Class 9 Science · Chapter 7
Short answer:
Chapter 7 — "Work, Energy, and Simple Machines" Class 9 Science ki Exploration textbook ki woh chapter hai jahan Physics rozmarra ki zindagi se seedha judti hai — trolley dhakelna, cricket ball fekna, ya seedhi (inclined plane) se saaman upar chadhana, sab is chapter ke concepts se explain hota hai. Purana "Work and Energy" chapter ab expand hoke ismein levers, pulleys aur inclined planes (simple machines) bhi add ho gaye hain.
Yaad rakhne wali core cheezein: Work (W = Fs cosθ), Kinetic Energy (½mv²), Potential Energy (mgh), Power (P = W/t), Law of Conservation of Energy, aur simple machines ka Mechanical Advantage + Velocity Ratio + Efficiency. Yahi "work energy and simple machines class 9 numericals" ka backbone hai — inhi formulas par baar-baar practice karna sabse zyada faydemand rehta hai.
Class 9 Science ki Exploration textbook (NCF-SE 2023 ke hisaab se poori naye sirey se likhi gayi, 13 chapters wali) me Chapter 7 hai — "Work, Energy, and Simple Machines". Agar aap "class 9 science new syllabus 2026-27 chapter list" dhoondh rahe ho, toh physics section me ye teesra chapter hai — pehle Motion (jiske "class 9 science motion chapter formulas" aap already revise kar chuke honge), phir Force and Laws of Motion, aur ab Work-Energy-Simple Machines. Iske baad Sound aata hai.
Purane "Work and Energy" chapter me sirf work, kinetic/potential energy, power aur conservation of energy tha. Naye rationalised syllabus ne isme Simple Machines (lever, pulley, inclined plane) merge kar diya hai — matlab ab Mechanical Advantage (MA), Velocity Ratio (VR) aur efficiency ke numericals bhi is single chapter ka part hain.
Neeche har in-text aur exercise question ka step-by-step solution diya gaya hai, poori working ke saath — taaki revision ke time formula yaad karne ke saath-saath ye bhi samajh aaye ki numerical ko step-by-step kaise solve karte hain. Agar aap poore chapter-wise revision plan bana rahe ho, toh "class 9 science all chapters notes pdf" ki jagah har chapter ko is tarah topic-by-topic solve karna zyada effective rehta hai, kyunki generic PDF me step-wise working aksar miss hoti hai.
Chapter 7 Summary — 5 Minute Revision
Is chapter me humne dekha ki work tabhi hota hai jab kisi object par force lage aur object us force ki direction me displace ho — W = Fs cosθ, aur agar force aur displacement same direction me hon toh W = Fs. Force displacement ke perpendicular ho (jaise satellite ka circular motion, ya suitcase ko sar par uthaye khada rehna) toh work zero hota hai.
Energy kaam karne ki capacity hai. Movement ki wajah se energy Kinetic Energy (KE = ½mv²) kehlati hai, aur position/height ki wajah se energy Potential Energy (PE = mgh) kehlati hai. Law of Conservation of Energy kehta hai ki energy na banayi ja sakti hai na khatam ki ja sakti hai — sirf ek form se doosre form me convert hoti hai; total energy hamesha constant rehti hai.
Power kaam karne ki rate hai — P = W/t, SI unit watt (1 W = 1 J/s). Commercial unit hai kilowatt-hour (kWh), jise "unit" bhi kehte hain — 1 kWh = 3.6 × 10⁶ J.
Simple machines (lever, pulley, inclined plane) kaam ko aasan banate hain — force ki direction badal kar ya required effort kam kar ke, energy input kam nahi karte. Mechanical Advantage (MA = Load/Effort) batata hai machine kitna force multiply kar rahi hai, Velocity Ratio (VR = distance moved by effort/distance moved by load) batata hai geometry ka ratio, aur Efficiency (η = MA/VR × 100) batata hai kitna input work useful output work me convert hua — real machines me friction ki wajah se efficiency hamesha 100% se kam hoti hai.
In-Text Questions — Solutions
Ek force 7 N kisi object par lagta hai aur object us force ki direction me 8 m displace hota hai. Is case me kitna work hua?
Diya hai: F = 7 N, s = 8 m, force aur displacement same direction me hain (θ = 0°).
W = F × s = 7 × 8 = 56 J
Is object par 56 joule work kiya gaya.
Hum kab kehte hain ki work hua hai?
Work tabhi hota hai jab do conditions ek saath poori ho:
- Kisi object par force lage.
- Object us force ki direction me (ya us direction ke component me) displace ho.
Agar force lage lekin displacement zero ho, ya displacement force ke bilkul perpendicular ho, toh work zero maana jaata hai.
Jab force object ki displacement ki direction me kaam kar rahi ho, uska expression likho.
W = F × s
Jahan W = work done, F = applied force, s = displacement force ki direction me. Agar force aur displacement ke beech θ angle ho toh general expression:
W = F × s × cos θ
1 joule work ki definition do.
1 joule work tab hota hai jab 1 newton ka force kisi object par lage aur object us force ki direction me 1 meter displace ho.
1 J = 1 N × 1 m
Do bailon ki jodi ek hal (plough) par 140 N ka force lagati hai. Khet 15 m lamba hai. Poora khet jotne me kitna work hua?
Diya hai: F = 140 N, s = 15 m, force aur displacement same direction me.
W = F × s = 140 × 15 = 2100 J
Poora khet jotne me 2100 joule work kiya gaya.
Kisi object ki kinetic energy kya hoti hai?
Kisi gatishil (moving) object me uski gati ki wajah se jo energy hoti hai, use kinetic energy kehte hain. Jitni zyada speed, utni zyada kinetic energy — object ko rokne ke liye utna hi zyada work karna padega.
Kisi object ki kinetic energy ka expression likho.
KE = ½ m v²
Jahan m = object ka mass (kg), v = velocity (m/s). Unit joule (J) hai.
Ek object (mass m) ki 5 m/s velocity par kinetic energy 25 J hai. Agar velocity double kar di jaaye toh KE kitni hogi? Aur agar velocity teen guna kar di jaaye?
KE, velocity ke square ke proportional hoti hai (KE ∝ v²), mass constant rehte hue.
Velocity double (2v) hone par:
KE_new = 25 × (2)² = 25 × 4 = 100 J
Velocity teen guna (3v) hone par:
KE_new = 25 × (3)² = 25 × 9 = 225 J
Power kya hoti hai?
Kaam karne ki rate ko power kehte hain — yani ek unit time me kitna work kiya gaya.
P = W/t
1 watt power ki definition do.
1 watt power tab hoti hai jab 1 joule work 1 second me kiya jaaye.
1 W = 1 J/s
Ek lamp 10 s me 1000 J electrical energy consume karta hai. Uski power kitni hai?
Diya hai: W = 1000 J, t = 10 s.
P = W/t = 1000/10 = 100 W
Lamp ki power 100 watt hai.
Lever ka mechanical advantage kya batata hai, aur ye load arm/effort arm se kaise juda hai?
Mechanical Advantage (MA) batata hai machine effort ko kitna multiply karke load uthati hai — MA = Load/Effort. Lever ke principle of moments (torque balance) se:
Effort × Effort arm = Load × Load arm
Isse:
MA = Effort arm / Load arm
Matlab jitna lamba effort arm load arm ke comparison me, utna zyada MA milega — kam effort se zyada load uthaya ja sakta hai.
Ek single fixed pulley aur ek single movable pulley ka ideal mechanical advantage kitna hota hai, aur kyun?
Single fixed pulley: MA = 1. Ye sirf force ki direction badalta hai (neeche kheenchne se load upar jaata hai), force multiply nahi karta.
Single movable pulley: MA = 2 (ideal condition me). Load ko do string segments support karte hain, isliye effort load ka aadha hi lagta hai.
Exercise Questions — Solutions (Q1–Q15)
Ek object ke paas 1 J energy hai. Iska kya matlab hai?
Iska matlab hai ki us object me itni capacity hai ki wo kisi doosre object par 1 N ka force lagakar use 1 m tak apni force ki direction me displace kar sake — yani 1 J energy = 1 J work karne ki capacity.
Mass m, velocity v se move kar rahe object ki kinetic energy kya hogi?
KE = ½ m v²
10 kg mass ko 5 m height tak uthane me kitna work hua? (g = 9.8 m/s²)
Diya hai: m = 10 kg, h = 5 m, g = 9.8 m/s².
Height tak uthane me object ke against gravity work karna padta hai:
W = mgh = 10 × 9.8 × 5 = 490 J
490 joule work kiya gaya.
Prithvi ka chakkar laga rahe satellite par gravity ki force kitna work karti hai? Justify karo.
Satellite ki circular orbit me, gravity ki force hamesha center (Earth) ki taraf hoti hai — yani displacement (jo orbit ke tangent direction me hota hai) ke perpendicular. Chunki θ = 90°, aur cos 90° = 0:
W = F × s × cos 90° = 0
Isliye gravity satellite par zero work karti hai.
Kya kisi object me force ke bina displacement ho sakta hai? Discuss karo.
Newton ke pehle law ke hisaab se, agar koi object uniform velocity se move kar raha hai aur usme koi net (unbalanced) force nahi lag rahi, toh bhi wo displace hota rahega — inertia ki wajah se. Toh haan, net force zero hone par bhi displacement ho sakta hai, bashart object already motion me ho. Lekin motion shuru karne ke liye kabhi na kabhi force lagni padti hai.
Ek insaan 30 minute tak suitcase apne sar par uthaye khada rehta hai aur thak jaata hai. Kya usne work kiya? Justify karo.
Physics ki definition me nahi — kyunki suitcase ka displacement zero hai (wo khada hai, hila nahi raha), isliye W = F × s = F × 0 = 0. Lekin insaan thakta isliye hai kyunki uski muscles internally energy consume kar rahi hain (biological/chemical energy loss), physical work na hone ke bawajood.
Ek electric heater 1500 W ka rated hai. 10 ghante me ye kitni energy use karega?
Diya hai: P = 1500 W = 1.5 kW, t = 10 h.
Energy = P × t = 1.5 kW × 10 h = 15 kWh
Joule me convert karein toh: 15 × 3.6 × 10⁶ = 5.4 × 10⁷ J.
Ek pendulum bob ko ek taraf kheenchkar chhod dene par oscillate karta hai — energy changes discuss karo. Bob aakhir kaam rukk kyun jaata hai? Kya ye conservation of energy ka violation hai?
Bob ko ek taraf kheenchne par usme potential energy maximum hoti hai (mean position se sabse zyada height par). Chhodne par ye PE gradually KE me convert hoti hai, aur mean position par PE minimum, KE maximum hoti hai. Doosri taraf jaate hue phir KE, PE me convert hoti hai.
Waqt ke saath air resistance aur pivot ki friction ki wajah se mechanical energy (KE+PE) dheere-dheere heat aur sound energy me convert hokar surroundings me chali jaati hai, isliye bob rukk jaata hai.
Ye conservation of energy ka violation nahi hai — total energy (mechanical + heat + sound) hamesha constant rehti hai, sirf form change hoti hai.
1 watt power ki definition do.
1 watt power tab hoti hai jab 1 second me 1 joule work kiya jaata hai (1 W = 1 J/s).
Ek ghar ne ek mahine me 250 units energy consume ki. Ye kitni joule energy hai?
1 unit = 1 kWh = 3.6 × 10⁶ J.
250 units = 250 × 3.6 × 10⁶ = 9 × 10⁸ J
Total energy = 9 × 10⁸ joule.
40 kg mass ke object ko ground se 5 m upar uthaya gaya hai. Uski potential energy kitni hai? Agar object neeche girne diya jaaye toh aadhe raaste par uski kinetic energy kitni hogi? (g = 10 m/s²)
Total PE at top:
PE = mgh = 40 × 10 × 5 = 2000 J
Aadhe raaste (h = 2.5 m) par: PE remaining = mg(2.5) = 40 × 10 × 2.5 = 1000 J. Conservation of energy se, jo PE khatam hui wo KE ban gayi:
KE at halfway = Total energy − PE remaining = 2000 − 1000 = 1000 J
Aadhe raaste par KE = 1000 J (PE ke barabar, kyunki halfway par energy equally split hoti hai free fall me).
200 N ka load uthane ke liye ek pulley system me 50 N effort lagta hai. Iska Mechanical Advantage nikaalo.
Diya hai: Load = 200 N, Effort = 50 N.
MA = Load/Effort = 200/50 = 4
Mechanical Advantage = 4.
Ek inclined plane (seedha ramp) ki length 5 m hai aur height 1 m hai. Iska ideal Mechanical Advantage nikaalo, aur agar load 500 N hai toh required (ideal, friction-less) effort kitna hoga?
Ideal frictionless inclined plane ke liye:
MA = Length/Height = l/h = 5/1 = 5
Required effort:
Effort = Load/MA = 500/5 = 100 N
Ideal MA = 5, required effort = 100 N.
Ek machine me effort, load se 4 gunaa distance move karta hai. Agar MA = 2.5 hai, toh iski efficiency nikaalo.
Diya hai: VR (Velocity Ratio) = distance moved by effort/distance moved by load = 4, MA = 2.5.
Efficiency (η) = (MA/VR) × 100 = (2.5/4) × 100 = 62.5%
Efficiency = 62.5% — matlab ye machine ideal (100%) nahi hai, friction ki wajah se kuch input work loss hota hai.
Ek lever me effort arm 2 m hai aur load arm 0.5 m hai. Iska mechanical advantage nikaalo, aur bataao ye kaun-si class ka lever ho sakta hai agar fulcrum beech me ho.
Diya hai: Effort arm = 2 m, Load arm = 0.5 m.
MA = Effort arm/Load arm = 2/0.5 = 4
MA = 4 (matlab is lever se 4 guna zyada load, kam effort se uthaya ja sakta hai). Agar fulcrum load aur effort ke beech me ho (jaise seesaw), toh ye Class I lever hai.
Important Equations — Ek Nazar Me
| Concept | Formula / Rule |
|---|---|
| Work (general) | W = F × s × cos θ (θ = force aur displacement ke beech angle) |
| Work (force aur displacement same direction) | W = F × s; SI unit: joule (J) = N·m |
| Work zero kab hota hai | Displacement = 0, ya force displacement ke perpendicular (θ = 90°) |
| Kinetic Energy | KE = ½ m v² (m = mass in kg, v = velocity in m/s) |
| Potential Energy (gravitational) | PE = m g h (h = height above reference level) |
| Law of Conservation of Energy | Total energy (isolated system) hamesha constant — sirf ek form se doosre me convert hoti hai |
| Power | P = W/t; SI unit: watt (W) = J/s |
| Commercial unit of energy | 1 kWh (1 "unit") = 3.6 × 10⁶ J |
| Horsepower (reference) | 1 HP ≈ 746 W |
| Mechanical Advantage (MA) | MA = Load (L) / Effort (E) |
| Velocity Ratio (VR) | VR = distance moved by effort / distance moved by load |
| Efficiency (η) | η = (MA/VR) × 100 = (Output work/Input work) × 100 — real machines me hamesha < 100% |
| Lever principle | Effort × Effort arm = Load × Load arm ⇒ MA = Effort arm/Load arm |
| Inclined plane (ideal, frictionless) | MA = Length of incline (l) / Height (h) |
| Single fixed pulley | MA = 1 (sirf force ki direction badalta hai) |
| Single movable pulley (ideal) | MA = 2 (load do string segments se supported hota hai) |
↔ Table ko side me swipe karein
Common Mistakes — Yahan Marks Kat te Hain
- Har force ko "work karta hai" maan lena — displacement zero ho ya force displacement ke perpendicular ho toh work zero hota hai, jaise satellite ka orbit ya static load uthaye khada rehna.
- Work ke formula me cos θ bhool jaana jab force aur displacement same direction me na ho — sirf W = Fs likh dena galat hai jab tak θ = 0° na ho.
- Kinetic energy ke numericals me velocity change hone par KE ko linearly scale kar dena — KE ∝ v² hai, isliye velocity double karne par KE 2x nahi, 4x hoti hai.
- kWh aur kW ko joule/watt samajh kar directly mix kar dena — energy (kWh, joule) aur power (kW, watt) alag quantities hain, unit conversion (1 kWh = 3.6×10⁶ J) miss ho jaata hai.
- Simple machine ka MA aur VR ko efficiency samajh lena — MA = Load/Effort hai, VR = distance ratio hai, aur efficiency dono ka ratio (MA/VR × 100) hai; teeno alag hain.
- Ideal machine assume kar lena real life me bhi — friction ki wajah se real machines ki MA aur efficiency hamesha ideal (frictionless) value se kam hoti hai, ye exam me clarify karna zaroori hai.
Board-Style Important Questions
- 1 mark: SI unit of power kya hai?
- 2 marks: Ek object ki kinetic energy uski velocity ke saath kaise vary karti hai? Formula ke saath samjhao.
- 3 marks: Work-energy theorem likho aur ek example se samjhao ki kaise net work done object ki kinetic energy me change ke barabar hota hai.
- 3 marks: Simple pendulum ke oscillation ke dauraan energy conservation ko step-by-step samjhao.
- 5 marks: Ek numerical: 60 kg mass ka vyakti 10 m height wali seedhi 20 second me chadhta hai. Uski power calculate karo. (g = 10 m/s²)
- 5 marks: Lever, pulley aur inclined plane teeno simple machines ka mechanical advantage kaise nikaala jaata hai — har ek ke liye formula aur ek numerical example do.
Aksar Poochhe Jaane Wale Sawaal
Class 9 Science Chapter 7 'Work, Energy, and Simple Machines' me kitne topics cover hote hain?
Is chapter me Work (definition + formula), Kinetic Energy, Potential Energy, Law of Conservation of Energy, Power aur commercial unit of energy, plus Simple Machines (lever, pulley, inclined plane) ke MA/VR/Efficiency — ye sab cover hote hain. Purane 'Work and Energy' chapter ke comparison me Simple Machines wala part naya add hua hai.
Kya ye chapter Class 9 Science 'class 9 science exploration book pdf download' me available hai ya alag se milega?
Ye chapter naye 'Exploration' textbook ka hi part hai (Chapter 7, 13-chapter edition), koi alag booklet nahi hai. Textbook download ya reference ke liye hamesha apne school ya official publisher source se hi verify karna best rehta hai, kyunki third-party PDFs me content mismatch ho sakta hai.
Work, Energy aur Simple Machines chapter ke numericals ke liye sabse zyada kya practice karni chahiye?
'work energy and simple machines class 9 numericals' me KE/PE calculation, power (P=W/t), kWh-to-joule conversion, aur simple machine ke MA/VR/efficiency wale sawal common types hain jo baar-baar practice maangte hain. Roz 3-4 mixed numericals solve karna best strategy hai.
Ye chapter Class 9 Science ke Motion chapter se kaise connected hai?
Motion chapter me velocity aur acceleration concepts cover hote hain — 'class 9 science motion chapter formulas' (v = u+at, s = ut+½at² etc.) yahan Kinetic Energy (KE = ½mv²) ke numericals me directly use hote hain, isliye Motion chapter revise karke aana helpful hota hai.
Work-Energy-Simple Machines chapter ke baad kaunsa chapter aata hai, aur uski taiyari kaise shuru karein?
Iske baad Sound chapter aata hai. Agar 'class 9 science sound chapter important questions' abhi se dekhna chahte ho, toh wave properties (frequency, wavelength, amplitude) ka basic revision Work-Energy chapter khatam hone ke baad shuru kar sakte ho — dono chapters energy transfer ke concept se related hain (sound bhi energy ka ek form hai).
Simple Machines wale numericals me sabse common confusion kya hoti hai?
Sabse common confusion Mechanical Advantage (MA), Velocity Ratio (VR), aur Efficiency ko ek doosre se mix kar dena hoti hai. Yaad rakho: MA = Load/Effort (force ratio), VR = distance moved by effort/load (geometry ratio), aur Efficiency = (MA/VR)×100 (dono ka combined output) — teeno alag formulas hain, exam me galat formula use karna sabse common mistake hai.
Class 9 Science — Saare Chapters
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