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 ↓


The concept of force


In the fascinating field of physics, force is a fundamental concept that helps us understand how objects interact with each other and with the world around them. At its core, force is any interaction that will change the motion of an object without opposition. In simple terms, force can change the velocity of an object with mass, i.e. speed it up. Force can also change the direction or shape of an object.

What is force?

Force refers to the push or pull on an object that results from the object's interaction with another object. Whenever two objects interact, forces act on each object. These forces can cause a change in the object's state of motion and are measured in newtons (N), named after Sir Isaac Newton, who formulated the three basic laws of motion.

Force (F) = Mass (m) × Acceleration (a)
  F = m × a

In this formula, F means force, m means mass, and a means acceleration. One newton is defined as the amount of force needed to accelerate one kilogram of mass through one meter per square second.

Types of forces

Forces can be broadly classified into two categories: contact forces and non-contact forces.

1. Contact force:

  • Applied force: This is the force applied to an object by someone or another object. For example, pushing a grocery cart is an applied force.
  • Friction force: It is the force exerted by an object on a surface when another object moves over it. For example, it takes friction to pull a sled over snow.
  • Tension force: It is the force transmitted through a string, rope, cable or wire when it is pulled by forces acting from opposite ends.
  • Normal force: The base force applied to an object in contact with another stationary object, such as a book on a table.
  • Air resistance force: A type of frictional force that acts on objects as they travel through the air, such as a parachute slowing down a skydiver's speed.

2. Non-contact forces:

  • Gravitational force: The force by which the Earth, the Moon, or any other massive object attracts another object. It is the force that gives weight to physical objects and makes them fall to the ground.
  • Electromagnetic force: It involves the force of attraction or repulsion between electrically charged particles. It is responsible for electric forces and magnetic forces.
  • Nuclear force: A strong force that acts within the atomic nucleus, holding the protons and neutrons together. This force is much more powerful than electromagnetic forces.

Newton's laws of motion

Sir Isaac Newton laid the foundation for classical mechanics with his three laws of motion, which describe how forces and objects interact. These laws are crucial for understanding and predicting motion.

Newton's first law of motion (law of inertia):

Unless acted upon by an external force, an object will remain at rest or in uniform motion in a straight line. This means that if no external force acts on an object, its velocity will remain constant. This is often referred to as the law of inertia, where inertia is the resistance of any physical object to a change in its state of motion.

Example: Consider a snowflake sliding on a perfectly smooth ice surface. It will continue moving in a straight line unless an external force, such as friction or a player's stick, acts on it to stop it or change its direction.

Ice Puck

Newton's second law of motion:

The acceleration of an object depends directly on the total force applied to the object, and inversely on the mass of the object. The formula for this law is:

F = m × a

What this law means is that greater force will result in greater acceleration and greater mass will result in less acceleration.

Example: Imagine you are pulling two sleds, one of which is much heavier than the other. If you apply the same force to both, the lighter sled will accelerate faster, while the heavier sled will accelerate slower.

Heavy sled Light Sled

Newton's third law of motion:

For every action there is an equal and opposite reaction. This means that forces always occur in pairs. If one object exerts a force on another object, the second object exerts a force of equal magnitude and opposite direction on the first object.

Example: When you jump from a small boat to a pier, you will notice that the boat moves away from the pier. This happens because when you push the boat to jump, the boat also pushes back with the same force.

boat push

Balanced and unbalanced forces

Forces can be balanced or unbalanced. Balanced forces are equal in size but opposite in direction; they do not change the state of motion of an object. Unbalanced forces, on the other hand, are not equal and opposite, and they cause a change in the motion of the object.

  • Balanced forces: When two forces acting on an object are equal in size but act in opposite directions, they are balanced forces. An object with balanced forces will remain at rest or keep moving at the same speed and in the same direction. For example, a book placed on a table experiences balanced forces because the gravitational force pulling it down is balanced by the normal force pushing it up.
  • Unbalanced forces: If one of the forces acting on an object is stronger than the other, the forces are unbalanced. They cause objects to start moving, stop moving, or change direction. For example, if you kick a soccer ball, the force from your foot overcomes the forces of friction and gravity, causing the ball to move.
ball

Force and motion: everyday examples

Let's look at how forces manifest in real-world scenarios:

  • Driving a car: The engine generates a force that moves the car forward, overcoming both air resistance and friction. The steering wheel and brakes are applications of force to change direction and speed.
  • Playing the game: Kicking a football applies force, which makes the ball fly. The amount of force depends on how far and how fast the ball goes.
  • Rowing a boat: When you row a boat, you push against the water; the water pushes back and pushes you forward with equal force.
  • Pushing a swing: Applying force to the swing causes it to move. Due to inertia, the motion continues until air resistance and friction slow it down.

Conclusion

The concept of force is central to understanding the physical world. By studying the interactions that occur between objects through forces, we can develop insights about motion, predict behavior, and create solutions in technology and everyday life. From Newton's groundbreaking laws to practical applications across a variety of disciplines, force remains a vital element in science, influencing everything from space exploration to the devices we use daily.


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The concept of force

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