Important Questions Class 9 Science Chapter 7 cover Work, Energy, and Simple Machines as per the CBSE 2026 syllabus. Work is done when a force moves an object in the direction of the force.
Students will find class 9 science chapter 7 important questions with answers on work, kinetic energy, potential energy, power, simple machines, mechanical advantage, numericals, and assertion-reason practice.
A ball rolling down a slope, a person climbing stairs, and a pulley lifting a bucket all show the same idea: energy helps work get done. Class 9 Science Chapter 7 teaches students how to measure this work, calculate energy, compare power, and understand why machines reduce effort. The chapter has many numericals, so students need clear formulas, correct units, and step-by-step practice for CBSE 2026 exams.
Key Takeaways
| Detail |
Information |
| Chapter |
Class 9 Science Chapter 7 |
| Topic |
Work, Energy, and Simple Machines |
| Syllabus |
CBSE 2026 |
| Question Types |
VSA, Short Answer, Long Answer, Numericals, Assertion-Reason, Case Study |
| Key Topics |
Work Done, Kinetic Energy, Potential Energy, Power, Simple Machines, Mechanical Advantage |
Class 9 Science Chapter 7 Work, Energy, and Simple Machines: Topics Covered
The work energy and simple machines class 9 important questions in this article cover the full chapter scope. Revise the concepts first, then practise numericals.
- Work done by a force
- Positive work, negative work, and zero work
- SI unit of work and energy
- Work-energy theorem
- Kinetic energy and its formula
- Potential energy and its formula
- Conservation of mechanical energy
- Power, watt, and horsepower
- Simple machines: pulley, inclined plane, lever
- Mechanical advantage
- Three classes of levers
Class 9 Science Chapter List
Important Questions Class 9 Science Chapter 7 with Answers
Important Questions Class 9 Science Chapter 7 with answers should be practised in exam order. Start with definitions, move to short answers, and then solve numericals.
The class 9 science chapter 7 question answer format below covers formulas, reasoning, application, and case-based questions.
Very Short Answer Important Questions Class 9 Science Chapter 7
Q1. Define work done by a force.
Ans. Work is done when a force acts on an object and the object moves in the direction of the force.
Formula: Work = Force × Displacement
SI unit: joule (J)
Q2. What is the SI unit of energy?
Ans. The SI unit of energy is the joule (J). One joule is the work done when a force of one newton moves an object by one metre.
Q3. When is work done zero?
Ans. Work done is zero when there is no displacement or when force and displacement are perpendicular.
Example: A person pushing a wall does zero work because the wall does not move.
Q4. What is kinetic energy?
Ans. Kinetic energy is the energy possessed by a body due to motion.
Formula: KE = ½mv²
Q5. What is potential energy?
Ans. Potential energy is the energy stored in a body due to its position or configuration.
Formula: PE = mgh
Q6. Define power.
Ans. Power is the rate of doing work.
Formula: Power = Work done / Time taken
SI unit: watt (W)
Q7. What is a simple machine?
Ans. A simple machine is a device that makes work easier by changing the magnitude or direction of force.
Examples include levers, pulleys, and inclined planes.
Q8. What is mechanical advantage?
Ans. Mechanical advantage is the ratio of load to effort.
Formula: MA = Load / Effort
Short Answer Questions Class 9 Science Chapter 7 with Answers
Short answers in this chapter test definitions with examples. Write the formula wherever possible.
Q1. Distinguish between positive work and negative work.
Ans. Positive work is done when force and displacement act in the same direction. Example: gravity acting on a falling apple.
Negative work is done when force and displacement act in opposite directions. Example: friction acting on a sliding block.
Q2. State the work-energy theorem.
Ans. The work-energy theorem states that the net work done on an object equals the change in its kinetic energy.
Work done = Final KE - Initial KE
Q3. What is the law of conservation of mechanical energy?
Ans. The total mechanical energy remains constant if only conservative forces act on a system.
KE + PE = constant
Q4. Differentiate between kinetic energy and potential energy.
| Basis |
Kinetic Energy |
Potential Energy |
| Meaning |
Energy due to motion |
Energy due to position |
| Formula |
KE = ½mv² |
PE = mgh |
| Example |
Moving ball |
Raised stone |
Q5. What is the difference between work and power?
Ans. Work is the total energy transferred when force causes displacement.
Power is the rate at which work is done. A person who completes the same work faster has more power.
Q6. What are the three classes of levers?
Ans.
| Lever Class |
Arrangement |
Examples |
| Class 1 |
Fulcrum between load and effort |
Seesaw, scissors |
| Class 2 |
Load between fulcrum and effort |
Wheelbarrow, bottle opener |
| Class 3 |
Effort between fulcrum and load |
Tweezers, fishing rod |
Long Answer Important Questions Class 9 Science Chapter 7
Long answers should include concept, process, example, and conclusion. Use tables where comparison is needed.
Q1. Explain energy transformations in a simple pendulum.
Ans. A pendulum swings between two extreme positions with a mean position at the centre.
At the extreme position, the bob is at maximum height. Potential energy is maximum and kinetic energy is zero.
As the bob moves toward the mean position, potential energy changes into kinetic energy. At the mean position, speed is maximum and kinetic energy is maximum.
As the bob moves to the other extreme, kinetic energy changes back into potential energy.
This shows conservation of mechanical energy. In real life, air resistance and friction reduce mechanical energy gradually.
Q2. Describe the three classes of levers with examples and mechanical advantage.
Ans. A lever has three parts: fulcrum, load, and effort. The arrangement of these parts decides the class of lever.
Class 1 lever has the fulcrum between load and effort. Examples are seesaw, crowbar, and scissors. Its mechanical advantage can be greater than, equal to, or less than 1.
Class 2 lever has the load between fulcrum and effort. Examples are wheelbarrow, bottle opener, and nutcracker. Its mechanical advantage is always greater than 1.
Class 3 lever has the effort between fulcrum and load. Examples are tweezers, fishing rod, and human forearm. Its mechanical advantage is less than 1, but it increases speed and range of movement.
Work and Energy Class 9 Questions
Work and energy class 9 questions test formulas, sign of work, energy conversion, and conservation. Students should always mention the unit in the final answer.
This section covers the most common theory and numerical formats.
Work Done Questions Class 9
Q1. Give two examples of positive work and negative work.
Ans.
Positive work:
- A person kicking a football forward
- Gravity pulling a falling apple downward
Negative work:
- Friction acting on a moving object
- A goalkeeper stopping a ball
Q2. Can work done by a force be negative?
Ans. Yes. Work is negative when force acts opposite to displacement.
Friction does negative work because it opposes motion.
Q3. Define one joule of work.
Ans. One joule is the work done when a force of one newton moves an object through one metre in the direction of force.
1 J = 1 N × 1 m
Q4. A weightlifter holds a 100 kg barbell above his head for 10 seconds. How much work is done on the barbell?
Ans. Work done is zero because displacement is zero.
The weightlifter applies force, but the barbell does not move.
Numericals Class 9 Science Chapter 7
Numericals from this chapter require formula, substitution, calculation, and final unit. Do not skip steps in CBSE 2026 exams.
Work Done Numericals Class 9 Science Chapter 7
Q1. A force of 50 N moves an object by 4 m in the direction of force. Calculate work done.
Ans.
W = F × d
W = 50 × 4
W = 200 J
Q2. A person carries a 20 kg box horizontally across a room. How much work is done against gravity?
Ans. Work done against gravity is zero.
Gravity acts vertically downward, while displacement is horizontal. Force and displacement are perpendicular.
Q3. A force of 30 N acts at 0° to a displacement of 6 m. Find the work done.
Ans.
W = F × d × cos θ
W = 30 × 6 × cos 0°
W = 180 J
Kinetic Energy Numericals Class 9
Kinetic energy numericals class 9 use KE = ½mv². Square the velocity before multiplying.
Q1. A car of mass 1000 kg moves at 20 m/s. Calculate its kinetic energy.
Ans.
KE = ½mv²
KE = ½ × 1000 × 20²
KE = 500 × 400
KE = 2,00,000 J
Q2. A ball of mass 0.5 kg is thrown at 10 m/s. Calculate its kinetic energy.
Ans.
KE = ½mv²
KE = ½ × 0.5 × 10²
KE = 25 J
Q3. If velocity doubles, how does kinetic energy change?
Ans. KE depends on v².
If velocity becomes 2v, kinetic energy becomes four times the original value.
Potential Energy Numericals Class 9
Potential energy numericals class 9 use PE = mgh. Convert mass to kg and height to metre before solving.
Q1. A stone of mass 2 kg is lifted to a height of 5 m. Calculate its potential energy. Take g = 10 m/s².
Ans.
PE = mgh
PE = 2 × 10 × 5
PE = 100 J
Q2. A book of mass 500 g is kept on a shelf 1.5 m high. Find its potential energy. Take g = 10 m/s².
Ans.
Mass = 500 g = 0.5 kg
PE = mgh
PE = 0.5 × 10 × 1.5
PE = 7.5 J
Q3. An object has potential energy of 300 J at a height of 6 m. Find its mass. Take g = 10 m/s².
Ans.
PE = mgh
300 = m × 10 × 6
m = 300 / 60
m = 5 kg
Power Numericals Class 9
Power numericals class 9 use P = W/t. Work should be in joules and time in seconds.
Q1. A machine does 5000 J of work in 25 seconds. Calculate power.
Ans.
P = W/t
P = 5000 / 25
P = 200 W
Q2. A person of mass 60 kg climbs a staircase 10 m high in 20 seconds. Calculate power. Take g = 10 m/s².
Ans.
W = mgh
W = 60 × 10 × 10
W = 6000 J
P = W/t
P = 6000 / 20
P = 300 W
Q3. Convert 5 horsepower into watts.
Ans.
1 horsepower = 746 W
5 horsepower = 5 × 746
5 horsepower = 3730 W
Mechanical Advantage Numericals Class 9
Mechanical advantage questions class 9 test whether students can identify load and effort correctly.
Q1. A lever lifts a load of 400 N with an effort of 100 N. Calculate mechanical advantage.
Ans.
MA = Load / Effort
MA = 400 / 100
MA = 4
Q2. A pulley uses an effort of 250 N to lift a load of 1000 N. Find mechanical advantage.
Ans.
MA = Load / Effort
MA = 1000 / 250
MA = 4
Q3. A machine has MA = 3. If effort is 60 N, what load can it lift?
Ans.
Load = MA × Effort
Load = 3 × 60
Load = 180 N
Kinetic and Potential Energy Questions Class 9
Kinetic and potential energy questions often appear as formula-based and reasoning questions. Learn the formula and the condition where each energy is maximum.
Kinetic Energy Formula Questions Class 9
Q1. Write the formula for kinetic energy and define each term.
Ans. KE = ½mv², where m is mass in kg and v is velocity in m/s.
Kinetic energy is measured in joules.
Q2. How does kinetic energy change if mass doubles?
Ans. KE = ½mv².
If mass doubles and velocity remains the same, kinetic energy also doubles.
Potential Energy Formula Questions Class 9
Q1. Write the formula for gravitational potential energy.
Ans. PE = mgh, where m is mass, g is acceleration due to gravity, and h is height.
Potential energy is measured in joules.
Q2. At what position is gravitational potential energy maximum?
Ans. Gravitational potential energy is maximum at the highest point.
At this position, height is maximum.
Kinetic Energy vs Potential Energy Questions Class 9
Q1. At what point in a pendulum is KE maximum and PE maximum?
Ans. KE is maximum at the mean position because speed is maximum.
PE is maximum at the extreme positions because height is maximum.
Conservation of Mechanical Energy Questions Class 9
Conservation of mechanical energy questions test energy conversion between KE and PE. The total remains constant when no non-conservative force acts.
Q1. Why does a pendulum eventually stop?
Ans. A real pendulum loses energy due to air resistance and friction at the support.
This energy changes into heat and sound. So mechanical energy gradually decreases.
Q2. A pendulum bob of mass 0.2 kg reaches a height of 0.5 m. Calculate maximum KE at the lowest point. Take g = 10 m/s².
Ans.
Maximum KE = PE at height
KE = mgh
KE = 0.2 × 10 × 0.5
KE = 1 J
Q3. A stone of mass 1 kg is dropped from 10 m. Find KE just before hitting the ground. Take g = 10 m/s².
Ans.
KE gained = PE lost
KE = mgh
KE = 1 × 10 × 10
KE = 100 J
Q4. Why does a roller coaster move fastest at the lowest point?
Ans. At the top, potential energy is maximum. As it moves down, potential energy changes into kinetic energy.
At the lowest point, kinetic energy is maximum. So speed is highest.
Power Important Questions Class 9 Science
Power questions test rate of doing work. Students should remember watt, horsepower, and the formula P = W/t.
Power Formula Questions Class 9 Science
Q1. Define power and write its formula.
Ans. Power is the rate at which work is done.
Formula: P = W/t
SI unit: watt
Q2. Two students carry identical boxes up the same staircase. A takes 20 seconds and B takes 40 seconds. Compare their power.
Ans. Both do the same work.
A takes half the time, so A has double the power of B.
Watt and Horsepower Questions Class 9
Q1. Define one watt.
Ans. One watt is the power of a machine that does one joule of work in one second.
1 W = 1 J/s
Q2. What is horsepower?
Ans. Horsepower is a unit of power used for engines and motors.
1 horsepower = 746 W
Simple Machines Important Questions Class 9
Simple machines questions class 9 appear in VSA, short answer, diagram-based, and numerical formats. Learn each machine with function, example, and mechanical advantage.
Pulley Questions Class 9 Science
Q1. What is a pulley? State its mechanical advantage.
Ans. A pulley is a grooved wheel with a rope.
A fixed pulley has MA = 1 and changes the direction of effort. A movable pulley has MA = 2 and reduces effort.
Q2. Why is a combination of pulleys useful?
Ans. A combination of pulleys increases mechanical advantage.
It reduces the effort needed to lift a heavy load.
Inclined Plane Questions Class 9 Science
Q1. What is an inclined plane?
Ans. An inclined plane is a flat surface placed at an angle to the horizontal.
It makes work easier by increasing the distance over which force is applied.
Q2. Why is pushing a box up a ramp easier than lifting it vertically?
Ans. A ramp spreads the work over a longer distance.
This reduces the force needed at any one point.
Lever Questions Class 9 Science
Q1. What are the three parts of a lever?
Ans. The three parts are fulcrum, load, and effort.
Their arrangement decides the lever class.
Q2. Write the principle of moments formula for a lever.
Ans.
Load × Load arm = Effort × Effort arm
When this condition is satisfied, the lever is balanced.
Classes of Levers Questions Class 9
Q1. Classify the following: scissors, nutcracker, tweezers, crowbar, wheelbarrow.
Ans.
| Object |
Lever Class |
| Scissors |
Class 1 |
| Nutcracker |
Class 2 |
| Tweezers |
Class 3 |
| Crowbar |
Class 1 |
| Wheelbarrow |
Class 2 |
Class 9 Science Chapter 7 Assertion Reason Questions
Class 9 science chapter 7 assertion reason questions test whether students understand the cause behind each concept. Read both statements separately before choosing the answer.
Directions:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Q1. Assertion (A): A satellite in circular orbit does no work against gravity.
Reason (R): Gravitational force is perpendicular to the satellite’s direction of motion.
Ans. (a) Work done is zero because force and displacement are perpendicular.
Q2. Assertion (A): A body at rest can have energy.
Reason (R): Potential energy depends on position, not motion.
Ans. (a) A raised object can have potential energy even at rest.
Q3. Assertion (A): Kinetic energy is always positive.
Reason (R): Kinetic energy depends on velocity squared.
Ans. (a) Since v² is always positive, kinetic energy is positive.
Q4. Assertion (A): A Class 2 lever has MA greater than 1.
Reason (R): In a Class 2 lever, effort arm is longer than load arm.
Ans. (a) Less effort is needed to balance a larger load.
Q5. Assertion (A): Friction reduces machine efficiency.
Reason (R): Friction changes some input energy into heat.
Ans. (a) This reduces useful output.
Case Study Questions Class 9 Science Chapter 7
Case studies test application. Read the situation first, then identify the concept.
Case Study 1: Escape Ramp
A runaway truck loses its brakes on a highway. An escape ramp is built uphill. The truck moves up the ramp and slows down.
Q1. What energy transformation occurs?
Ans. Kinetic energy changes into gravitational potential energy.
Q2. Why does the truck slow down without brakes?
Ans. The truck does work against gravity.
Its kinetic energy decreases as potential energy increases.
Q3. Which simple machine does the ramp represent?
Ans. The ramp represents an inclined plane.
It helps stop the truck over a longer distance.
Case Study 2: Watermill
A watermill uses falling water to rotate a wheel and grind grain.
Q1. Identify the energy transformation.
Ans. Gravitational potential energy changes into kinetic energy.
Kinetic energy turns the wheel and becomes mechanical energy.
Q2. What principle is shown here?
Ans. It shows conservation of energy.
Energy changes form but is not destroyed.
Q3. Why does a real watermill lose some energy?
Ans. A real watermill loses energy due to friction and air resistance.
Some energy changes into heat and sound.
MCQ Practice Questions Class 9 Science Chapter 7
MCQs help students revise formulas and concepts quickly.
Q1. The SI unit of work is:
a) Watt
b) Joule
c) Newton
d) Metre
Ans. b) Joule
Q2. Kinetic energy depends on:
a) Height
b) Velocity squared
c) Time only
d) Direction only
Ans. b) Velocity squared
Q3. The formula for power is:
a) P = W/t
b) P = mgh
c) P = ½mv²
d) P = Load/Effort
Ans. a) P = W/t
Q4. A fixed pulley mainly changes:
a) Direction of force
b) Mass of load
c) Energy used
d) Weight of object
Ans. a) Direction of force
Q5. A Class 2 lever has:
a) Fulcrum between load and effort
b) Load between fulcrum and effort
c) Effort between fulcrum and load
d) No fulcrum
Ans. b) Load between fulcrum and effort
Important Definitions and Formulas Class 9 Science Chapter 7
| Term |
Definition / Formula |
| Work |
W = F × d |
| Kinetic Energy |
KE = ½mv² |
| Potential Energy |
PE = mgh |
| Power |
P = W/t |
| Mechanical Advantage |
MA = Load / Effort |
| 1 Horsepower |
746 W |
| 1 Joule |
Work done by 1 N force over 1 m |
| Conservation of Energy |
KE + PE = constant |
| Efficiency |
Useful output / Total input × 100 |
Marks Distribution for Class 9 Science Chapter 7
| Question Type |
Topics Usually Tested |
| 1 mark |
Work, energy, power, joule, watt, MA |
| 2 marks |
Positive work, negative work, work-energy theorem |
| 3 marks |
Conservation of energy, levers, simple machines |
| Numericals |
Work, KE, PE, power, mechanical advantage |
| Assertion-reason |
Work, KE, PE, friction, levers |
| Case study |
Ramps, pendulum, watermill, pulley systems |