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 → Work, Energy and Power ↓


Concept of work


In everyday language, "work" can mean different things. For example, doing homework, cleaning the room, or working a job all fall into the broad category of work. But in physics, work has a very specific meaning. Understanding its definition is the key to understanding the interrelationship between work, energy, and power in mechanics.

Definition of work in physics

In physics, work is done when a force is applied to an object and the object moves in the direction of the force. The basic idea is that work involves the transfer of energy. The amount of work done depends on two things:

  • The magnitude of the force applied.
  • The distance over which the force is applied.

The formula used to calculate the work is:

Work = Force × Distance × cos(θ)

Where:

  • Work is the energy transferred when a force moves an object.
  • Force is the push or pull applied to an object.
  • Distance represents the distance over which the force is applied.
  • θ (theta) is the angle between the force and the direction of motion.

Work is measured in joules (J), force in newtons (N) and distance in meters (m).

Understanding the formula

The formula may look complicated because of the cosine component, but it reflects the idea that not all force contributes to the work done. Only the component of the force that acts in the direction of motion does work. If the force and motion are in the same direction, cos(θ) is 1, because cos(0°) = 1, and the work done is maximum.

Let us understand this with an example:

Force distance

Imagine a box being pushed on the floor. The force applied will be in the direction of motion of the box.

If you apply a force of 10 N to push the box 5 m, the work done can be calculated as:

Work = 10 N × 5 m × cos(0°) = 50 J

In this scenario, 50 joules of work is done to move the box. This is straightforward because the force is aligned with the motion.

Work done with angles

Things get a little more complicated when the force and motion are not perfectly aligned. For example, if the force is applied at an angle, you should only consider the component of the force that acts in the direction of motion. This is where cos(θ) factor becomes important.

Consider another case:

θ

If you pull an object moving 5 m at an angle of 45° to the horizontal with a force of 10 N, then the work done is calculated as:

Work = 10 N × 5 m × cos(45°) ≈ 35.36 J

Here, because the force is applied at an angle (45°), only the horizontal component of the force contributes to the work done.

When is the work not completed?

There are situations where force is applied but no work is done. This happens mainly when:

  • The object does not move - no displacement, no work.
  • The force is perpendicular to the direction of motion - cos(90°) = 0, so no work.

Imagine you are pushing a solid wall. You apply force, but if the wall doesn't move, then according to physics no work is done.

Some examples to know what is the real meaning of work

Pushing the car

You may have seen a situation where people are pushing a stationary car to start it. Suppose a force of 200N is applied to start the car, and the car moves 10 m. The work done is:

Work = 200 N × 10 m = 2000 J

Picking up a book

If you lift a book weighing 2 kg from the floor and place it on a shelf 1.5 m high, you apply a force equal to the weight of the book. The force of gravity acts as follows:

Force = Mass × Gravity = 2 kg × 9.8 m/s² = 19.6 N

The work done against gravity is:

Work = 19.6 N × 1.5 m = 29.4 J

Different types of work

When people refer to work in different contexts, it can be useful to categorize it:

  • Positive work: Force and displacement are in the same direction.

    Example: Pushing a box across the floor.

  • Negative work: Force and displacement are in opposite directions.

    Example: When you apply brakes to a moving car, you apply a force opposite to the direction of motion to stop it.

  • Zero work: There is no displacement, or the force is perpendicular to the displacement.

    Example: Holding a heavy bag in your hand in a steady position.

Connection to energy

When work is done, energy is often transferred from one form to another. This is where the energy relationship begins:

For example, picking up a book increases its gravitational potential energy. The work you do on it is stored as potential energy, and when you drop it, that potential energy turns into kinetic energy.

In another example, when you ride a bicycle, the chemical energy obtained from your muscles is converted into kinetic energy, and the bicycle moves forward through the work you do.

Summary

In physics, understanding work is important because it lays the foundation for understanding energy and power. Work refers to the process of energy transfer; it is the measure of the force required to move an object a distance. The mathematical expression Work = Force × Distance × cos(θ) captures the essence of how forces and directions affect the amount of work done.

Through various examples, from simple pushing of a box to lifting objects against gravity, the concept of work is explained as how energy is transferred and transformed. Delving deeper into the types of work (positive, negative, zero) and understanding its relation to energy forms further strengthens our understanding of physics in daily life.


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Concept of work

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