⚡ Class 9 Science – Work, Energy, and Simple Machines Original Visual Revision Notes
Aligned with the Class 9 Science syllabus. Independently created study material. Not affiliated with or endorsed by NCERT.
When you push a book across a table, you do work. When you hold a heavy bag perfectly still without moving, scientists say you do no work at all — even though your muscles feel exhausted! In science, work has a very precise meaning: a force must actually cause movement through a distance. This distinction between the everyday meaning and the scientific meaning of "work" is one of the most interesting surprises in physics.
Energy is the ability to do this work. It comes in many forms — kinetic energy (energy of motion), potential energy (stored energy due to position), heat energy, light energy, electrical energy, and more. One of the most powerful ideas in all of science is the Law of Conservation of Energy: energy can never be created or destroyed, it only changes from one form to another. When you drop a ball, gravitational potential energy converts to kinetic energy. When you charge your phone, electrical energy converts to chemical energy stored in the battery.
In this chapter, we also explore simple machines — levers, pulleys, and inclined planes — tools that make our work easier by changing the size or direction of a force. By the end, you will understand why a small child can lift a heavy adult on a see-saw, how a pulley makes lifting feel lighter, and what the difference between a 60-watt and a 100-watt bulb really means!
💪 Topic 1: What is Work in Science
Work means a force moves an object some distance.
Formula: Work equals force times distance.
W = F × s is the short form.
The SI unit of work is the joule (J).
No movement at all means no work is done.
⚖ Topic 2: Positive and Negative Work
Positive work happens when force helps the motion.
Force and movement point the same way.
Negative work happens when force opposes motion.
Force and movement point in opposite ways.
Friction usually does negative work on moving things.
🔋 Topic 3: Energy and Its Forms
Energy is the ability to do work.
Mechanical energy comes from motion and position.
Heat energy is also called thermal energy.
Light and sound are forms of energy too.
Chemical and electrical energy power many devices.
🏃 Topic 4: Kinetic Energy
Kinetic energy is the energy of motion.
Formula: KE equals half of mass times speed squared.
KE = ½mv² is the short form.
Faster objects carry much more kinetic energy.
A resting object has zero kinetic energy.
⛰ Topic 5: Potential Energy
Potential energy is energy stored in an object.
Height above the ground gives potential energy.
Formula: PE equals mass times gravity times height.
PE = mgh is the short form.
Stretched or squeezed springs also store energy.
♻ Topic 6: Conservation of Energy
Energy cannot be created or destroyed at all.
Energy only changes from one form to another.
A falling object turns potential energy into kinetic.
Total mechanical energy stays the same without friction.
This idea is the law of conservation of energy.
⚡ Topic 7: Power
Power is the rate of doing work.
Formula: Power equals work divided by time.
P = W/t is the short form.
The SI unit of power is the watt (W).
One watt equals one joule done every second.
⛳ Topic 8: Simple Machines — Pulley
A pulley is a wheel that a rope runs over.
Pulleys can change the direction of a force.
A single fixed pulley has mechanical advantage of one.
Pulleys make lifting heavy loads more convenient.
Many pulleys together can reduce the effort needed.
📏 Topic 9: Simple Machines — Inclined Plane
An inclined plane is a slanted flat surface.
It helps move a load using less force.
A longer ramp needs even less effort force.
Mechanical advantage equals ramp length divided by height.
Loading ramps use this idea to lift trucks.
🔧 Topic 10: Simple Machines — Lever
A lever is a rigid bar that turns on a point.
This fixed turning point is called the fulcrum.
Effort force lifts the load around the fulcrum.
Mechanical advantage equals effort arm over load arm.
Levers help lift heavy loads with less effort.
🎨 Section 2 — Easy Illustrations
Fig 1: Work is force multiplied by the distance moved
Fig 2: Energy changes form as the ball rolls down
Fig 3: Total energy stays the same as the pendulum swings
Fig 4: Pulley, inclined plane, and lever make work easier
Fig 5: Levers differ by where the fulcrum, load, and effort sit
🤔 Section 3 — Worked Examples
💡 Example 1: Finding Work Done
A worker pushes a crate with a force of 40 N across a distance of 8 m. Find the work done.
1
Write down the known values: F = 40 N, s = 8 m.
2
Multiply force by distance: W = 40 × 8.
W = F × s
Answer: The work done is 320 J.
💡 Example 2: Finding Kinetic Energy
A bicycle of mass 15 kg moves at a speed of 6 m/s. Find its kinetic energy.
1
Write down mass and speed: m = 15 kg, v = 6 m/s.
2
Square the speed: 6 × 6 = 36.
3
KE = ½ × 15 × 36.
KE = ½mv²
Answer: The kinetic energy is 270 J.
💡 Example 3: Mechanical Advantage of a Lever
A lever has an effort arm of 90 cm and a load arm of 30 cm. It lifts a load of 60 N. Find the mechanical advantage and the effort needed.
1
Divide effort arm by load arm: 90 ÷ 30.
2
Mechanical advantage MA = 3.
3
Effort = Load ÷ MA = 60 ÷ 3.
MA = Effort Arm ÷ Load Arm
Answer: MA = 3, and the effort needed is 20 N.
📚 Section 4 — Cheat Sheet
Term
Meaning
Work
Force applied over a distance moved.
Joule (J)
The SI unit of work and energy.
Force
A push or a pull on an object.
Displacement
Distance moved in a straight direction.
Positive Work
Force and motion point the same way.
Negative Work
Force and motion point opposite ways.
Energy
The ability to do work.
Kinetic Energy
Energy that a moving object has.
Potential Energy
Energy stored due to position or height.
Power
The rate at which work is done.
Watt (W)
The SI unit of power.
Simple Machine
A tool that makes work easier to do.
Mechanical Advantage
Load divided by the effort used.
Fulcrum
The fixed point a lever turns on.
Inclined Plane
A slanted surface used to lift loads.
✍ Section 5 — Take-Home Handout
Part A: Fill in the Blanks
1. Work is done when a force moves an object through a _______.
2. The SI unit of work is the _______.
3. Kinetic energy depends on mass and _______.
4. Potential energy depends on height and _______.
5. Power is work done divided by _______.
Part B: True or False
1. Work is zero if there is no displacement.
2. Kinetic energy increases when speed increases.
3. A pulley cannot change the direction of force.
4. Power tells us how fast work is done.
5. Potential energy is highest at ground level.
Part C: Solve These
1. A force of 25 N moves a box 4 m. Find the work done.
2. A ball of mass 2 kg falls from a height of 5 m. Find its potential energy. (g = 10 m/s²)
3. A machine does 500 J of work in 10 seconds. Find its power.
Part D: Match the Columns
Column A
Column B
Your Answer
1. Kinetic Energy
A. mgh
2. Power
B. ½mv²
3. Pulley
C. W/t
4. Lever
D. Changes direction of force
5. Potential Energy
E. Rigid bar on a fulcrum
Part E: Draw and Label
Draw a simple lever below. Label the fulcrum, load, and effort.
Work is done when a force moves an object through a distance. (No displacement = no work in science.)
The SI unit of work is the joule (J). One joule = one newton of force moving an object one metre.
Kinetic energy depends on mass and velocity (speed). Formula: KE = ½mv². Doubling speed quadruples KE!
Potential energy depends on height and mass (and also gravity). Formula: PE = mgh.
Power is work done divided by time. Formula: P = W ÷ t. Unit is the watt (W).
Part B — True or False (Answers)
True. Work = Force × displacement. If displacement is zero, work = zero. Holding a heavy bag still means no work is done in science.
True. KE = ½mv². Speed is squared, so as speed increases, kinetic energy increases much faster.
False. A pulley CAN change the direction of force — that is one of its main advantages! A single fixed pulley lets you pull down to lift something up.
True. Power = Work ÷ Time. A more powerful machine does the same work in less time, or more work in the same time.
False. Potential energy is highest at maximum height, not at ground level. At ground level (h = 0), PE = mgh = 0.
Part C — Solve (Step-by-Step)
C1. Force = 25 N, distance = 4 m. Find work done.
1Formula: W = F × s
2= 25 × 4 = 100
✔ Work done = 100 joules (J)
C2. Ball: mass = 2 kg, height = 5 m. Find potential energy. (g = 10 m/s²)
1Formula: PE = mgh
2= 2 × 10 × 5 = 100
✔ Potential energy = 100 joules (J)
C3. Machine does 500 J in 10 s. Find power.
1Formula: P = W ÷ t
2= 500 ÷ 10 = 50
✔ Power = 50 watts (W)
Part D — Match the Columns (Answers)
Kinetic Energy → B. ½mv² — Energy of motion; depends on mass and square of velocity.
Power → C. W/t — Rate of doing work; unit is the watt.
Pulley → D. Changes direction of force — A single fixed pulley lets you pull down to lift something up.
Lever → E. Rigid bar on a fulcrum — A stiff rod that pivots on a fixed point to change force.
Potential Energy → A. mgh — Stored energy due to height; zero at ground level.
Part E — Draw a Lever (Guidance)
A simple Class I lever (like a see-saw) should show:
Lever bar: Draw a long horizontal line (the rigid rod).
Fulcrum (▲): A triangle shape in the middle of the bar — this is the pivot point. Label it "Fulcrum".
Load (L): A heavy weight on one end. Draw a downward arrow. Label it "Load".
Effort (E): A force applied on the other end. Draw a downward arrow (smaller than load arrow). Label it "Effort".
Label the two distances: "Load arm" (fulcrum to load) and "Effort arm" (fulcrum to effort).
Mechanical Advantage = Load arm ÷ Effort arm. Longer effort arm = less effort needed!
🃏 Section 6 — Revision Flashcards
Light side = Question. Dark side = Answer.
What is the formula for work?
W = F × s
What is the SI unit of energy?
Joule (J)
What is the formula for kinetic energy?
KE = ½mv²
What is the formula for potential energy?
PE = mgh
What happens to energy during conservation?
It stays constant, only its form changes.
What is the formula for power?
P = W/t
What is mechanical advantage?
Load divided by effort.
What are the three classes of levers?
Class I, Class II, and Class III.
🧠 Section 7 — Mind Map
Fig 6: Mind map of work, energy, and simple machines
⚠ This is independently created study material aligned with the Class 9 Science syllabus. Not affiliated with or endorsed by NCERT or any government body.