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Physics · Class 9 · Chapter 8: Work, Power and Energy

8 min

Key points

Work is done when a force acts on a body and the body moves through some distance in the direction of force
Work = Force × Distance (W = F × d) — only when force and displacement are in same direction
Energy is the capacity of a body to do work — measured in Joules
Kinetic Energy is the energy possessed by a body due to its motion (KE = ½mv²)
Potential Energy is the energy possessed by a body due to its position or state (PE = mgh)
Power is the rate of doing work or rate of transfer of energy (P = W/t)
Law of Conservation of Energy: Energy can neither be created nor destroyed, it can only be converted from one form to another
At Tarbela Dam, potential energy of water is converted to kinetic energy, then to electrical energy
SI unit of work and energy is Joule (J), SI unit of power is Watt (W)

Definitions

Work: Work is said to be done when a force acts on a body and the body moves through some distance in the direction of the force.
Energy: Energy is defined as the capacity of a body to do work.
Power: Power is defined as the rate of doing work or the rate of transfer of energy.
Kinetic Energy: The energy possessed by a body due to its motion is called kinetic energy.
Potential Energy: The energy possessed by a body due to its position or state is called potential energy.

Worked example

A boy pushes a box with a force of 50 N through a distance of 10 m in the direction of force. Calculate the work done. Also define power. (5 marks)

Given: Force = 50 N, Distance = 10 m Work = Force × Distance Work = 50 × 10 = 500 J Power is defined as the rate of doing work or the work done per unit time.

Common mistakes

Students forget that work is zero when force is perpendicular to displacement
Confusing kinetic energy formula (½mv²) with potential energy formula (mgh)
Not writing proper units — always write Joules for work/energy and Watts for power
Forgetting to square the velocity in kinetic energy calculations
Writing definitions without exact textbook words — BSEK expects word-for-word definitions

Quick recap

Work is force multiplied by distance moved in direction of force, measured in Joules. Power is rate of doing work, measured in Watts. Energy is capacity to do work - kinetic energy is energy due to motion, potential energy is energy due to position. Energy can neither be created nor destroyed, only converted from one form to another (Law of Conservation of Energy).

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