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CBSE Class 9 — Notes, Chapters & Practice Quizzes

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Science · 13 chapters
Summary, key terms, extra questions with answers and a practice quiz with AI diagnosis for each.

Chapter 7: Work, Energy, and Simple MachinesClass 9 Science — summary, notes, extra questions & NCERT solutions

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In science, work is done only when a force produces:

Summary

This chapter gives the scientific meanings of work, energy and power and links them through energy conservation, then introduces simple machines. In science, work is done only when a force moves an object along its direction: \(W = F \times s\), measured in joules (J); no work is done if there is no displacement or if the force is perpendicular to the motion. Energy is the capacity to do work, also measured in joules. Kinetic energy is the energy of motion, \(KE = \tfrac{1}{2}mv^2\), while gravitational potential energy is the energy of position, \(PE = mgh\). The law of conservation of energy states that energy can neither be created nor destroyed but only transformed; for a freely falling or sliding body the sum of kinetic and potential energy stays constant, which is why two children of different masses reach the bottom of the same frictionless slide with the same speed. Power is the rate of doing work, \(P = \dfrac{W}{t}\), measured in watts (W); commercial electrical energy is sold in kilowatt-hours (units). Simple machines — the lever, pulley, inclined plane, wheel and axle, wedge and screw — do not create energy but make work easier by changing the size or direction of the effort needed, illustrating mechanical advantage in everyday tools.

Scientific meaning of workKinetic and potential energyConservation and transformation of energyPower and commercial energy (kWh)Simple machines and mechanical advantage

Key terms

Work
Done when a force moves an object along its own direction, \(W = F \times s\); measured in joules (J).
Kinetic energy
The energy possessed by a body because of its motion, \(KE = \tfrac{1}{2}mv^2\).
Potential energy
The energy possessed by a body because of its position or configuration, e.g. gravitational \(PE = mgh\).
Law of conservation of energy
Energy can neither be created nor destroyed, only changed from one form to another; the total stays constant.
Power
The rate of doing work or transferring energy, \(P = \dfrac{W}{t}\); measured in watts (W).
Simple machine
A device such as a lever, pulley or inclined plane that makes work easier by changing the magnitude or direction of the effort.

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Kinetic energy
Kinetic energy =9J
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Done when a force moves an object along its own direction, \(W = F \times s\); measured in joules (J).
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Practice quiz · Work, Energy, and Simple Machines

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