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Science Class 9 - Work And-Energy Notes

Comprehensive study notes for Class 9 - Work And-Energy olympiad preparation

Work and Energy

Welcome to the chapter on Work and Energy for Class 9. In this chapter, you will learn what work and energy mean in science, how they are measured, and how they are related. By the end of this chapter, you will be able to solve problems and understand the importance of work and energy in daily life and nature.

Key Concepts

  • Work: Work is done when a force moves an object in the direction of the force.
  • Energy: Energy is the ability to do work.
  • Joule: The SI unit of work and energy.

Work

Work is done when a force acts on an object and moves it. If there is no movement, no work is done.

  • Formula: Work = Force × Distance
  • Work is measured in joules (J).
  • If force is 5 N and distance is 2 m, Work = 5 × 2 = 10 J

Energy

Energy is the ability to do work. There are many forms of energy, such as:

  • Kinetic Energy (energy of moving objects)
  • Potential Energy (energy stored in objects)
  • Heat Energy
  • Light Energy
  • Chemical Energy

Kinetic Energy

Kinetic energy is the energy of a moving object.

  • Formula: Kinetic Energy = ½ × mass × velocity²
  • If mass is 2 kg and velocity is 3 m/s, KE = ½ × 2 × 3² = 9 J

Potential Energy

Potential energy is stored energy due to position or shape.

  • Formula: Potential Energy = mass × gravity × height
  • If mass is 1 kg, gravity is 10 m/s², and height is 5 m, PE = 1 × 10 × 5 = 50 J

Law of Conservation of Energy

Energy cannot be created or destroyed. It can only change from one form to another.

  • Example: When you drop a ball, potential energy changes to kinetic energy.

Applications of Work and Energy

  • Machines use energy to do work.
  • Our bodies use food energy to move and grow.
  • Electricity is a form of energy used in homes and schools.

Practice Questions

  1. Calculate the work done when a force of 10 N moves an object 3 m.
  2. What is the kinetic energy of a 4 kg object moving at 2 m/s?
  3. Find the potential energy of a 2 kg object at a height of 6 m (g = 10 m/s²).
  4. Name two forms of energy.
  5. State the law of conservation of energy.

Challenge Yourself

  • List three examples where energy changes from one form to another.
  • Explain why no work is done if you push a wall and it does not move.

Did You Know?

  • The food we eat gives us chemical energy!
  • Solar panels change sunlight into electrical energy.

Glossary

  • Force: A push or pull on an object.
  • Joule: The unit of work and energy.
  • Kinetic Energy: Energy of a moving object.
  • Potential Energy: Stored energy due to position.

Answers to Practice Questions

  1. Work = 10 × 3 = 30 J
  2. KE = ½ × 4 × 2² = 8 J
  3. PE = 2 × 10 × 6 = 120 J
  4. Heat energy, light energy (any two).
  5. Energy cannot be created or destroyed, only changed from one form to another.

Understanding work and energy helps you see how things move and change in the world around you!

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