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 ↓


Positive and negative actions


Introduction

In the study of physics, particularly in the areas of work, energy, and power, it is fundamental to understand the concept of work. Work is defined as the measure of energy transfer that occurs when an object is moved over a distance by an external force. In mechanics, the idea of work is crucial to understanding how energy is used and transferred in various systems. It is important to understand two specific types of work: positive work and negative work. These concepts help us understand how forces can assist or oppose motion and how energy is gained or lost in a system.

What is the work?

In physics, work is a measure of energy transfer. When a force is applied to an object to cause a displacement, work is done on that object. Work is calculated using the formula:

W = F × d × cos(θ)

Where:

  • W is the work done (in joules).
  • F is the force applied to the object (in Newtons).
  • d is the displacement of the object (in meters).
  • θ is the angle between the direction of force and displacement.

Understanding affirmative action

Positive work is done when a force applied to an object causes the object to move in the direction of the force. In simple terms, if an effort causes a displacement that is in the same direction as the force, the work done is positive. This indicates that energy is being transferred to the object, resulting in an increase in the kinetic energy of the object.

Example of affirmative action

Imagine you are pushing a box across the floor. If you apply force in the direction of the box's motion, you are doing positive work on the box. As long as the force you apply is in the same direction as the box's motion, the work is considered positive.

Force Agitation

In the example shown above, the blue arrow (force) points in the same direction as the black arrow (speed), indicating that you are doing positive work on the box.

Understanding negative work

Negative work occurs when a force applied to an object causes the object to move in the opposite direction of the force. If a force opposes the motion of an object, it can slow the object down, which indicates that energy is being taken away from the object.

Example of negative work

Suppose a car slows down as it moves on a road. The friction force between the car's tires and the road surface does negative work because it acts opposite to the direction of the car's motion, reducing its speed.

Clash Agitation

In this illustration, the red arrow (friction) points in the opposite direction of the black arrow (speed), indicating that negative work is being done on the car, which slows it down.

Mathematical representation

To distinguish between positive or negative work, consider the angle θ in the work calculation formula:

W = F × d × cos(θ)

  • If 0° ≤ θ < 90°, cos(θ) is positive, so the function is positive.
  • If θ = 90°, cos(θ) = 0, and no work is done.
  • If 90° < θ ≤ 180°, cos(θ) is negative, so the function is negative.

Applications of positive and negative work

Positive actions in everyday life

Positive work is common in situations where energy is transferred to an object to increase its speed or lift it against gravity. Common examples include:

  • Lifting weights: When you lift dumbbells, you apply upward force, which is in the same direction as the movement of the weight.
  • Riding a bicycle: When you press down on the pedals, you apply a force that pushes the bicycle forward, performing positive work.

Negative work in everyday life

Negative work is seen in situations where energy is removed from the system, often causing an object to slow down or stop. Examples include:

  • Braking a car: When brakes are applied to a car, friction acts in the direction opposite to the motion of the car, doing negative work and slowing down the car.
  • When going down a hill, the force of gravity does negative work, because it acts opposite to the upward force applied by your legs.

Conclusion

Understanding positive and negative work helps us understand how forces affect the energy levels of a system. Whether an object is moved in the direction of a force (positive work) or against it (negative work), these concepts are fundamental to describing the dynamics of energy transfer and motion in physics.

By analyzing the direction of forces and their relationship to speed, we can better understand how different systems function and how work, energy, and power are interconnected in the world around us.


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Positive and negative actions

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