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Physics · Class 9 · Chapter 9: Simple Machines

8 min

Key points

A machine is a device that makes work easier by changing the magnitude or direction of force
Simple machines include lever, pulley, wheel and axle, inclined plane, wedge, and screw
Mechanical Advantage (MA) = Load/Effort — it tells us how many times a machine multiplies the effort
Velocity Ratio (VR) = Distance moved by effort/Distance moved by load
Efficiency = (MA/VR) × 100% — no machine is 100% efficient due to friction
First class lever has fulcrum between effort and load (scissors, crowbar)
Second class lever has load between fulcrum and effort (wheelbarrow, nutcracker)
Third class lever has effort between fulcrum and load (fishing rod, human arm)
Fixed pulley changes direction of force, movable pulley provides mechanical advantage
Inclined plane reduces effort force but increases distance — used in loading trucks at Karachi port

Definitions

Machine: A device which makes our work easier either by changing the magnitude of force or by changing the direction of force or both.
Mechanical Advantage: The ratio of load to the effort is called mechanical advantage. MA = Load/Effort
Velocity Ratio: The ratio of distance moved by effort to the distance moved by load is called velocity ratio. VR = Distance moved by effort/Distance moved by load
Efficiency: The ratio of mechanical advantage to velocity ratio expressed as percentage is called efficiency. Efficiency = (MA/VR) × 100%
Lever: A rigid bar which can rotate about a fixed point called fulcrum is known as lever.

Worked example

Define simple machine. Calculate the mechanical advantage of a lever if effort arm is 60cm and load arm is 15cm. Also find the effort required to lift a load of 200N. (6 marks)

Simple machine is a device that makes our work easier by changing the direction, magnitude or point of application of force. MA = Effort arm/Load arm = 60cm/15cm = 4. Since MA = Load/Effort, therefore 4 = 200N/Effort, so Effort = 200N/4 = 50N

Common mistakes

Students confuse the positions of effort, load, and fulcrum in different classes of levers
Forgetting to write the formula before solving numerical problems
Mixing up mechanical advantage and velocity ratio definitions
Not converting units properly in numerical problems
Drawing incorrect diagrams for different types of pulleys

Quick recap

Simple machines make work easier by changing force direction, magnitude or application point. Six types are lever, pulley, wheel-axle, inclined plane, wedge and screw. Mechanical advantage equals load divided by effort, while efficiency equals work output divided by work input multiplied by 100. Remember the three classes of levers and their real-life examples for exam answers.

Generated by AI from Sindh Textbook Board material; review state: ai_unreviewed. MCQs, past-paper references and examiner notes are held back until a teacher has checked them.