Class 9 Science Important Questions and Answers: Work, Energy and Simple Machines
Premium NCERT, Board Exam, Exemplar and Olympiad-level question answers for Class 9 Science. Learn work done by constant force, zero work, positive and negative work, work-energy theorem, kinetic energy, potential energy, conservation of mechanical energy, power and simple machines.
Short Introduction
Work and energy are important concepts in Physics. Work is said to be done when a force produces displacement in the body. Energy is the capacity of a body to do work. These ideas help us understand motion, machines, power, lifting objects, falling bodies and many daily-life activities.
This chapter also explains simple machines such as levers, pulleys and inclined planes. Simple machines help us perform work more easily by changing the direction or magnitude of force.
Exam focus: Work done, zero work, positive and negative work, kinetic energy derivation, potential energy derivation, conservation of mechanical energy, power and numerical problems are very important.
Chapter Overview
1. Work Done
Work is done when force causes displacement in the direction of force.
2. Positive, Negative and Zero Work
Work depends on the angle between force and displacement.
3. Energy
Energy is the capacity to do work and exists in many forms.
4. Mechanical Energy
Mechanical energy is the sum of kinetic energy and potential energy.
5. Power
Power is the rate of doing work or rate of energy transfer.
6. Simple Machines
Pulley, inclined plane and lever make work easier.
Important Keywords
Important Formulae
Work Done
W = F × s
Work Done at Angle
W = Fs cos θ
Kinetic Energy
KE = 1/2 mv2
Potential Energy
PE = mgh
Mechanical Energy
ME = KE + PE
Power
P = W ÷ t
Power Using Energy
P = E ÷ t
Commercial Unit
1 kWh = 3.6 × 106 J
Mechanical Advantage
MA = Load ÷ Effort
Symbols: W = work done, F = force, s = displacement, θ = angle between force and displacement, m = mass, v = velocity, g = acceleration due to gravity, h = height, P = power, t = time.
Concept Map: Work, Energy and Simple Machines
Work Done by Constant Force
Work is done when a constant force produces displacement in its direction.
Formula: W = F × s
Mechanical Energy
Mechanical energy is the energy possessed by a body due to its motion or position.
Formula: ME = KE + PE
Simple Machines
Simple machines help us do work more easily by changing force, distance or direction.
Examples: Lever, pulley, inclined plane.
Important Very Short Answer Questions
Q1. Define work.
Answer: Work is said to be done when a force acts on a body and produces displacement in the body.
Q2. Write the formula for work done by a constant force.
Answer: Work done = Force × Displacement, or W = F × s.
Q3. What is the SI unit of work?
Answer: The SI unit of work is joule, written as J.
Q4. Define one joule of work.
Answer: One joule is the work done when a force of 1 N displaces a body by 1 m in the direction of force.
Q5. When is work done equal to zero?
Answer: Work done is zero when displacement is zero or when force acts perpendicular to displacement.
Q6. What is positive work?
Answer: Work is positive when force and displacement are in the same direction.
Q7. What is negative work?
Answer: Work is negative when force and displacement are in opposite directions.
Q8. Define energy.
Answer: Energy is the capacity of a body to do work.
Q9. What is kinetic energy?
Answer: Kinetic energy is the energy possessed by a body due to its motion.
Q10. What is potential energy?
Answer: Potential energy is the energy possessed by a body due to its position or configuration.
Q11. What is gravitational potential energy?
Answer: Gravitational potential energy is the energy possessed by a body due to its height above the ground.
Q12. Define power.
Answer: Power is the rate of doing work or rate of transfer of energy.
Q13. What is the SI unit of power?
Answer: The SI unit of power is watt, written as W.
Q14. What is a simple machine?
Answer: A simple machine is a device that makes work easier by changing the magnitude or direction of force.
Q15. Give three examples of simple machines.
Answer: Lever, pulley and inclined plane are examples of simple machines.
Short Answer Questions
Q1. What are the two necessary conditions for work to be done?
Answer:
- A force must act on the body.
- The body must be displaced in the direction of force or in a direction having a component of force.
Q2. Give examples of zero work done.
Answer:
- A person pushing a wall does no work if the wall does not move.
- A coolie carrying a load horizontally does no work against gravity because force and displacement are perpendicular.
- Earth does no work on a body moving in a horizontal circular path if gravitational force is perpendicular to displacement.
Q3. Differentiate between positive work and negative work.
Answer:
| Positive Work | Negative Work |
|---|---|
| Force and displacement are in the same direction. | Force and displacement are in opposite directions. |
| Work done is positive. | Work done is negative. |
| Example: A body falling freely under gravity. | Example: Friction acting on a moving object. |
Q4. State the work-energy theorem.
Answer: The work-energy theorem states that the work done by a force on a body is equal to the change in its kinetic energy.
Q5. Write different forms of energy.
Answer: Different forms of energy are:
- Mechanical energy
- Heat energy
- Light energy
- Sound energy
- Chemical energy
- Electrical energy
- Nuclear energy
Q6. Differentiate between kinetic energy and potential energy.
Answer:
| Kinetic Energy | Potential Energy |
|---|---|
| Energy possessed by a body due to motion. | Energy possessed by a body due to position or configuration. |
| It depends on mass and velocity. | Gravitational potential energy depends on mass, gravity and height. |
| Formula: KE = 1/2 mv2 | Formula: PE = mgh |
| Example: A moving car. | Example: Water stored in a dam. |
Q7. What is conservation of mechanical energy?
Answer: Conservation of mechanical energy means that the sum of kinetic energy and potential energy remains constant when only conservative forces like gravity act on a body.
Q8. Why is an inclined plane used?
Answer: An inclined plane is used to lift a heavy load to a height by applying a smaller effort over a longer distance.
Q9. What is the function of a pulley?
Answer: A pulley helps in lifting loads by changing the direction of effort. In some systems, it also reduces the effort required.
Q10. What is a lever?
Answer: A lever is a rigid bar that turns about a fixed point called fulcrum and is used to lift or move loads with less effort.
Derivations: Kinetic Energy and Potential Energy
Q1. Derive the expression for kinetic energy.
Answer:
Let a body of mass m be initially at rest.
Initial velocity, u = 0
Final velocity, v = v
Acceleration = a
Displacement = s
Force acting on the body, F = ma
Work done, W = F × s
So, W = ma × s
Using the equation of motion:
v2 – u2 = 2as
Since u = 0, v2 = 2as
Therefore, as = v2 ÷ 2
W = m × v2 ÷ 2
Kinetic Energy = 1/2 mv2
Q2. Derive the expression for gravitational potential energy.
Answer:
Let a body of mass m be lifted through a height h above the ground.
Force required to lift the body is equal to its weight.
Weight of body = mg
Work done against gravity = Force × Displacement
W = mg × h
This work done is stored in the body as gravitational potential energy.
Gravitational Potential Energy = mgh
Q3. Explain conservation of mechanical energy for a freely falling body.
Answer:
- At the highest point, the body has maximum potential energy and zero kinetic energy if released from rest.
- As the body falls, its height decreases, so potential energy decreases.
- At the same time, velocity increases, so kinetic energy increases.
- Just before reaching the ground, kinetic energy is maximum and potential energy is minimum.
- If air resistance is neglected, the total mechanical energy remains constant.
Long Answer Questions
Q1. Explain work done by a constant force. When can work be positive, negative or zero?
Answer: Work done by a constant force is the product of force and displacement in the direction of force.
Formula: W = F × s
If force makes an angle θ with displacement, then W = Fs cos θ.
- Positive work: Work is positive when force and displacement are in the same direction. Example: gravity doing work on a falling body.
- Negative work: Work is negative when force and displacement are in opposite directions. Example: friction acting on a moving object.
- Zero work: Work is zero when displacement is zero or force is perpendicular to displacement. Example: pushing a wall that does not move.
Q2. Explain the work-energy theorem with an example.
Answer: The work-energy theorem states that the work done by a force on a body is equal to the change in kinetic energy of the body.
Mathematical form:
Work done = Change in kinetic energy
W = KEfinal – KEinitial
W = 1/2 mv2 – 1/2 mu2
Example: When a force is applied to a stationary trolley, the trolley starts moving. The work done by the force appears as kinetic energy of the trolley.
Q3. Explain power and its units.
Answer: Power is the rate of doing work or rate of energy transfer.
Formula: Power = Work done ÷ Time taken
P = W ÷ t
- The SI unit of power is watt.
- One watt is the power when 1 joule of work is done in 1 second.
- 1 kilowatt = 1000 watt.
- Commercial unit of electrical energy is kilowatt hour.
- 1 kWh = 3.6 × 106 J.
Q4. Explain simple machines and their importance.
Answer: Simple machines are basic devices that make work easier by changing the magnitude or direction of force.
Importance of simple machines:
- They help in lifting heavy loads with less effort.
- They can change the direction of applied force.
- They can increase speed or distance of movement.
- They make daily work easier and more efficient.
Examples: Lever, pulley and inclined plane.
Q5. Explain lever, pulley and inclined plane.
Answer:
- Lever: A lever is a rigid bar that rotates about a fixed point called fulcrum. Example: seesaw, crowbar, scissors.
- Pulley: A pulley is a wheel with a grooved rim around which a rope passes. It is used to lift loads or change direction of effort.
- Inclined plane: An inclined plane is a sloping surface used to raise heavy objects with less effort. Example: ramp.
Important Numericals with Solutions
Numerical 1: A force of 20 N displaces a body by 5 m in the direction of force. Find work done.
Solution:
Force, F = 20 N
Displacement, s = 5 m
Work done, W = F × s
W = 20 × 5 = 100 J
Numerical 2: A person applies a force of 50 N on a wall, but the wall does not move. Find work done.
Solution:
Force, F = 50 N
Displacement, s = 0 m
Work done, W = F × s
W = 50 × 0 = 0 J
No work is done because displacement is zero.
Numerical 3: A body of mass 4 kg moves with velocity 5 m/s. Find its kinetic energy.
Solution:
Mass, m = 4 kg
Velocity, v = 5 m/s
KE = 1/2 mv2
KE = 1/2 × 4 × 52
KE = 2 × 25 = 50 J
Numerical 4: Calculate the kinetic energy of a 1000 kg car moving with a speed of 20 m/s.
Solution:
m = 1000 kg
v = 20 m/s
KE = 1/2 mv2
KE = 1/2 × 1000 × 202
KE = 500 × 400 = 200000 J
Numerical 5: Find the potential energy of a 10 kg object raised to a height of 5 m. Take g = 10 m/s2.
Solution:
m = 10 kg
g = 10 m/s2
h = 5 m
PE = mgh
PE = 10 × 10 × 5 = 500 J
Numerical 6: A body has mass 2 kg and is moving with velocity 10 m/s. Find the work required to stop it.
Solution:
m = 2 kg
v = 10 m/s
Initial kinetic energy = 1/2 mv2
KE = 1/2 × 2 × 102 = 100 J
To stop the body, work done against motion = 100 J
The work done by stopping force is negative.
Numerical 7: A machine does 600 J of work in 20 seconds. Find its power.
Solution:
Work done, W = 600 J
Time, t = 20 s
Power, P = W ÷ t
P = 600 ÷ 20 = 30 W
Numerical 8: A motor has power 500 W. How much work does it do in 10 seconds?
Solution:
Power, P = 500 W
Time, t = 10 s
P = W ÷ t
W = P × t
W = 500 × 10 = 5000 J
Numerical 9: Convert 2 kWh into joules.
Solution:
1 kWh = 3.6 × 106 J
2 kWh = 2 × 3.6 × 106 J
2 kWh = 7.2 × 106 J
Numerical 10: A load of 400 N is lifted using an effort of 100 N. Find mechanical advantage.
Solution:
Load = 400 N
Effort = 100 N
Mechanical advantage = Load ÷ Effort
MA = 400 ÷ 100 = 4
Numerical 11: A force of 30 N acts opposite to the displacement of 4 m. Find work done.
Solution:
Force, F = 30 N
Displacement, s = 4 m
Since force is opposite to displacement, work done is negative.
W = -F × s
W = -30 × 4 = -120 J
Numerical 12: A body of mass 5 kg is at a height of 20 m. Find its potential energy. Take g = 9.8 m/s2.
Solution:
m = 5 kg
g = 9.8 m/s2
h = 20 m
PE = mgh
PE = 5 × 9.8 × 20 = 980 J
Case-Study Based Questions
Case Study 1: Work Done in Daily Life
A student pushes a heavy box on the floor. The box moves 3 m in the direction of the applied force. Later, the student pushes a wall with the same force, but the wall does not move.
Q1. In which case is work done?
Answer: Work is done when the box moves because force produces displacement.
Q2. Why is no work done on the wall?
Answer: No work is done because displacement of the wall is zero.
Q3. Write the formula for work done.
Answer: Work done = Force × Displacement.
Q4. What is the SI unit of work?
Answer: The SI unit of work is joule.
Case Study 2: Falling Stone
A stone is held at a height above the ground. When released, it falls freely. As it falls, its speed increases and its height decreases. Air resistance is neglected.
Q1. Which energy is maximum at the highest point?
Answer: Potential energy is maximum at the highest point.
Q2. Which energy increases as the stone falls?
Answer: Kinetic energy increases as the stone falls.
Q3. What happens to total mechanical energy?
Answer: Total mechanical energy remains constant if air resistance is neglected.
Q4. Name the principle involved.
Answer: The principle involved is conservation of mechanical energy.
Case Study 3: Simple Machines
A worker uses a ramp to push a heavy box into a truck. Another worker uses a pulley to lift a bucket. A third worker uses a crowbar to lift a heavy stone.
Q1. Which simple machine is used as a ramp?
Answer: The ramp is an inclined plane.
Q2. Which simple machine helps lift the bucket?
Answer: A pulley helps lift the bucket.
Q3. Which simple machine is a crowbar?
Answer: A crowbar is a lever.
Q4. Why are simple machines used?
Answer: Simple machines are used to make work easier by changing force or direction of effort.
Assertion-Reason Questions
Choose the correct option:
A. Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
B. Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
C. Assertion is true but Reason is false.
D. Assertion is false but Reason is true.
Q1. Assertion: Work done is zero when displacement is zero.
Reason: Work done is the product of force and displacement in the direction of force.
Answer: A. Both are true and Reason correctly explains Assertion.
Q2. Assertion: Work done by friction on a moving body is generally negative.
Reason: Friction acts opposite to the direction of motion.
Answer: A. Both are true and Reason correctly explains Assertion.
Q3. Assertion: A moving body possesses kinetic energy.
Reason: Kinetic energy is the energy possessed by a body due to its motion.
Answer: A. Both are true and Reason correctly explains Assertion.
Q4. Assertion: Potential energy of a body increases when it is raised to a greater height.
Reason: Gravitational potential energy is given by PE = mgh.
Answer: A. Both are true and Reason correctly explains Assertion.
Q5. Assertion: Power is the total work done.
Reason: Power is the rate of doing work.
Answer: D. Assertion is false but Reason is true.
Q6. Assertion: An inclined plane reduces the effort needed to raise a load.
Reason: It increases the distance over which effort is applied.
Answer: A. Both are true and Reason correctly explains Assertion.
Exam Tips
- Write the two conditions of work done clearly: force and displacement.
- Remember that work is zero if displacement is zero or force is perpendicular to displacement.
- Use signs carefully: positive work for same direction and negative work for opposite direction.
- For kinetic energy numericals, always square the velocity.
- For potential energy numericals, write PE = mgh before substitution.
- Learn the derivation of KE = 1/2 mv2 using equations of motion.
- Learn the derivation of PE = mgh using work done against gravity.
- For power numericals, check whether time is in seconds.
- Remember 1 kWh = 3.6 × 106 J.
- For simple machines, focus on pulley, lever, inclined plane, effort, load and fulcrum.
Quick Revision Box
Work
Work is done when force produces displacement.
Zero Work
Work is zero when displacement is zero or force is perpendicular.
Positive Work
Force and displacement are in the same direction.
Negative Work
Force and displacement are in opposite directions.
Kinetic Energy
Energy due to motion: KE = 1/2 mv2.
Potential Energy
Energy due to position: PE = mgh.
Mechanical Energy
Sum of kinetic energy and potential energy.
Power
Rate of doing work: P = W/t.
Lever
Rigid bar rotating about a fulcrum.
Pulley
Wheel and rope system used to lift loads.
Inclined Plane
Sloping surface used to lift loads with less effort.
Simple Machine
Device that makes work easier.
FAQ Section
What is the main focus of Work, Energy and Simple Machines?
The main focus is work done by force, forms of energy, kinetic energy, potential energy, conservation of mechanical energy, power and simple machines.
What is work done by a constant force?
Work done by a constant force is the product of force and displacement in the direction of force.
When is work done equal to zero?
Work done is zero when displacement is zero or when force acts perpendicular to displacement.
What is positive work?
Positive work is done when force and displacement are in the same direction.
What is negative work?
Negative work is done when force and displacement are in opposite directions.
What is the work-energy theorem?
The work-energy theorem states that work done on a body is equal to the change in its kinetic energy.
What is kinetic energy?
Kinetic energy is the energy possessed by a body due to its motion.
What is gravitational potential energy?
Gravitational potential energy is the energy possessed by a body due to its height above the ground.
What is conservation of mechanical energy?
It means the total mechanical energy, that is kinetic energy plus potential energy, remains constant if only conservative forces act.
What is power?
Power is the rate of doing work or rate of energy transfer.
What are simple machines?
Simple machines are devices that make work easier by changing the magnitude or direction of force.
Give examples of simple machines.
Lever, pulley and inclined plane are examples of simple machines.
Final Conclusion
Work, Energy and Simple Machines is a scoring chapter in Class 9 Science because it connects formulas with daily-life situations. Students should understand when work is positive, negative or zero and should practise numericals based on work, kinetic energy, potential energy and power.
Simple machines such as pulleys, levers and inclined planes show how Physics helps us reduce effort and perform work more efficiently. For CBSE exams, focus on definitions, formula-based numericals, derivations and application-based questions.
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