NCERT Class 9 Science Work, Energy and Simple Machines Class 9 Solutions
Short Intro
This chapter explains the concepts of work, energy, power, and simple machines with practical examples and numerical problems. Students will learn how forces perform work, how energy changes from one form to another, and how machines make our work easier.
Quick Information Box
| Topic | Details |
|---|---|
| Chapter Name | Work, Energy and Simple Machines |
| Class | Grade 9 |
| Subject | Science / Physics |
| Important Topics | Work, Energy, Power, Kinetic Energy, Potential Energy, Machines |
| SI Unit of Work | Joule (J) |
| SI Unit of Power | Watt (W) |
| Key Formula | W = F × s |
Concepts Used (Topics Covered)
- Work Done by Force
- Positive and Negative Work
- Work-Energy Theorem
- Forms of Energy
- Kinetic Energy
- Potential Energy
- Conservation of Mechanical Energy
- Power
- Pulley
- Inclined Plane
- Lever and Mechanical Advantage
Important Formulas
- Work Done
W = F × s - Kinetic Energy
KE = ½ mv² - Potential Energy
PE = mgh - Power
P = W / t - Mechanical Advantage
MA = Load / Effort - Inclined Plane
MA = Length / Height - Lever Principle
Effort × Effort Arm = Load × Load Arm
Questions & Step-by-step Solutions with Explanation
Q1. State whether True or False.
(i) Work is said to be done when a force is applied, even if the object does not move.
Solution:
Work is done only when displacement occurs.
If displacement = 0, then work done = 0.
Answer:
False
(ii) Lifting a bucket vertically upward results in positive work done on the bucket.
Solution:
Force and displacement are in the same direction.
Therefore, work done is positive.
Answer:
True
(iii) The SI unit for both work and energy is joule (J).
Solution:
Both work and energy are measured in joules.
Answer:
True
(iv) A motionless stretched rubber band has kinetic energy.
Solution:
The rubber band is not moving, so it has no kinetic energy.
It stores potential energy.
Answer:
False
(v) Energy can change from one form to another.
Solution:
Electrical energy changes into light and heat energy in a bulb.
Answer:
True
Q2. Fill in the blanks.
(i) Work done = ______ × ______
Answer:
Force × Displacement
(ii) 1 joule of work is done when a force of ______ newton displaces an object by 1 metre.
Answer:
1 newton
(iii) Expression for kinetic energy is ______
Answer:
½ mv²
(iv) Potential energy of an object is ______
Answer:
mgh
(v) Power is defined as the ______ at which work is done.
Answer:
Rate
Q3. Ball Reaches Highest Point
Correct Statements:
- Kinetic energy is zero
- Potential energy is maximum
Explanation:
At the highest point velocity becomes zero, therefore kinetic energy becomes zero.
Q4. Energy Transformations
| Situation | Energy Conversion |
|---|---|
| Truck moving uphill | Kinetic → Potential |
| Watch spring unwinding | Potential → Kinetic |
| Photosynthesis | Light → Chemical |
| Water flowing from dam | Potential → Kinetic |
| Burning matchstick | Chemical → Heat and Light |
| Firecracker explosion | Chemical → Sound, Light, Heat |
| Microphone | Sound → Electrical |
| Electric bulb | Electrical → Light |
| Solar panel | Light → Electrical |
Q5. Student Lifted to Top Floor
Given:
m = 50 kg
h = 72.5 m
g = 10 m/s²
Formula:
PE = mgh
Calculation:
PE = 50 × 10 × 72.5
PE = 36250 J
Answer:
(i) Gain in PE = 36250 J
(ii) Gain in PE while climbing stairs = 36250 J
(iii) Potential energy depends only on height, not on path.
Q6. Crane Lifting Mass
Solution:
Potential energy depends on height.
20th floor height is double of 10th floor.
Therefore:
- Energy required becomes double.
- Time also becomes double.
Power = Work / Time
Hence power remains same.
Answer:
- Energy required = Double
- Power required = Same
Q7. Factors Affecting Energy in Raising Flag
Solution:
Energy depends on:
- Mass of flag
- Height of pole
- Gravitational acceleration
Raising slowly or quickly does not change work done.
If speed doubles, power requirement doubles.
Q8. Fuel Ratio Problem
Day 1 Total Mass:
60 + 100 = 160 kg
Day 2 Total Mass:
60 + 40 + 100 = 200 kg
Kinetic energy ∝ mass
Fuel consumption ratio:
160 : 200
Simplified Ratio:
4 : 5
Q9. Balanced Seesaw
Adult weight = 2 × Child weight
To balance:
Adult must sit at half the distance from fulcrum compared to child.
Q10. Ball Thrown Upwards
Given:
m = 2 kg
u = 20 m/s
(i) Sign of Work Done by Gravity
- Upward motion → Negative
- Downward motion → Positive
(ii) Work Done by Air Resistance
Initial KE:
= ½ × 2 × 20²
= 400 J
Potential Energy at top:
= 2 × 10 × 19.4
= 388 J
Energy lost:
= 400 − 388
= 12 J
Answer:
Work done by air resistance = −12 J
Q11. Variable Force Problem
(i) Speed at 0 m
KE = 180 J
180 = ½ × 10 × v²
v² = 36
v = 6 m/s
(ii) Speed at 4 m
Work done = Area under graph
Area = 50 × 4 = 200 J
Final KE:
= 180 + 200
= 380 J
380 = ½ × 10 × v²
v² = 76
v ≈ 8.7 m/s
Negative acceleration?
No, force is always positive.
Q12. Ball on Moon
Moon gravity = Earth gravity / 6
Height ∝ 1/g
Height on Moon:
= 8 × 6
= 48 m
Answer:
48 m
Q13. Car Braking Problem
(i) Motion Between A and B
Car moves with constant speed.
(ii) Kinetic Energy at A
m = 1000 kg
v = 35 m/s
KE = ½ × 1000 × 35²
KE = 612500 J
(iii) Work Done by Brakes
Work done = Change in KE
= −612500 J
(iv) Energy Transformation
Kinetic energy converts into heat due to friction.
Q14. Potential Energy Graph
Mechanical energy remains constant.
Total energy = 30 J
Use:
KE = Total Energy − PE
Then:
KE = ½ mv²
Students can calculate velocity using graph values.
Q15. Coconut Falling Problem
Given:
m = 1.5 kg
h = 10 m
(i) Velocity Before Hitting Sand
v² = 2gh
v² = 2 × 10 × 10
v² = 200
v = 14.14 m/s
(ii) Depth of Depression
Potential Energy:
= 1.5 × 10 × 10
= 150 J
Force = 3000 N
Depth:
s = W/F
s = 150 / 3000
s = 0.05 m
Answer:
Depth = 5 cm
Common Mistakes
- Forgetting SI units
- Using wrong formula for energy
- Ignoring direction in work done
- Confusing power with energy
- Not converting km/h into m/s
Exam Tips
- Learn all formulas carefully.
- Draw diagrams for machines.
- Always write units in answers.
- Use proper sign convention for work done.
- Practice numerical problems regularly.
Practice MCQs
1. SI unit of work is:
A. Newton
B. Joule
C. Watt
D. Pascal
Answer:
B. Joule
2. Energy possessed due to motion is:
A. Potential Energy
B. Chemical Energy
C. Kinetic Energy
D. Heat Energy
Answer:
C. Kinetic Energy
3. Formula for power is:
A. F × s
B. mgh
C. W/t
D. ½mv²
Answer:
C. W/t
4. Mechanical advantage of a fixed pulley is:
A. 0
B. 1
C. 2
D. 10
Answer:
B. 1
5. Energy stored in stretched spring is:
A. Sound Energy
B. Potential Energy
C. Heat Energy
D. Light Energy
Answer:
B. Potential Energy
FAQ Section
Q1. What is work in physics?
Work is done when force causes displacement in the direction of force.
Q2. What is the SI unit of energy?
Joule (J)
Q3. What is kinetic energy?
Energy possessed by a moving object.
Q4. What is potential energy?
Stored energy due to position or shape.
Q5. What is a simple machine?
A device that makes work easier by changing force direction or magnitude.
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