NCERT Class 9 Science Chapter 10 – Work and Energy

NCERT Class 9 Science Chapter 10 explains the scientific meaning of work and introduces the concept of energy as the capacity to do work. NCERT Class 9 Science Chapter 10 builds strong conceptual clarity about kinetic energy, potential energy, mechanical energy and power, which are essential for higher physics.

The chapter defines work as the product of force and displacement in the direction of force, expressed as W = F × s, where the SI unit of work is joule (J). One joule is the work done when a force of 1 newton displaces an object by 1 metre.

NCERT Class 9 Science Chapter 10 derives the formula of kinetic energy as ½mv² using equations of motion and explains gravitational potential energy as mgh, where g = 9.8 m/s². It also introduces the law of conservation of energy, which states that energy can neither be created nor destroyed but only transformed from one form to another.

The chapter further explains power as the rate of doing work, expressed as Power = Work/Time, with SI unit watt (W). One watt equals one joule per second, and 1 kilowatt-hour (kWh) = 3.6 × 10⁶ joules.

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1. Introduction

• In daily language, work means any activity involving effort, but in physics work has a specific scientific meaning.
• Work is said to be done only when a force applied on an object produces displacement.
• The concept of work connects force and motion and leads to idea of energy.
• Energy is the capacity to do work.
• The SI unit of work and energy is joule (J).

2. Work done by a force

• Work is done when a force causes displacement in direction of force.
• Work = Force × Displacement (W = F × s).
• If force and displacement are in same direction, W = Fs.
• If displacement is zero, work done is zero.
• SI unit of work is joule (J).
• 1 joule = Work done when 1 N force displaces object by 1 m.

3. Conditions when work is zero

• If displacement is zero, work = 0.
• Example: Pushing a wall that does not move.
• If force applied but displacement is perpendicular, work = 0.
• Example: Carrying a bag horizontally.

4. Positive and negative work

• Work is positive when force and displacement are in same direction.
• Work is negative when force and displacement are opposite.
• Example: Friction does negative work.
• Example: Gravitational force does positive work during free fall.

5. Energy

• Energy is capacity to do work.
• Object possessing energy can do work.
• Forms include kinetic energy, potential energy, heat, light, sound and electrical energy.
• SI unit of energy is joule (J).

6. Kinetic energy

• Energy possessed due to motion.
• Formula: KE = ½ mv².
• Depends on mass and square of velocity.
• SI unit is joule.
• Greater velocity means much greater kinetic energy.

7. Derivation of kinetic energy formula

• Consider object of mass m moving with velocity u.
• After applying force F, velocity becomes v.
• Work done = Force × displacement.
• From Newton’s second law, F = ma.
• Using equation v² – u² = 2as.
• Work = mas = m(v² – u²)/2.
• If object starts from rest (u=0), KE = ½ mv².

8. Potential energy

• Energy possessed due to position or configuration.
• Gravitational potential energy = mgh.
• Where m = mass, g = 9.8 m/s², h = height.
• Depends on mass, gravity and height.
• SI unit is joule.

9. Work done by gravity

• When object falls from height h, work done by gravity = mgh.
• Potential energy decreases while kinetic energy increases.
• At ground level, potential energy becomes zero.

10. Mechanical energy

• Sum of kinetic and potential energy.
• Mechanical energy = KE + PE.
• In absence of friction, mechanical energy is conserved.
• This is called law of conservation of energy.

11. Law of conservation of energy

• Energy can neither be created nor destroyed.
• Energy only changes from one form to another.
• Example: In free fall, PE converts into KE.
• Total energy remains constant.

12. Power

• Power is rate of doing work.
• Power = Work / Time.
• SI unit is watt (W).
• 1 watt = 1 joule per second.
• Commercial unit of energy is kilowatt-hour (kWh).
• 1 kWh = 3.6 × 10⁶ joules.

13. Work-energy theorem

• States that work done on object equals change in its kinetic energy.
• If net work done is W, then W = change in KE.
• Derived from Newton’s second law and equations of motion.
• Important for solving motion problems.

14. Numericals

• If 10 N force moves object 5 m, work = 50 J.
• KE of 2 kg object moving at 3 m/s = 9 J.
• PE of 5 kg object at 10 m height = 490 J.
• Power if 100 J work done in 5 s = 20 W.
• Electrical energy of 1 kW device used for 2 hours = 2 kWh.

15. Units and conversions

• 1 Joule = 1 N × 1 m.
• 1 Watt = 1 J/s.
• 1 kW = 1000 W.
• 1 kWh = 3.6 × 10⁶ J.

16. Conclusion

• Work is done only when force causes displacement.
• Kinetic energy depends on mass and velocity.
• Potential energy depends on position.
• Energy is conserved.
• Power measures rate of energy transfer.

17. Exam oriented facts

• Work formula – W = F × s.
• Kinetic energy – ½ mv².
• Potential energy – mgh.
• Power – Work/Time.
• 1 Watt – 1 J/s.
• 1 kWh – 3.6 × 10⁶ J.
• Law of conservation of energy – Energy cannot be destroyed.
• Work-energy theorem – Work equals change in KE.

Understanding NCERT Class 9 Science Chapter 10 is essential for connecting force, motion and energy transformation in physical systems.

This chapter forms the base for advanced mechanics, electricity and thermodynamics in senior secondary classes.

For school examinations, students must focus on formulas for work, kinetic energy, potential energy, power, conservation of energy and derivation of ½mv².

For competitive examinations like JEE and NDA, NCERT Class 9 Science Chapter 10 provides fundamental clarity required for solving numerical problems involving energy and power.

Continue reading NCERT Class 9 Science Chapter 11 – Sound to understand wave motion, frequency and speed of sound.

FAQs

Q1. What is NCERT Class 9 Science Chapter 10 about?
It explains the scientific meaning of work, different forms of energy, kinetic and potential energy, power and the law of conservation of energy.

Q2. What is the formula for kinetic energy?
The formula for kinetic energy is ½mv², where m is mass and v is velocity of the object.

Q3. What is potential energy?
Potential energy is the energy possessed by an object due to its position or configuration, and gravitational potential energy is expressed as mgh.

Q4. What is 1 kilowatt-hour (kWh)?
One kilowatt-hour is the energy consumed when a device of 1 kilowatt operates for 1 hour, and it equals 3.6 × 10⁶ joules.

Q5. Why is NCERT Class 9 Science Chapter 10 important for competitive exams?
It builds the foundation for energy transformation, conservation laws and numerical problem-solving, which are essential topics in engineering entrance examinations.

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