CLASS 9 WORK AND ENERGY

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Chapter 11 – Work and Energy

This chapter explains the fundamental physics concepts of work, energy, and power, clarifying how they are defined scientifically (which differs from everyday usage) and how they are interconnected.

1. Work

  • In science, work is done only when:

1.    A force acts on an object, and

2.    The object undergoes displacement in the direction of the force.

  • If there is no displacement (e.g., holding a load stationary or pushing a wall that doesn’t move), no work is done, even though effort is applied.
  • Formula:

W=F×sW = F \times sW=F×s

  • Unit: Joule (J)
    • 1 J = work done by a force of 1 N moving an object by 1 m.
  • Work can be:
    • Positive (force and displacement in same direction)
    • Negative (force opposite to displacement)
    • Zero (no displacement or no force)

2. Energy

  • Energy is the capacity to do work.
  • Objects that do work lose energy, while objects on which work is done gain energy.
  • Unit: Joule (J); larger unit: kilojoule (kJ)
  • Energy exists in many forms:
    • Mechanical (kinetic + potential)
    • Heat
    • Chemical
    • Electrical
    • Light

3. Kinetic Energy (KE)

  • Energy possessed by an object due to its motion.
  • Faster motion → more kinetic energy.
  • Formula:

KE=12mv2KE = \frac{1}{2}mv^2KE=21​mv2

  • KE equals the work done to bring an object from rest to a certain velocity.

4. Potential Energy (PE)

  • Energy possessed due to an object’s position or configuration.
  • Examples: stretched rubber band, compressed spring, raised object.
  • Gravitational Potential Energy:

PE=mghPE = mghPE=mgh

  • Depends on:
    • Mass
    • Height
    • Chosen reference (ground level)

5. Mechanical Energy

  • Sum of kinetic energy and potential energy:

Mechanical Energy=KE+PE\text{Mechanical Energy} = KE + PEMechanical Energy=KE+PE

6. Law of Conservation of Energy

  • Energy can neither be created nor destroyed, only converted from one form to another.
  • Total energy of a closed system remains constant.
  • Example: In a freely falling object,
    • Potential energy decreases
    • Kinetic energy increases
    • Total energy remains the same

7. Power

  • Power measures the rate of doing work or rate of energy transfer.
  • Formula:

P=WtP = \frac{W}{t}P=tW​

  • Unit: Watt (W)
    • 1 W = 1 J/s
  • Greater power means work is done faster, not necessarily more work.

8. Commercial Unit of Energy

  • Joule is too small for practical use.
  • Kilowatt-hour (kWh) is used in electricity billing.
    • 1 kWh = 3.6×1063.6 \times 10^63.6×106 J
    • Commonly called one “unit” of electricity

Key Takeaways

  • Not all effort results in scientific “work.”
  • Energy and work share the same unit.
  • Motion → kinetic energy; position/configuration → potential energy.
  • Energy transformations are essential for life and machines.
  • Power tells how fast, not how much, work is done.
  • Electricity consumption is measured in kWh, not joules.

 

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