1. What is Work?
In physics, **work is done only when a force moves an object** in the direction of force.
Formula: Work (W) = Force (F) × Displacement (s)
Unit: Joule (J)
Formula: Work (W) = Force (F) × Displacement (s)
Unit: Joule (J)
हिंदी: Work तभी होगा जब बल लगने पर वस्तु हिले।
धक्का लगाया लेकिन वस्तु नहीं हिली → Work = 0 (Physics rule).
2. Positive, Negative & Zero Work
✔ **Positive Work** – force & displacement in same direction
✔ **Negative Work** – force opposite to displacement (e.g., friction)
✔ **Zero Work** – no displacement
उदाहरण:
टेबल उठाकर ले जाना = Positive work
घर्षण (friction) = Negative work
दीवार को धक्का → नहीं हिली = Zero work
3. Energy (Ability to do Work)
Energy = capacity to do work.
Unit: **Joule (J)**
हिंदी: Energy वह क्षमता है जिससे काम होता है।
4. Potential Energy (PE)
Energy due to **position** or **height**.
PE = m × g × h
Examples:
PE = m × g × h
Examples:
- Water stored at height in a dam
- Stretched bow
- Lifted stone
ऊँचाई पर रखी वस्तु में PE होती है।
5. Kinetic Energy (KE)
Energy of a moving body.
KE = ½ × m × v²
Examples:
KE = ½ × m × v²
Examples:
- Moving car
- Flying bullet
- Flowing water
गति वाली वस्तु की ऊर्जा = Kinetic Energy
Formula याद रखें: ½mv²
6. Law of Conservation of Energy
Energy can neither be created nor destroyed.
It only changes from one form to another.
Example:
Example:
- Dams → PE of water → KE → Electricity
- Pendulum → PE ↔ KE continuously
ऊर्जा कभी खत्म नहीं होती, बस रूप बदलती है।
7. Power
Power = rate of doing work.
Power (P) = Work / Time Unit: **Watt (W)**
Power (P) = Work / Time Unit: **Watt (W)**
हिंदी: Power = काम करने की गति
1 Watt = 1 Joule/second
8. Gravitation (NDA Very Important)
Every object attracts every other object with a force called **gravitational force**.
Weight (W) = m × g Where g = 9.8 m/s²
Weight (W) = m × g Where g = 9.8 m/s²
हिंदी: गुरुत्वाकर्षण हमें पृथ्वी की ओर खींचता है → यही वजन कहलाता है।
9. Simple Machines (Everyday NDA Physics)
Machines make work easier by **multiplying force** or **changing direction of force**.
Examples:
Examples:
- Lever – seesaw, pliers
- Pulley – well pulley, cranes
- Inclined Plane – ramps
- Wheel & Axle – screwdriver, steering wheel
- Hydraulic Brakes – Pascal’s law
- Jack – multiplying force
हिंदी: Simple machines = वे उपकरण जो काम को आसान करते हैं।
Lever, pulley, jack – ये NDA के लिए बहुत पूछे जाते हैं।
10. Everyday Science Applications (UPSC Combined)
- Pressure Cooker – higher pressure → higher boiling point → faster cooking
- Thermos Flask – prevents heat loss (conduction, convection, radiation)
- Hydraulic Brake – Pascal’s law (pressure transmitted equally)
- Pumps / Siphon – atmospheric pressure
- Fuse – melts to protect circuit
- Lever in scissors, pliers, nutcracker
ये practical devices UPSC को बहुत पसंद हैं —
इन्हें example के साथ याद रखें।
📘 Practice Questions (NDA Style)
1. Work is done only when:
a) Force is applied
b) Body moves
c) Force produces displacement
d) None
2. The unit of energy is:
a) Newton
b) Joule
c) Watt
d) Pascal
3. PE of a body depends on:
a) Height
b) Mass
c) Gravity
d) All of the above
4. Kinetic energy formula is:
a) mv
b) mgh
c) ½mv²
d) m/v
5. Pascal’s law is used in:
a) Pulley
b) Hydraulic brake
c) Lever
d) Screw gauge
6. The rate of doing work is:
a) Energy
b) Power
c) Momentum
d) Pressure
a) Force is applied
b) Body moves
c) Force produces displacement
d) None
2. The unit of energy is:
a) Newton
b) Joule
c) Watt
d) Pascal
3. PE of a body depends on:
a) Height
b) Mass
c) Gravity
d) All of the above
4. Kinetic energy formula is:
a) mv
b) mgh
c) ½mv²
d) m/v
5. Pascal’s law is used in:
a) Pulley
b) Hydraulic brake
c) Lever
d) Screw gauge
6. The rate of doing work is:
a) Energy
b) Power
c) Momentum
d) Pressure
Answers:
1 – c
2 – b
3 – d
4 – c
5 – b
6 – b
1 – c
2 – b
3 – d
4 – c
5 – b
6 – b
Quick Recap | सार
Now you should be able to:
- Recall the most important ideas of this topic.
- Solve typical NDA-style questions from this chapter.
- Spot and avoid common traps used in competitive exams.
Self-check: 3–4 random questions उठा कर देखिए – अगर आसानी से हो रहे हैं, तो chapter strong है।
Common Exam Mistakes | आम गलतियाँ
- Concept ठीक से clear न करना और सीधे रटने की कोशिश करना।
- Question पूरी तरह पढ़े बिना answer mark कर देना।
- Figures, dates, या formulas को last step में verify न करना।
NDA में smart strategy यह है कि आसान वाले सवाल पहले, doubtful बाद में – और silly mistakes से बचना सबसे ज़रूरी है।