Short, simple notes by SciNCERT Decoder by Kuldeep S. Take g = 10 m/s² in all calculations.
These Work Energy and Simple Machines Class 9 Notes cover the whole of CBSE Class 9 Science Chapter 7 in easy words, with diagrams, formulas and solved examples. Read them along with your NCERT Science textbook (the books are also on the official NCERT website), and find more chapters on our Class 9 study material page.
Work Energy and Simple Machines Class 9 Notes at a Glance
- Work = force × displacement in the direction of the force (W = F s cos θ). Unit: joule.
- Kinetic energy = ½ m v² and potential energy = m g h.
- Work–energy theorem: net work done = change in kinetic energy.
- Mechanical energy (K + U) stays constant when only gravity-type forces act.
- Power = work ÷ time. Unit: watt.
- Simple machines (pulley, inclined plane, lever) change the size or direction of a force. They never reduce the total work.
1. Work
(F = force in N, s = displacement in m, θ = angle between force and displacement)
If force and displacement are in the same direction, θ = 0°, so W = F × s.
Two conditions for work: (1) a force acts, and (2) the object moves.
Unit: joule (J). 1 J = 1 N × 1 m.
Work is a scalar. It has size but no direction.
Positive, negative and zero work
| Type | When | Example | Effect on energy |
|---|---|---|---|
| Positive | θ < 90° (force helps the motion) | Pushing a handcart forward | Energy increases |
| Negative | θ > 90° (force opposes the motion) | Friction on a sliding box | Energy decreases |
| Zero | F = 0, or s = 0, or θ = 90° | Pushing a fixed wall; carrying a bucket on a level road; centripetal force | No energy change |
Work from a graph
On a force–displacement graph, work = area under the graph.
2. Energy
Forms: mechanical, heat, light, sound, electrical, chemical, nuclear.
| Device | Energy change |
|---|---|
| Electric motor / ceiling fan | Electrical → Mechanical |
| Generator | Mechanical → Electrical |
| Bulb | Electrical → Light (and heat) |
| Dry cell | Chemical → Electrical |
| Solar cell | Light → Electrical |
| Plant (photosynthesis) | Light → Chemical |
3. Work–Energy Theorem
This theorem links the first two topics of the Work Energy and Simple Machines Class 9 Notes: work and energy.
- Positive net work → speed increases.
- Negative net work → speed decreases.
- Zero net work → speed stays the same.
4. Kinetic Energy (K)
(m in kg, v in m/s)
Quick proof: From rest, W = F × s = ma × (v²/2a) = ½mv². This work is stored as kinetic energy.
5. Potential Energy (U)
Gravitational potential energy:
(h = height above a chosen level)
- The value depends on the level you choose as zero.
- It depends only on the height gained, not on the path taken.
- Elastic potential energy is stored in a stretched or compressed spring, rubber band or bow.
6. Mechanical Energy and Its Conservation
Conservation of mechanical energy is the most useful idea in the Work Energy and Simple Machines Class 9 Notes for numerical questions.
A stone falling from height h
| Position | PE | KE | Total |
|---|---|---|---|
| Top | mgh | 0 | mgh |
| Halfway | ½ mgh | ½ mgh | mgh |
| Just before ground | 0 | mgh | mgh |
At the ground, v = √(2gh).
Simple pendulum
At the two ends, KE = 0 and PE is maximum. At the lowest point, KE is maximum and PE is minimum.
In real life: friction and air resistance turn some mechanical energy into heat and sound. So a pendulum stops after some time. The total energy is still conserved, but some is no longer useful.
7. Power
Power completes the energy part of the Work Energy and Simple Machines Class 9 Notes. The machines come next.
Unit: watt (W). 1 W = 1 J/s.
- 1 kW = 1000 W.
- 1 horsepower (hp) = 746 W.
- Also, P = F × v when force and velocity are in the same direction.
8. Simple Machines
In the Work Energy and Simple Machines Class 9 Notes, simple machines are all about trading force for distance. Learn the pulley, the inclined plane and the lever one by one.
Key terms:
- Effort (E): the force we apply.
- Load (L): the force we want to overcome.
- Mechanical advantage: MA = Load ÷ Effort
| MA | The machine is a… | Example |
|---|---|---|
| More than 1 | Force multiplier | Crowbar, car jack |
| Equal to 1 | Direction changer | Fixed pulley |
| Less than 1 | Speed/distance multiplier | Tweezers, broom |
Efficiency is always below 100%.
8.1 Pulley
A pulley is a grooved wheel with a rope.
- Fixed pulley: only changes the direction of the force. MA = 1. Example: drawing water from a well.
- Movable pulley: moves with the load. Ideally MA = 2, so a 200 N effort lifts a 400 N load. Cranes use pulley systems.
8.2 Inclined plane
A ramp lets us raise a load with less force over a longer distance.
(L = length of ramp, h = height). A longer, gentler ramp has a bigger MA and needs less effort.
8.3 Lever
A lever is a rigid bar that turns about a fixed point called the fulcrum.
MA = effort arm ÷ load arm
| Class | Who is in the middle? | Examples |
|---|---|---|
| I | Fulcrum | Seesaw, scissors, crowbar |
| II | Load | Wheelbarrow, bottle opener |
| III | Effort | Tweezers, tongs, broom |
A door handle is placed far from the hinges because a long effort arm gives a bigger turning effect from a small force.
9. Solved Examples
These solved examples from the Work Energy and Simple Machines Class 9 Notes show how to use each formula. Always write the formula, the values with units, and then the answer.
W = F × s = 80 × 25 = 2000 J
W = Fs cos θ = 50 × 10 × 0.5 = 250 J
Work done by friction = −20 × 3 = −60 J
K = ½ × 60 × 5² = 750 J. At 10 m/s, K = 3000 J, which is 4 times as much.
ΔK = 0 − ½ × 500 × 10² = −25,000 J. So F × 10 = 25,000, giving F = 2500 N
U = mgh = 2 × 10 × 5 = 100 J
v = √(2gh) = √(2 × 10 × 20) = 20 m/s
W = mgh = 100 × 10 × 10 = 10,000 J. P = 10,000 ÷ 50 = 200 W
MA = 3 ÷ 1 = 3. To lift a 600 N drum, ideal effort = 600 ÷ 3 = 200 N
MA = 1.5 ÷ 0.3 = 5. To lift 1000 N, effort = 1000 ÷ 5 = 200 N
Efficiency = (400 ÷ 500) × 100 = 80%
10. Formula Sheet
This sheet gives every key formula of the Work Energy and Simple Machines Class 9 Notes in one place. Revise it daily before the exam.
| Quantity | Formula | Unit |
|---|---|---|
| Work | W = F s cos θ | J |
| Kinetic energy | K = ½ m v² | J |
| Potential energy | U = m g h | J |
| Work-energy theorem | W(net) = K(final) − K(initial) | J |
| Mechanical energy | E = K + U | J |
| Power | P = W/t = F v | W |
| MA | Load ÷ Effort | no unit |
| MA of inclined plane | L ÷ h | no unit |
| MA of lever | Effort arm ÷ Load arm | no unit |
| Efficiency | (Output ÷ Input) × 100 | % |
11. Common Mistakes
Avoid these mistakes when you revise the Work Energy and Simple Machines Class 9 Notes and when you write your answers.
- Saying “work is done” when someone pushes a wall that does not move. Work is zero.
- Forgetting that energy and work have the same unit, the joule.
- Thinking a machine gives “free” work. It only trades force for distance.
- Using cm or g in formulas. Always convert to m and kg first.
- Saying “total energy is lost” with friction. Mechanical energy is lost as heat; total energy is conserved.
12. Test Yourself
Check how well you understood the Work Energy and Simple Machines Class 9 Notes. Tap a question to see its answer.
1. When is the work done by a force zero?
F = 0, s = 0, or the force is perpendicular to the motion.
2. What happens to the KE of a car when its speed becomes 3 times?
It becomes 9 times.
3. State the law of conservation of mechanical energy.
If only gravity-type forces act (no friction or air resistance), K + U stays constant.
4. A 40 kg girl climbs 15 steps of 0.2 m each in 6 s. Find her power.
h = 3 m; W = 40 × 10 × 3 = 1200 J; P = 1200 ÷ 6 = 200 W.
5. Why does a fixed pulley make work easier if MA = 1?
It changes the direction of the force to a more convenient one, so we can pull downwards.
13. Important Questions and Answers
These are the most asked questions from the Work Energy and Simple Machines Class 9 Notes. Learn the answers in your own words.
Work is done when a force moves an object in the direction of the force. It is calculated as W = F s cos θ, and its unit is the joule.
Work is a scalar. It has size but no direction. It can be positive, negative or zero, but the sign does not show a direction.
Work is negative when the force acts opposite to the displacement (θ is more than 90°). Friction on a sliding box is an example.
Kinetic energy is the energy of motion, K = ½ m v². Potential energy is the energy of position or shape, such as U = m g h. A moving auto-rickshaw has kinetic energy, and water in an overhead tank has potential energy.
Because K = ½ m v² depends on the square of the speed, and (2v)² = 4v².
The net work done on an object is equal to the change in its kinetic energy.
No. The height does not change, so the potential energy stays the same. The lifting force is also perpendicular to the motion, so it does no work.
Total energy is always conserved. Mechanical energy (K + U) is conserved only when there is no friction or air resistance.
Power is the rate of doing work, P = W/t. Its unit is the watt. 1 horsepower = 746 W.
A longer ramp has a bigger mechanical advantage (L ÷ h), so less effort is needed. But the effort acts over a longer distance, so the total work is the same in an ideal case.
Because energy cannot be created. Real machines lose some energy as heat and sound due to friction, so their efficiency is always below 100%.
Class III levers, such as tweezers and a broom, because their effort arm is shorter than their load arm.
14. Quick Summary
Work is force times displacement in the direction of the force, and energy is the capacity to do work. Kinetic energy depends on speed, potential energy depends on height, and their sum stays constant when no friction acts. Power tells how fast work is done, and simple machines only trade force for distance. Revise these Work Energy and Simple Machines Class 9 Notes with the diagrams and solved examples, and you will be ready for any question from this chapter.
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