Class 9 Science · Chapter 6
Short answer:
Class 9 Science Chapter 6 "How Forces Affect Motion" (naye "Exploration" textbook, session 2026-27) force, balanced-unbalanced forces, inertia, aur Newton ke teenon laws of motion cover karta hai — saath me momentum aur uski conservation bhi. Yehi chapter hai jahan se class 9 science motion chapter formulas ka sabse important hissa (F = ma, p = mv) aata hai, aur ye Chapter 4 "Motion" (jahan sirf speed/velocity/acceleration cover hue the) ka direct next-step hai — ab hum poochte hain "motion badalta kyu hai?"
Chapter 4 me humne motion ko describe karna seekha tha — distance, displacement, speed, acceleration. Chapter 6 "How Forces Affect Motion" us kahani ka agla part hai: ab hum poochte hain ki motion badalta kyun hai, aur uska jawaab hai — force.
Ye chapter Class 9 Physics ki asli neev hai, kyunki isme jo Newton ke teen laws of motion aap seekhte ho, wahi laws Class 11 Mechanics, JEE ki poori Kinematics-Dynamics section, aur real-engineering (seat belts, rockets, crash safety) ki base bante hain. Naye NCERT "Exploration" textbook (First Edition, April 2026) me ye topic purane "Force and Laws of Motion" chapter se rename/restructure hoke Chapter 6 ban gaya hai — content mostly wahi hai (balanced/unbalanced force → inertia → F=ma → momentum conservation), sirf naming aur presentation NCF-SE 2023 ke hisaab se refresh hui hai.
Agar aap poore class 9 science new syllabus 2026-27 chapter list me apni jagah check kar rahe ho, to ye chapter physics ke teen chapters (Motion → How Forces Affect Motion → Work, Energy and Simple Machines) ka middle piece hai. Neeche in-text questions, exercise solutions, formula table, common mistakes, aur exam-pattern questions — sab step-by-step working ke saath diye gaye hain.
Chapter 6 Summary — 5 Minute Revision
Force ek push ya pull hai jo kisi object ki state of motion badal sakta hai (ya shape). Jab forces balanced hoti hain, net force zero hota hai aur object ki motion state (rest ya constant velocity) nahi badalti — yahi Newton's First Law (Law of Inertia) hai. Inertia object ka wo natural tendency hai jo apni current state (rest/motion) ko maintain karna chahta hai, aur mass hi inertia ka measure hai — jitna zyada mass, utna zyada inertia.
Newton's Second Law quantitative relation deta hai: net force momentum ke rate of change ke barabar hota hai, jo simplify hoke F = ma banta hai — isi se force ka SI unit newton (N) define hota hai. Newton's Third Law batata hai ki har action ka equal-opposite reaction hota hai, lekin ye do alag objects par lagta hai (isliye net force cancel nahi hota). Isi third law se conservation of momentum derive hota hai — kisi bhi isolated system (jaise gun-bullet ya do gaadiyon ki collision) ka total momentum collision se pehle aur baad me same rehta hai.
In-Text Questions — Solutions
Ek football zameen par static padi hai. Kya wo apne aap motion me aa sakti hai? Apna jawaab reason ke saath do.
Nahi. Newton ke first law (law of inertia) ke according, koi bhi object apni rest state me hi rahega jab tak koi unbalanced (net) external force uspar act na kare. Football me khud motion start karne ki capability nahi hai — usse kick (push force) lagani padegi tabhi wo motion me aayegi.
Bus achanak brake lagati hai to andar khade passengers aage ki taraf jhuk jaate hain. Kyun?
Jab bus brake lagati hai, bus (aur uske saath passengers ke pair floor ke through) turant ruk jaati hai, lekin passenger ke upper body ka wahi purana inertia of motion continue karta hai — wo apni original velocity se aage badhna chahta hai. Isi wajah se body aage ki taraf jhuk jaati hai jab tak koi force (seat, haath ki pakad) use na roke.
Do gaadiyon me se ek khaali hai aur doosri poori tarah loaded hai. Dono ko same force se push kiya jaaye to kaunsi jyada aasani se move hogi aur kyun?
Khaali gaadi zyada aasani se move hogi. Loaded gaadi ka mass zyada hai, aur mass hi inertia ka measure hota hai — isliye uska inertia (motion resist karne ki tendency) bhi zyada hai. Same force (F) lagane par, a = F/m ke hisaab se zyada mass wali gaadi ka acceleration kam hoga.
Ek carrom striker ko flick karne par coins bikhar jaate hain lekin queen apni jagah se thodi hi hilti hai jab tak striker directly na maare. Ye kis concept ka example hai?
Ye inertia of rest ka example hai — striker se force sirf jis coin ko directly touch karta hai usi par lagta hai, baaki coins apni rest state me inertia ki wajah se stay karte hain jab tak unpar direct force na lage.
2 kg mass ki trolley 3 m/s² se accelerate ho rahi hai. Uspar lagne wala net force nikaalo.
Diya hai: m = 2 kg, a = 3 m/s²
Newton's second law se, F = m × a
F = 2 × 3 = 6 N
Trolley par lagne wala net force 6 newton hai, jo acceleration ki hi direction me hoga.
5 kg ki ek gendh 2 m/s se move kar rahi hai. Uska momentum nikaalo.
Diya hai: m = 5 kg, v = 2 m/s
Momentum, p = m × v
p = 5 × 2 = 10 kg m/s
Gendh ka momentum 10 kg m/s hai, jo velocity ki direction me hi hota hai.
0.5 kg ki cricket ball 20 m/s se aane wale bhi bat se takraakar 15 m/s se ulti direction me wapas jaati hai. Agar contact time 0.02 s ho to bat dwara ball par lagaya gaya average force nikaalo.
Initial velocity, u = +20 m/s (ball aane ki direction positive lo)
Final velocity, v = −15 m/s (opposite direction me wapas)
Change in momentum, Δp = m(v − u) = 0.5 × (−15 − 20) = 0.5 × (−35) = −17.5 kg m/s
F = Δp / t = −17.5 / 0.02 = −875 N
Force ka magnitude 875 N hai, negative sign batata hai ki force bat ki taraf (initial motion ke opposite) lagi.
Do trolleys collide karti hain — ek 3 kg ki 4 m/s se aur doosri 2 kg ki rest me hai. Collision ke baad dono ek saath 3 m/s se move karti hain. Kya ye momentum conservation follow karta hai? Check karo.
Before collision: total momentum = (3 × 4) + (2 × 0) = 12 + 0 = 12 kg m/s
After collision (dono jud kar move): total mass = 3 + 2 = 5 kg
After collision momentum = 5 × 3 = 15 kg m/s
Before (12 kg m/s) aur after (15 kg m/s) barabar nahi hain, isliye ye given data momentum conservation follow nahi karta — isse pata chalta hai ki final common velocity 2.4 m/s honi chahiye thi (12/5), 3 m/s nahi.
Ek naav (canoe) me baitha aadmi pani ko peeche ki taraf paddle se dhakelta hai, aur naav aage badh jaati hai. Isse Newton ka kaunsa law explain hota hai?
Ye Newton's Third Law of Motion ka example hai. Aadmi paddle se pani ko peeche push karta hai (action force), aur reaction me pani naav ko equal-and-opposite force se aage push karta hai. Dono forces equal magnitude ki hoti hain lekin do alag objects (pani aur naav) par act karti hain, isliye ye cancel nahi hoti.
Ek bandook se goli chalayi jaati hai to bandook peeche ki taraf jhatka (recoil) khaati hai. Explain karo kyun.
Chalne se pehle bandook + goli system ka total momentum zero tha (dono rest me). Goli firing ke baad aage badhti hai (forward momentum), to Newton's third law + momentum conservation ke hisaab se bandook ko equal-magnitude, opposite-direction momentum milta hai — isi wajah se bandook peeche recoil karti hai. Bandook ka mass zyada hone ki wajah se uski recoil velocity goli ki velocity se kaafi kam hoti hai.
Ek trolley ko level floor par constant velocity se push karne ke liye ek constant horizontal force F lagani padti hai. Iska matlab kya trolley par net force zero hai ya F ke barabar?
Constant velocity ka matlab acceleration zero hai, aur Newton's second law (Fnet = ma) ke according, jab a = 0 tab Fnet bhi zero hota hai. Isliye applied force F, floor se lagne wale friction force ke exactly barabar aur opposite hoti hai — dono net me cancel ho jaate hain.
Karate player ek moti lakdi ki takhti (wooden slab) ko ek hi jhatke me tod deta hai. Isme kaunsa physics principle kaam kar raha hai?
Karate chop me haath ko bahut chhote time interval me rokna hota hai — isse momentum change ka rate (Δp/Δt) bahut bada ho jaata hai, aur Newton's second law (F = Δp/Δt) ke hisaab se takhti par ek bahut bada force lagta hai jo usse tod deta hai.
High jump ya long jump players cushioned mats/sand pits par kyu jump karte hain, hard ground par nahi?
Cushioned surface landing ke time ko badha deti hai (soft material zyada der tak compress hoti hai). Newton's second law se, force = change in momentum / time — jab time badhta hai to landing force kam ho jaata hai, isse athlete ke joints aur bones ko chot lagne ka risk kam hota hai.
Car me seat belts kyu lagayi jaati hain — physics ke terms me explain karo.
Sudden collision me car turant ruk jaati hai, lekin passenger ka body apne inertia of motion ki wajah se aage badhna chahta hai. Seat belt is forward motion ko rokte waqt stopping time ko thoda badha deti hai aur force ko body ke strong hissों (chest, pelvis) par distribute kar deti hai, jisse F = Δp/Δt kam hokar injury ka risk ghat jaata hai.
Exercise Questions — Solutions (Q1–Q20)
Force ki definition do. Iske SI unit ka naam aur symbol likho.
Force ek push ya pull hai jo kisi object par lagne se uski rest ya motion ki state ko badal sakta hai, ya uski shape/size ko badal sakta hai. Force ka SI unit newton hai, jiska symbol N (capital) hota hai. 1 newton wo force hai jo 1 kg mass ke object me 1 m/s² ka acceleration produce kare.
Balanced forces aur unbalanced forces me antar batao, ek-ek example ke saath.
Balanced forces: Jab kisi object par lagne wali sabhi forces ka net effect zero hota hai, object apni state (rest ya constant velocity) me hi rehta hai. Example: table par rakhi kitaab par gravity aur normal reaction force balanced hoti hain, isliye kitaab static rehti hai.
Unbalanced forces: Jab net force zero nahi hota, object accelerate karta hai — speed, direction ya dono badalte hain. Example: gaadi ka engine force jab friction se zyada ho, gaadi speed pakadti hai.
Newton ke first law of motion ko likho. Ise 'law of inertia' kyu kaha jaata hai?
Newton's First Law: Koi bhi object apni rest state ya uniform motion (straight line, constant speed) me tab tak rahega jab tak koi unbalanced (net) external force uspar act na kare.
Ise law of inertia isliye kaha jaata hai kyunki ye directly kisi object ki inertia (apni current state ko maintain karne ki natural tendency) ko define karta hai.
Inertia ki teen types likho, har ek ka ek daily-life example ke saath.
1. Inertia of rest: Table cloth ko jhatke se kheenchne par uspar rakhe bartan apni jagah rehte hain (object rest me rehna chahta hai).
2. Inertia of motion: Chalti bus me achanak brake lagne par passenger aage ki taraf gir jaata hai (object motion me hi rehna chahta hai).
3. Inertia of direction: Chalti gaadi jab sharp mod le to andar baithe log opposite side ki taraf dhakele jaate hain (object apni original direction me hi jaana chahta hai).
Mass aur inertia ka kya relation hai? Do objects jinka mass alag hai, unme se kis me inertia zyada hoga?
Mass kisi object ke inertia ka direct measure hai — jitna zyada mass, utna zyada inertia (yaani uski state badalna utna hi mushkil). Do objects me se jiska mass zyada hoga, uska inertia bhi zyada hoga — usse accelerate karna ya rokna dono zyada mushkil honge.
Momentum define karo. Iska SI unit aur direction bhi batao.
Momentum kisi moving object ki motion ki quantity hai, jo uske mass aur velocity ke product ke barabar hota hai: p = m × v. Iska SI unit kg m/s hai, aur ye ek vector quantity hai — iski direction hamesha object ki velocity ki direction ke same hoti hai.
Newton ke second law of motion ko state karo aur ismein se F = ma kaise derive hota hai, dikhao.
Newton's Second Law: Kisi object par lagne wala net force uske momentum ke rate of change ke seedha proportional hota hai, aur force ki direction me hi momentum change hota hai.
F ∝ (change in momentum) / time = (mv − mu) / t = m(v − u)/t
Chunki (v − u)/t = acceleration (a), isliye:
F ∝ ma ⟹ F = k·ma
Newton ko is tarah define kiya gaya hai ki proportionality constant k = 1 ho jaaye, isliye final relation hai: F = ma.
1 newton force ki SI definition do, using F = ma.
F = ma me m = 1 kg aur a = 1 m/s² rakhne par:
F = 1 kg × 1 m/s² = 1 N
1 newton wo force hai jo 1 kg mass ke object me 1 m/s² ka acceleration produce kare.
1000 kg ki ek car rest se 20 m/s ki speed 10 second me pakad leti hai. Car par lagne wala average force nikaalo.
Diya hai: m = 1000 kg, u = 0, v = 20 m/s, t = 10 s
Acceleration, a = (v − u)/t = (20 − 0)/10 = 2 m/s²
Force, F = m × a = 1000 × 2
F = 2000 N
Car par average force 2000 N (2 kN) lagta hai.
Ek 60 kg ki sledge 10 m/s se move kar rahi hai. Uska momentum nikaalo, aur agar wo 5 second me ruk jaaye to usme lagne wala retarding force nikaalo.
Momentum, p = m × v = 60 × 10 = 600 kg m/s
Ruk jaane par final velocity v = 0, initial u = 10 m/s, t = 5 s
a = (v − u)/t = (0 − 10)/5 = −2 m/s²
F = m × a = 60 × (−2) = −120 N
Sledge ka momentum 600 kg m/s hai; retarding (rokne wala) force 120 N hai, negative sign motion ke opposite direction dikhata hai.
Newton ke third law of motion ko state karo. Kya action-reaction pair ek doosre ko cancel karte hain? Reason do.
Newton's Third Law: Har action force ke liye ek equal magnitude aur opposite direction ka reaction force hota hai.
Nahi, ye ek doosre ko cancel nahi karte, kyunki action force aur reaction force do alag-alag objects par lagte hain, ek hi object par nahi. Cancel tabhi hota jab dono forces same object par lagti.
Conservation of momentum ke law ko state karo.
Jab do (ya zyada) objects ek isolated system me interact karte hain (jaise collision), to unke total momentum me koi external force nahi lagti, isliye collision se pehle ka total momentum aur baad ka total momentum barabar rehta hai — bashart external force zero ho.
Newton ke third law se momentum conservation ka law kaise derive hota hai, short me dikhao.
Do objects A aur B collide karte hain time t ke liye. A, B par force FAB lagata hai; B, A par FBA lagata hai.
Third law se: FAB = −FBA
Second law se: F = Δp/t, isliye ΔpA/t = −ΔpB/t ⟹ ΔpA = −ΔpB
Yaani ΔpA + ΔpB = 0 ⟹ total momentum change zero
Isse sabit hota hai ki system ka total momentum conserved rehta hai.
4 kg ki ek gun se 0.05 kg ki bullet 100 m/s se fire hoti hai. Gun ki recoil velocity nikaalo.
Firing se pehle total momentum = 0 (dono rest me)
Momentum conservation: mgunvgun + mbulletvbullet = 0
4 × vgun + 0.05 × 100 = 0
4 × vgun = −5
vgun = −1.25 m/s
Gun ki recoil velocity 1.25 m/s hai, negative sign batata hai ki ye bullet ki direction ke opposite (peeche) hai.
Do objects, mass 100 g aur 200 g, opposite directions me 2 m/s aur 1 m/s se move karte hue collide karte hain aur collision ke baad chipak jaate hain. Combined velocity nikaalo.
m₁ = 0.1 kg, u₁ = +2 m/s; m₂ = 0.2 kg, u₂ = −1 m/s (opposite direction negative)
Before collision total momentum = (0.1 × 2) + (0.2 × −1) = 0.2 − 0.2 = 0 kg m/s
After collision: (m₁+m₂) × v = 0 ⟹ 0.3 × v = 0
v = 0 m/s
Collision ke baad combined object rest me aa jaayega (v = 0), kyunki dono ke momentum equal-opposite the.
Rocket propulsion Newton ke kaunse law(s) par based hai? Short me explain karo.
Rocket propulsion Newton's Third Law aur momentum conservation par based hai. Rocket apne fuel ki hot gases ko high speed se peeche (niche) ki taraf expel karta hai (action), aur reaction me gases rocket ko upar/aage push karte hain equal-opposite force se. Total system (rocket + expelled gases) ka momentum conserved rehta hai.
Ek 50 kg ka athlete high jump me landing karta hai. Explain karo padded mat force ko kaise kam karta hai (F = Δp/Δt ka use karke).
Landing par athlete ki momentum change (Δp) fixed hai — usse zero velocity tak lana hi hai. Force, F = Δp/Δt hai. Padded mat athlete ke stop hone ka time (Δt) badha deti hai (compress hoti hai gradually), isliye same Δp ke liye F kam lagta hai — hard ground par Δt bahut kam hota, isliye F bahut zyada lagta aur injury ka risk badh jaata.
Kya ek akela force kabhi 'balanced' ho sakta hai? Apna jawaab justify karo.
Nahi. Balanced/unbalanced sirf tab meaningful hai jab do ya zyada forces ek hi object par simultaneously act kar rahi hoon aur unka net effect compare kiya ja raha ho. Ek akela force apne-aap balanced nahi ho sakta — usse balance karne ke liye kam-se-kam ek equal-opposite doosri force zaroori hai.
Ek 0.15 kg ki cricket ball ko bowler 108 km/h ki speed se throw karta hai. Iski velocity ko m/s me convert karo aur momentum nikaalo.
108 km/h ko m/s me convert: 108 × (5/18) = 30 m/s
Momentum, p = m × v = 0.15 × 30
p = 4.5 kg m/s
Ball ka momentum 4.5 kg m/s hai. (Note: km/h se m/s convert karte time 5/18 factor use karna zaroori hai — ye common mistake area hai.)
Explain karo ki heavy truck ko push karne me zyada force kyu lagti hai compared to ek chhoti car, dono ko same acceleration dene ke liye.
Newton's second law ke hisaab se, F = m × a. Same acceleration (a) ke liye, force directly mass ke proportional hoti hai. Truck ka mass car se kaafi zyada hota hai, isliye same acceleration produce karne ke liye truck ko zyada force chahiye hoti hai.
Important Equations — Ek Nazar Me
| Concept | Formula | SI Unit / Note |
|---|---|---|
| Force (Newton's Second Law) | F = m × a | newton (N); 1 N = 1 kg m/s² |
| Force via momentum change | F = (mv − mu) / t = Δp / t | N; direction = momentum change ki direction |
| Momentum | p = m × v | kg m/s; vector quantity, velocity ki direction me |
| Acceleration | a = (v − u) / t | m/s² |
| Conservation of momentum (two-body collision) | m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂ | Sirf tab jab koi external force na lage |
| Newton's First Law | ΣF = 0 ⟹ v = constant (rest ya uniform motion) | Law of inertia |
| Newton's Third Law | FAB = −FBA | Alag objects par act karta hai, cancel nahi hota |
| Speed unit conversion | km/h → m/s: multiply by 5/18 | Numericals me common step |
↔ Table ko side me swipe karein
Common Mistakes — Yahan Marks Kat te Hain
- Mass aur weight ko same maan lena — mass kisi object ka fixed matter-quantity hai (kg), weight gravity ki wajah se lagne wala force hai (N); inertia hamesha mass se related hai, weight se nahi.
- Ye sochna ki 'heavier object zyada tezi se girta hai' — free fall me air resistance ignore karein to sab objects same acceleration (g) se girte hain, chahe mass kuch bhi ho.
- Action-reaction pair ko ek hi object par lagta maan lena aur unhe 'cancel' kar dena — ye dono forces hamesha do alag objects par lagti hain, isliye kabhi cancel nahi hoti.
- F = ma me units mismatch karna — mass ko gram me hi rakh dena bina kg me convert kiye, ya velocity ko km/h me hi rakh dena F=ma/momentum calculation me daalte waqt.
- Momentum ko sirf magnitude tak treat karna aur direction (sign) bhool jana — collision ya recoil problems me opposite-direction velocities ko negative sign dena zaroori hai, warna conservation equation galat aa jaata hai.
- 'Zyada force = zyada motion badegi hamesha' jaisa galat generalisation — agar object par forces balanced hain (net force zero) to chahe individual forces bade ho, motion state nahi badlegi.
Board-Style Important Questions
- SI unit of force kya hai aur ise kis quantity ke terms me define kiya jaata hai?
- Inertia of rest aur inertia of motion me antar batao, ek-ek example ke saath.
- Newton ke third law of motion ko state karo aur samjhao ki action-reaction pair cancel kyu nahi hota.
- Newton ke second law of motion se F = ma kaise derive hota hai, step-by-step dikhao.
- Ek 4 kg ki gun se 0.02 kg ki bullet 150 m/s se fire hoti hai. Gun ki recoil velocity calculate karo, momentum conservation use karke.
- Momentum conservation ka law state karo aur Newton ke second aur third law se ise derive karo. Ek real-life application (jaise rocket propulsion ya gun recoil) diagram/steps ke saath explain karo.
Aksar Poochhe Jaane Wale Sawaal
Class 9 Science Chapter 6 'How Forces Affect Motion' me total kitne topics/sections hain?
Chapter broadly char parts me chalta hai: (1) Force, balanced/unbalanced forces aur friction, (2) Newton's First Law aur inertia, (3) Newton's Second Law, F=ma aur momentum, (4) Newton's Third Law aur conservation of momentum. First half conceptual hai, second half me numericals zyada aate hain — isliye practice zaroori hai.
Kya ye chapter purane 'Force and Laws of Motion' chapter jaisa hi hai?
Haan, core physics content (Newton ke teen laws, inertia, momentum, conservation) largely wahi hai — bas naye 'Exploration' textbook (session 2026-27) me chapter ka naam, numbering aur kuch presentation NCF-SE 2023 ke hisaab se update hui hai.
Is chapter ke saare formulas ek jagah kahan milenge — class 9 science motion chapter formulas ke saath compare karke?
Upar diye gaye 'Formulas' table me F=ma, p=mv, momentum conservation, aur unit-conversion sab ek jagah hain. Isse Chapter 4 'Motion' ke formulas (speed, velocity, acceleration equations) ke saath compare karna easy ho jaata hai, kyunki Chapter 6 unhi acceleration concepts ko force ke saath extend karta hai.
Class 9 science ka full syllabus aur baaki chapters ke notes kahan se milenge?
Agar aapko poore class 9 science all chapters notes pdf ya class 9 science new syllabus 2026-27 chapter list chahiye — to har chapter (Motion, How Forces Affect Motion, Work-Energy-Simple Machines, Sound, Atomic Foundations of Matter, Journey Inside the Atom, Cell, Tissues, Reproduction, Diversity and Classification, wagairah) ke solutions is series me alag-alag cover hote hain.
NCERT Exploration Class 9 Science ka official book PDF kahan se download karein?
Class 9 science exploration book pdf download ke liye official source ncert.nic.in hi use karo — wahan se free me textbook PDFs chapter-wise download ho sakti hain. Third-party sites se PDF lene se pehle verify kar lo ki wo latest 2026-27 'Exploration' edition hai, purani 'Science' edition nahi.
Chapter 6 ke baad agla chapter kya hai aur uski taiyari kaise start karein?
Agla chapter 'Work, Energy and Simple Machines' hai, jisme pulleys, levers, inclined planes ke saath work-energy numericals bhi add hue hain. Chapter 6 ke F=ma aur momentum concepts strong hone chahiye pehle, kyunki work energy and simple machines class 9 numericals me force aur motion ka use baar-baar aata hai.
Class 9 Science — Saare Chapters
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