This result is what we call the recoil of the gun.

Example 3: Swimming action

When a swimmer pushes against the water with his arms and legs, he exerts an action force on the water, causing the water to move backward. At the same time, the water exerts an equal and opposite reaction force on the swimmer, causing him to move forward.

Characteristics of action and reaction forces

It is necessary to understand some key features of action and reaction forces:

  1. Equal magnitude: The two forces are always equal in size. The magnitude of the action force will be equal to the magnitude of the reaction force.
  2. Opposite Direction: The forces act in exactly opposite directions.
  3. Act on different bodies: Action and reaction forces act on two different objects, never on the same object.
  4. Simultaneous action: These forces occur simultaneously; they are present simultaneously and there is no time lag between them.

Mathematical representation

The mathematical expression of action and reaction forces can be expressed as:

F AB = - F BA
    F AB = - F BA

Here, F AB represents the force exerted by object A on object B (action), and F BA is the force exerted by object B on object A (reaction). The negative sign indicates that these forces are equal in magnitude but opposite in direction.

Applications of action and reaction forces

Action and reaction forces come into play in countless everyday situations and scientific applications:

Rockets and space travel

Rockets work on the principle of action and reaction. When rocket engines throw gases downward, these gases exert an upward reaction force on the rocket, causing the rocket to move into space.

Walking

When you walk, you push the ground backward with your feet (action), and as a result, the ground pushes you forward (reaction), making you move forward.

Thought experiment: the floating astronaut

Suppose an astronaut is floating in space holding a tool. If the astronaut throws the tool in one direction (action), the astronaut will move in the opposite direction (reaction) due to conservation of momentum.

Study of action and reaction using balloon

A simple way to visualize action and reaction forces is to use a balloon. Inflate the balloon and let it go without tying it. As the air escapes from the balloon, it moves in the opposite direction. The expelled air is the action force, and the balloon moving in the opposite direction is the reaction force.

BalloonAir (action) Balloon (reaction)

Clarifying misunderstandings

Sometimes people mistakenly assume that if the action and reaction forces cancel each other out, they should result in no motion. But remember, these forces act on different objects; therefore, their effects do not cancel out but create motion.

Real life example of acting forces

Consider a car moving on a road. The tires push on the road with a certain force (action), and the road pushes on the tires with an equal and opposite force (reaction), which moves the car forward.

carfeedbackaction

Summary

Understanding action and reaction forces is crucial to understanding how objects in our world interact and move. From the smallest glance at a book to the vast complexities of space travel, these forces are fundamental to countless aspects of physics. Introducing this concept builds a bridge to more complex analyses in physics, where force, momentum, and dynamics play important roles.


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action and reaction force

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