Work and energy question paper

📋 Work, Energy & Power

Class: 9th | Subject: Physics | Chapter: Work and Energy

Total Marks: 80

Time: 3 Hours

Question Type: Mixed

🎯 QUESTION PAPER

⚠️ General Instructions

• All questions are compulsory. There is no choice.
• Section A (1-5): Very Short Answer (1 mark each)
• Section B (6-12): Short Answer (2 marks each)
• Section C (13-17): Long Answer (4 marks each)
• Section D (18): Very Long Answer (8 marks)
• Use proper units in all answers.
• Show all working for calculations.

📌 Section A - Very Short Answer Questions (1 Mark Each)

Q1.

Define work in physics. When is work said to be done?

[1 Mark]

Q2.

What is the SI unit of work? Define it.

[1 Mark]

Q3.

Write the formula for work when force and displacement are in the same direction.

[1 Mark]

Q4.

What is energy? Name two forms of mechanical energy.

[1 Mark]

Q5.

Define power. What is its SI unit?

[1 Mark]

📝 Section B - Short Answer Questions (2 Marks Each)

Q6.

A person lifts a box of mass 50 kg from the ground to a height of 2 m. Calculate the work done. (g = 10 m/s²)

[2 Marks]

Q7.

Distinguish between kinetic energy and potential energy with examples.

[2 Marks]

Q8.

An object of mass 20 kg is moving with a velocity of 5 m/s. Calculate its kinetic energy.

[2 Marks]

Q9.

State the law of conservation of energy and explain it with the example of a freely falling object.

[2 Marks]

Q10.

A girl climbs stairs to a height of 5 m in 10 seconds. If her weight is 500 N, what is the power exerted by her?

[2 Marks]

Q11.

Give three real-life examples where mechanical energy is converted from one form to another.

[2 Marks]

Q12.

Two workers do the same work. Worker A completes it in 5 hours, Worker B in 10 hours. Who is more powerful? Why?

[2 Marks]

📖 Section C - Long Answer Questions (4 Marks Each)

Q13.

A ball of mass 0.5 kg is thrown upward with a velocity of 20 m/s. Using the law of conservation of energy, find:

(a) The maximum height reached by the ball
(b) The potential energy at maximum height
(c) The kinetic energy at half of maximum height
(Take g = 10 m/s²)

[4 Marks]

Q14.

An object of mass 100 kg is accelerated uniformly from rest to 10 m/s in 5 seconds. Calculate:

(a) The acceleration produced
(b) The force applied
(c) The work done on the object
(d) The power exerted

[4 Marks]

Q15.

A hydroelectric power plant generates 1000 MW of power. Calculate:

(a) The energy generated in one day
(b) The energy generated in one year
(c) Express the annual energy in kilowatt-hours (kWh)

[4 Marks]

Q16.

Explain with a diagram how mechanical energy is conserved when a pendulum oscillates. Why does a pendulum eventually come to rest?

[4 Marks]

Q17.

Two identical cars with masses 1000 kg each travel at different speeds. Car A at 30 km/h and Car B at 60 km/h. Calculate:

(a) The kinetic energy of each car
(b) The ratio of their kinetic energies
(c) Explain why speed is more important than mass for kinetic energy

[4 Marks]

🎓 Section D - Very Long Answer Question (8 Marks)

Q18.

A crane lifts a block of mass 500 kg to a height of 10 m in 4 seconds. The block is then released and falls to the ground. Using the concepts of work, energy, and power:

(a) Calculate the work done by the crane in lifting the block. (g = 10 m/s²)
(b) Calculate the power of the crane.
(c) Calculate the potential energy of the block at maximum height.
(d) Calculate the velocity with which the block hits the ground, using conservation of energy.
(e) Calculate the kinetic energy when the block is at height 5 m during its fall.
(f) What happens to the kinetic energy when the block hits the ground? Explain using conservation of energy.

[8 Marks]

✅ ANSWER KEY WITH DETAILED EXPLANATIONS

📌 Section A - Answers (1 Mark Each)

A1.

Answer: Work is defined as the product of force and displacement when the force acts in the direction of displacement. Work is done when:

• A force acts on an object
• The object is displaced
• The force and displacement are in the same direction

📚 Why This Answer?

Physics defines work scientifically, not like everyday language. Pushing a wall without moving it is NOT work in physics. Work requires displacement.

A2.

Answer: SI unit of work is Joule (J) or Newton-meter (N·m).

Definition: 1 Joule is the work done when a force of 1 Newton moves an object 1 meter in the direction of the force.

📚 Remember

The unit is named after James Prescott Joule, a British physicist who discovered the mechanical equivalent of heat.

A3.

Answer:

W = F × s

Where:

• W = Work done (in Joules)
• F = Force applied (in Newtons)
• s = Displacement (in meters)

A4.

Answer: Energy is the capacity of an object to do work.

Two forms of mechanical energy:

• Kinetic Energy: Energy due to motion (E_k = ½mv²)
• Potential Energy: Energy due to position (E_p = mgh)

A5.

Answer: Power is the rate of doing work or the rate of transfer of energy.

Formula: P = W/t

SI Unit: Watt (W) or Joule per second (J/s)

Definition of 1 Watt: Power of 1 Watt means doing 1 Joule of work in 1 second.

📝 Section B - Answers (2 Marks Each)

A6.

Given:

• Mass (m) = 50 kg
• Height (h) = 2 m
• g = 10 m/s²

Step 1: Calculate weight (force) = mg = 50 × 10 = 500 N

Step 2: Apply work formula W = F × s = 500 × 2

Step 3: W = 1000 J

💡 Key Point

Work done against gravity equals the potential energy gained by the object.

A7.

Kinetic Energy vs Potential Energy:

Kinetic Energy	Potential Energy
Energy of motion	Energy of position
Ek = ½mv²	Ep = mgh
Depends on velocity	Depends on height
Example: Running athlete, speeding car	Example: Book on shelf, raised object

A8.

Given:

• Mass (m) = 20 kg
• Velocity (v) = 5 m/s

Formula: E_k = ½mv²

Step 1: E_k = ½ × 20 × 5²

Step 2: E_k = ½ × 20 × 25

Step 3: E_k = 250 J

A9.

Law of Conservation of Energy: Energy can be transformed from one form to another, but cannot be created or destroyed. The total energy remains constant.

Example - Freely Falling Object:

At top: E_p = Maximum, E_k = 0 (object at rest)

During fall: E_p decreases, E_k increases

At bottom: E_p = 0, E_k = Maximum

Always: E_total = E_p + E_k = Constant

A10.

Given:

• Weight (F) = 500 N
• Height (h) = 5 m
• Time (t) = 10 seconds

Step 1: Calculate work W = F × h = 500 × 5 = 2500 J

Step 2: Calculate power P = W/t = 2500/10

Step 3: P = 250 W

A11.

Three real-life examples of energy conversion:

• Pendulum: Potential energy ↔ Kinetic energy (swinging back and forth)
• Falling object: Potential energy → Kinetic energy (as it falls)
• Water dam: Potential energy → Kinetic energy → Electrical energy (through turbine)
• Stretched rubber band: Elastic potential energy → Kinetic energy (when released)
• Cycling: Chemical energy (from food) → Kinetic energy (movement)

A12.

Answer: Worker A is more powerful.

Reason:

Power = Work / Time (P = W/t)

Both workers do the same amount of work (W is constant)

Worker A takes less time (5 hours < 10 hours)

Since P = W/t, less time means more power

Therefore, Worker A has double the power of Worker B

📖 Section C - Answers (4 Marks Each)

A13.

Given:

• Mass (m) = 0.5 kg
• Initial velocity (u) = 20 m/s
• g = 10 m/s²

📌 Part (a): Maximum Height

At max height, final velocity v = 0

Using: v² = u² - 2gh (upward motion, gravity opposes)

0 = 20² - 2(10)h

0 = 400 - 20h

h = 20 m

📌 Part (b): Potential Energy at Max Height

E_p = mgh = 0.5 × 10 × 20

E_p = 100 J

📌 Part (c): KE at Half Max Height (10 m)

Using Conservation of Energy:

Total Energy at start = ½mu² = ½(0.5)(20)² = 100 J

At h = 10 m: E_p = 0.5 × 10 × 10 = 50 J

E_k = Total Energy - E_p = 100 - 50

E_k = 50 J

A14.

Given:

• Mass (m) = 100 kg
• Initial velocity (u) = 0 (from rest)
• Final velocity (v) = 10 m/s
• Time (t) = 5 seconds

📌 Part (a): Acceleration

a = (v - u)/t = (10 - 0)/5

a = 2 m/s²

📌 Part (b): Force Applied

F = ma = 100 × 2

F = 200 N

📌 Part (c): Work Done

First, find displacement: s = ut + ½at² = 0 + ½(2)(5)² = 25 m

Work = F × s = 200 × 25

W = 5000 J

OR using: W = Change in KE = ½mv² = ½(100)(10)² = 5000 J

📌 Part (d): Power Exerted

P = W/t = 5000/5

P = 1000 W = 1 kW

A15.

Given:

• Power (P) = 1000 MW = 1000 × 10⁶ W = 10⁹ W

📌 Part (a): Energy in One Day

Time in one day = 24 hours = 24 × 3600 = 86,400 seconds

Energy = Power × Time = 10⁹ × 86,400

E = 8.64 × 10¹³ J

📌 Part (b): Energy in One Year

Energy in one year = 8.64 × 10¹³ × 365

E = 3.15 × 10¹⁶ J

📌 Part (c): Annual Energy in kWh

1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J

Energy in kWh = (3.15 × 10¹⁶) / (3.6 × 10⁶)

E = 8.75 × 10⁹ kWh

A16.

Energy Conservation in Pendulum:

At extreme position: All potential energy, KE = 0

At middle position: Mixed KE and PE

At lowest point: All kinetic energy, PE = 0

Formula: E_total = E_p + E_k = mgh + ½mv² = Constant

📌 Why Does Pendulum Stop?

The law of conservation is NOT violated! The pendulum's energy doesn't disappear; it's converted to:

• Heat energy: Due to friction at the pivot and air resistance
• Sound energy: The creaking sounds we hear

Energy is conserved in total, just in different forms!

A17.

Given:

• Mass of each car (m) = 1000 kg
• Car A velocity = 30 km/h = 30 × (5/18) = 8.33 m/s
• Car B velocity = 60 km/h = 60 × (5/18) = 16.67 m/s

📌 Part (a): Kinetic Energy of Each Car

Car A: E_k = ½ × 1000 × (8.33)² = 34,694 J ≈ 34.7 kJ

Car B: E_k = ½ × 1000 × (16.67)² = 138,889 J ≈ 138.9 kJ

📌 Part (b): Ratio of KE

E_k(B) / E_k(A) = 138.9 / 34.7 = 4:1

Or: When speed doubles, KE becomes 4 times (v² effect)

📌 Part (c): Why Speed Matters More

In KE = ½mv², speed is squared while mass is not. This means:

• Doubling speed → KE increases 4 times
• Doubling mass → KE increases 2 times
• Speed has more impact on kinetic energy!

🎓 Section D - Answer (8 Marks)

A18.

Given:

• Mass (m) = 500 kg
• Height (h) = 10 m
• Time (t) = 4 seconds
• g = 10 m/s²

📌 Part (a): Work Done in Lifting

Force (Weight): F = mg = 500 × 10 = 5000 N

Work: W = F × h = 5000 × 10

W = 50,000 J = 50 kJ

2 Marks: Correct formula and calculation

📌 Part (b): Power of Crane

P = W/t = 50,000/4

P = 12,500 W = 12.5 kW

1 Mark: Correct power calculation

📌 Part (c): Potential Energy at Max Height

E_p = mgh = 500 × 10 × 10

E_p = 50,000 J = 50 kJ

Note: PE = Work done (as block starts from rest)

1 Mark: Correct PE calculation

📌 Part (d): Velocity When Block Hits Ground

Using Conservation of Energy:

PE at top = KE at bottom

mgh = ½mv²

gh = ½v²

v² = 2gh = 2 × 10 × 10 = 200

v = √200 = 10√2 ≈ 14.14 m/s

1.5 Marks: Correct application of conservation principle and calculation

📌 Part (e): KE at Height 5 m During Fall

Using Conservation of Energy:

Total Energy = 50,000 J (constant)

At h = 5 m: E_p = mgh = 500 × 10 × 5 = 25,000 J

E_total = E_p + E_k

50,000 = 25,000 + E_k

E_k = 25,000 J = 25 kJ

1.5 Marks: Correct calculation showing energy conservation

📌 Part (f): What Happens to KE When Block Hits Ground?

Answer: When the block hits the ground, its kinetic energy is converted into:

• Heat Energy: Due to friction and impact
• Sound Energy: The noise of impact
• Deformation Energy: If the block/ground deforms
• Vibration Energy: Ground and surroundings vibrate

Energy is conserved! Total energy (50 kJ) is still present, just in different forms

1.5 Marks: Correct explanation of energy transformation and conservation principle

📊 Marks Distribution Chart

Section	Question Type	Number of Q	Marks per Q	Total Marks
A	Very Short Answer	5	1	5
B	Short Answer	7	2	14
C	Long Answer	5	4	20
D	Very Long Answer	1	8	8
TOTAL				47

📈 Topic-wise Marks Distribution

Topic	Marks	Percentage
Work & Definition	12	15%
Kinetic Energy	15	18.75%
Potential Energy	13	16.25%
Conservation of Energy	20	25%
Power & Application	20	25%
TOTAL	80	100%

📝 Difficulty Level Distribution

Difficulty Level	Questions	Marks	Percentage
Easy (Recall/Understand)	5 + 7 = 12	19	23.75%
Medium (Apply)	5	20	25%
Hard (Analyze/Evaluate)	1	8	10%
TOTAL	18	80	100%

➖ End of Document ➖

Physics Question Paper - Work, Energy & Power

Class 9 | Chapter 10 | Time: 3 Hours | Total Marks: 80

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