Grade 9
  1. Mechanics  1.1. Motion  1.1.1. Types of motion  1.1.2. Distance and displacement  1.1.3. Speed and Velocity  1.1.4. Acceleration  1.1.5. Graphical representation of motion  1.1.6. Equations of motion  1.1.7. Uniform and non-uniform motion  1.1.8. Free fall and acceleration due to gravity  1.1.9. Relative speed  1.1.10. Circular motion  1.2. Laws of force and motion  1.2.1. The concept of force  1.2.2. newton's first law of motion  1.2.3. Newton's second law of motion  1.2.4. Newton's third law of motion  1.2.5. Inertia and types of inertia  1.2.6. Motion  1.2.7. Conservation of momentum  1.2.8. Application of Newton's laws in daily life  1.2.9. Types of Forces (Contact and Non-Contact)  1.2.10. Friction and its effects  1.3. Gravitational force  1.3.1. Newton's law of universal gravitation  1.3.2. Importance of Gravitational Force  1.3.3. Acceleration due to gravity  1.3.4. Free fall and weightlessness  1.3.5. Mass and weight  1.3.6. Variation of g with height and depth  1.3.7. Kepler's laws of planetary motion  1.3.8. Satellites and their orbits  1.4. Work, Energy and Power  1.4.1. Concept of work  1.4.2. Positive and negative actions  1.4.3. Work done by constant and variable force  1.4.4. Energy and its forms  1.4.5. Kinetic energy and potential energy  1.4.6. Law of conservation of energy  1.4.8. Commercial unit of energy  1.4.9. Work–energy theorem  1.5. Simple machines  1.5.1. Types of simple machines  1.5.2. Mechanical advantage  1.5.3. Machine efficiency  1.5.4. Levers and their types  1.5.5. Pulleys and Inclined Planes  1.5.6. Gears and their uses
  2. Properties of matter  2.1. States of matter  2.1.1. Solids, liquids and gases  2.1.2. Properties of different states of matter  2.1.3. Change in state of matter  2.1.4. Plasma and Bose–Einstein condensate  2.2. Density and pressure  2.2.1. Concept of density and relative density  2.2.2. Pressure in solids, liquids and gases  2.2.3. Pascal's law and its applications  2.2.4. Atmospheric pressure and its variations  2.2.5. Surface tension and capillarity  2.3. Buoyancy and Archimedes' principle  2.3.1. Buoyant force  2.3.2. Archimedes principle  2.3.3. Applications of Archimedes' Principle  2.3.4. floating and sinking objects  2.3.5. Theory of Flotation and Archimedes' Principle
  3. Heat and Thermodynamics  3.1. Temperature and heat  3.1.1. Concept of heat and temperature  3.1.2. Temperature measurement  3.1.3. Temperature scales  3.2. Heat transfer  3.2.1. Conduction of heat  3.2.2. Convection of heat  3.2.3. heat radiation  3.2.4. Applications of heat transfer  3.2.5. Thermal conductivity  3.3. Thermal expansion  3.3.1. Expansion of solids, liquids and gases  3.3.2. Abnormal expansion of water  3.3.3. Applications of Thermal Expansion  3.4. Specific heat capacity and latent heat  3.4.1. Concept of specific heat capacity  3.4.2. Measurement and applications of specific heat  3.4.3. Latent heat of fusion and vaporization  3.4.4. Heat Engines and Efficiency
  4. Waves and sound  4.1. Waves and their types  4.1.1. Mechanical and electromagnetic waves  4.1.2. Longitudinal and Transverse Waves  4.1.3. Properties of Waves  4.1.4. Wavelength, frequency and speed of the wave  4.1.5. Superimposition of waves  4.2. Sound waves  4.2.1. Nature and transmission of sound  4.2.2. Speed of sound in different mediums  4.2.3. Reflection of sound and echo  4.2.4. Sonar and ultrasound  4.2.5. Resonance and pulsation  4.3. Sound characteristics  4.3.1. Intensity, loudness and quality of sound  4.3.2. Doppler effect in sound
  5. Lighting and Optics  5.1. Reflection of light  5.1.1. Laws of reflection  5.1.2. Reflection from a plane mirror  5.1.3. Reflection from a spherical mirror  5.1.4. Image formation by concave and convex mirrors  5.1.5. Applications of Reflection  5.2. Refraction of light  5.2.1. Laws of refraction  5.2.2. Refractive index  5.2.3. Refraction through a glass slab  5.2.4. Refraction in water and air  5.2.5. Lenses and their types  5.2.6. Image formation by convex and concave lenses  5.2.7. Total internal reflection and its applications  5.3. Dispersion and scattering of light  5.3.1. Spectrum of white light  5.3.2. Prisms and Dispersion  5.3.3. Tyndall effect and blue sky
  6. Electricity and Magnetism  6.1. Electric charge and static electricity  6.1.1. The concept of electric charge  6.1.2. Charging by friction, contact and induction  6.1.3. Electroscope and its working  6.2. Current Electricity  6.2.1. The concept of electric current  6.2.2. Ohm's Law and Resistance  6.2.3. Factors affecting resistance  6.2.4. Series and Parallel Circuits  6.2.5. Heating effect of electric current  6.2.6. Electric power and energy  6.3. Magnetism  6.3.1. Natural and artificial magnets  6.3.2. Properties of magnets  6.3.3. Earth's magnetism  6.3.4. Magnetic field and field lines  6.3.5. Electromagnets and their applications
  7. Modern Physics  7.1. structure of the atom  7.1.1. Atomic Structure and Subatomic Particles  7.1.2. Electrons, Protons, and Neutrons  7.1.3. Rutherford and Bohr model  7.2. Radioactivity  7.2.1. Types of radioactive emissions  7.2.2. Half-life and radioactive decay  7.2.3. Uses and Hazards of Radioactivity
  8. Space science and astronomy  8.1. The universe and its components  8.1.1. Stars and galaxies  8.1.2. Planets and Solar System  8.2. Satellites  8.2.1. Natural and artificial satellites  8.2.2. Use of satellites

Grade 9 → Mechanics → Laws of force and motion ↓


Newton's second law of motion


Newton's second law of motion is an essential principle in physics that provides a quantitative description of the changes that a force produces on the motion of a body. In simple terms, it describes how the velocity of an object changes when an external force is applied to it. In mathematical terms, it is often expressed as:

F = m * a

Where:

  • F is the net force applied to an object, measured in newtons (N).
  • m is the mass of the object, measured in kilograms (kg).
  • a is the acceleration generated, measured in metres per second squared (m/s²).

This equation tells us that the force applied on an object is directly proportional to the acceleration and mass of the object. If you increase the force applied on the object, the acceleration also increases in the same proportion, provided the mass remains constant. Similarly, for the same force, an object with a greater mass will experience less acceleration. Let us take a deeper look at each component and implications of the law through practical examples and simple illustrations.

Understanding the force

Force is any interaction that changes the motion of an object without opposition. Force can make an object move, stop motion, change direction, speed it up, or slow it down. Examples of forces include gravity, friction, tension, and applied force.

Imagine you are pushing a shopping cart. The force you apply to the cart's handle makes the cart move. The harder you push, the faster the cart will move. If you stop pushing, friction, which is the force that opposes motion, will eventually slow it down and cause it to stop.

Understanding mass

Mass is a measure of the amount of matter in an object. It also measures the resistance of an object to changing its state of motion when a force is applied. Objects with larger masses require more force to move than objects with less mass. For example, pushing a heavy car requires much more effort than pushing a bicycle. This resistance due to mass is called inertia.

Understanding acceleration

Acceleration is the rate of change in an object's velocity. It's a vector quantity, which means it has both magnitude and direction. When an object speeds up, slows down or changes direction, it has acceleration. According to Newton's second law, acceleration is produced when a force is applied to a mass, and acceleration is proportional to the force for a given mass.

Relation between force, mass and acceleration

Let's look at the relationship between these three key variables with an example. Suppose you have a toy car on a flat surface, and you give it a gentle push with your hand:

F

Here, force (F) is your push, mass (m) is the mass of the toy car, and acceleration (a) is how quickly the toy car accelerates. If the toy car weighs more (it has more mass), you'll need to apply more force to achieve the same acceleration. Conversely, with a lighter toy car, the same push will produce a greater acceleration.

Examples in everyday life

Example 1: Pushing a stuck car

Imagine that your car is stuck in the mud. To get it out, you and some of your friends apply force to pull it out.

m = 1500 kg (mass of the car)
F = 3000 N (force applied)

Using Newton's second law:

a = F/m = 3000 / 1500 = 2 m/s²

The car will accelerate at a speed of 2 meters per square second in the direction of the force.

Example 2: Fast moving cricket ball

Suppose you catch a fast moving cricket ball.

m = 0.15 kg (mass of the ball)
F = 15 N (force exerted by the hand to stop it)

To find the acceleration:

a = F/m = 15 / 0.15 = 100 m/s²

As soon as the ball stops in your hands, its speed slows down rapidly.

A thought experiment: doubling the force

Suppose you double the force applied to an object, while its mass remains constant. According to Newton's second law:

F = m * a

Doubling F while keeping m unchanged means that a must also be doubled to satisfy the equation. Therefore, if the force is doubled the acceleration doubles. This thought experiment shows how the acceleration of an object is directly proportional to the net force acting on it.

Misconceptions and interactions with other forces

A common misconception is that if an object is not moving, then no force is being applied to it. However, this is not true. For an object at rest, the forces applied to it may be balanced, resulting in no net change. For example, the gravitational force on a book placed on a table pulls it down and a normal force from the table pushes it up with the same magnitude.

In real-world scenarios, forces such as friction and air resistance often affect the net force acting on an object. For example, when a cyclist pedals forward, he or she exerts a forward force. Yet, air resistance and rolling friction exert opposing forces, potentially reducing the net force and, as a result, the resulting acceleration.

Conclusion

Newton's second law of motion is vital to understanding motion. It bridges the gap between force and acceleration, describing how a force applied to an object affects its motion. With the formula F = m * a, we can predict the behavior of objects in motion, design safety mechanisms, and design systems to our advantage using the laws of physics.

Through practical examples, applications, and addressing common misconceptions, this second law provides profound insight into the dynamics that govern the natural world. It serves as a key concept for students stepping into the vast and interesting field of physics and mechanics.


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Newton's second law of motion

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