1.5.15 · D1 · Physics › Rotational Mechanics › Acceleration of rolling objects on inclines — comparison
Jab koi round object slope pe roll karta hai, gravity ki energy split ho jaati hai — aage badhne mein aur spin karne mein. Kitni energy spin mein jaayegi, yeh sirf object ki shape par depend karta hai, uske mass ya size par nahi — toh shapes compare karne se pehle humein har woh symbol samajhna hoga jo "aage ki motion", "spinning", aur "shape" ko measure karta hai.
Yeh page assume karta hai ki tumne kuch nahi dekha. Parent formula a = 1 + β g sin θ mein har letter yahan build kiya gaya hai, ek ek karke, har ek apne pehle waale par tikaa hua. Agar tum koi symbol pehle se jaante ho, toh skim karo — lekin neeche diye gaye pictures hi asli baat hain.
Koi bhi symbol aane se pehle, us scene ko dekho jis par poora topic tika hai.
Definition The incline (slope)
Ek flat ramp jo zameen se upar ki taraf tili hui hai. Ramp aur flat zameen ke beech ka angle between the ramp and the flat ground θ (Greek letter "theta") kehlata hai. Ek round object iske upar rakha hai aur use rest se chhod diya jaata hai.
Is page par baaki sab kuch isi picture ka measurement hai: kitna steep (θ ), kitna bhaari (m ), kitna bada (R ), kitni tez aage badhta hai (v , a ), kitni tez spin karta hai (ω , α ), aur mass kitna "spread out" hai (I , β ).
θ (theta) = tilt angle
Simple words: ramp kitna steep hai. Flat floor θ = 0 ∘ hai; vertical wall θ = 9 0 ∘ hai.
Picture: upar diye figure mein ramp aur zameen ke beech wala wedge-shaped corner.
Topic ko kyun chahiye: zyada steep slope object ko zyada tez neeche kheenchti hai, toh θ control karta hai ki gravity kitni help karti hai. Bada θ → zyada tez.
θ ki range
Ek physical incline sirf flat se vertical tak tilti hai, toh θ sirf ek interval 0 ∘ ≤ θ ≤ 9 0 ∘ mein rehta hai. Koi aur case nahi hain — koi "second" ya "third" region nahi sochna. Is poore interval mein sin θ smoothly aur steadily 0 se 1 tak badhta hai.
Hum θ ko directly use nahi karenge — hum sin θ use karte hain. Uska apna build chahiye.
Gravity ko slice karne se pehle, humein un do cheezein name karni hain jo gravity ka pull banati hain: kitna stuff hai, aur pull-per-kilogram kitna strong hai.
m = mass
Simple words: object mein kitna "stuff" hai, kilograms mein. Yeh ek saath do cheezein measure karta hai: gravity ise kitna tez kheenchti hai, aur yeh speed change karne mein kitna reluctant hai (iska inertia). Is double role ko yaad rakho — isi wajah se m eventually cancel ho jaata hai (tum ise literally §10 mein ek equation ke dono taraf dekhoge).
Picture: figure s01 mein ball ka size/heft.
g = gravitational field strength
Simple words: Earth ka pull per kilogram kitna strong hai. Earth par g ≈ 9.8 m/s 2 . Mass ko g se multiply karo aur milta hai weight , yaani neeche ki taraf force m g .
Picture: figure s02 mein m g length ka seedha-neeche ka arrow.
m g kyun, sirf g kyun nahi?
Ek force hi cheezein accelerate karti hai. Kisi object par gravity ki force uska mass times g hai, yaani uska weight m g . Yahi poora seedha-neeche ka pull hai jise hum agli section mein slice karenge.
Gravity ka weight m g seedha neeche kheenchta hai. Lekin object sirf ramp ke along move kar sakta hai. Toh humein us pull ka woh slice chahiye jo slope ke neeche ki taraf point karta hai. Wahi sin θ measure karta hai.
sin θ = opposite over hypotenuse
Ek right triangle draw karo jiske slope-side ki length 1 hai (hypotenuse, orange). Opposite side — us slope-length par vertical drop — ki length sin θ hai.
Simple words: "seedha neeche" ka woh fraction jo "slope ke along" lie karta hai.
Yeh tool kyun, koi aur kyun nahi? Hum specifically ramp ke neeche gravity ka component chahte hain. Seedhe-neeche ke pull ko "ramp ke along" aur "ramp ke andar" mein split karna exactly wahi hai jo sine aur cosine karte hain — sine along-ramp part leta hai kyunki woh part badhta hai jab ramp tilt hoti hai.
R = radius
Simple words: object ke centre se rim tak ki distance (metres mein). Rolling object ke liye yeh friction ka lever arm bhi hai aur spinning aur moving ke beech ki link bhi.
Picture: figure s03 mein centre se edge tak ki line (§6 mein dikhaya gaya).
Ab, "forward motion" ke liye do symbols. Kyunki hum §5 mein ek positive direction fix karte hain, dono ko signed numbers maana jaata hai, sirf magnitudes nahi:
Common mistake Speed aur acceleration mein confusion
Yeh sahi kyun lagta hai: roz ki boli mein "faster" dono matlab rakhta hai. Fix: v = tum abhi kitni tez ja rahe ho; a = tum kitni tez tez ho rahe ho. 100 km/h par cruise karne waali car ka v bada hai lekin a zero hai. Is topic mein hum a compare karte hain.
Forces aur motions ki direction hoti hai. Unhe bina guess kiye combine karne ke liye, pehle agree karna padega ki kaunsa way positive hai — straight-line aur spinning quantities dono ke liye.
Definition Slope ke along positive = slope ke neeche
Sabhi straight-line quantities (v , a , aur forces) ke liye down the slope ko + direction choose karo, kyunki object actually usi taraf accelerate karta hai. Tab:
Gravity ka pull m g sin θ slope ke neeche point karta hai → + sign ke saath enter karta hai.
Slope ke upar point karne wali force − sign ke saath enter karti hai.
Picture: figure s03 mein orange forward arrow + hai; ulti taraf point karne wala arrow subtract karta hai.
Definition Positive spin = woh spin jo ise aage roll karaaye
Rotational quantities ω aur α (§6 mein built) aur torque τ (§7 mein built) ke liye, woh spin sense positive maano jo object ko slope ke neeche le jaata hai — yaani object ka top aage move kare, bottom peeche. Us sense mein twist + torque hai; ulti sense − hai.
Ab kyun: jab hum baad mein a = α R aur τ = I α likhte hain, dono conventions fix hone se har sign pehle se unambiguous hoga — koi case-checking nahi chahiye.
Ek rolling object sirf slide nahi karta — woh turn karta hai. Us turning ke liye apne do symbols chahiye, exactly v aur a ke twins.
Intuition Translation vs rotation — same story, do languages
Straight-line world v , a use karta hai. Turning world ω , α use karta hai. Yeh mirror images hain, aur humne dono ke liye matching + directions fix ki hain. Rolling condition (§9) woh dictionary hai jo dono ke beech translate karti hai. Dekho Rolling Without Slipping .
Agar slope bilkul slippery hoti, toh object sirf slide karta, kabhi spin nahi karta. Friction hi rim ko grip karke ise turn karne par majboor karti hai.
f = friction force
Simple words: object aur slope ke beech sideways grip, slope ke upar point karta hai, toh §5 ke hisaab se yeh equations mein − sign ke saath enter karta hai. Yeh chhota hai, lekin akela force hai jo object ko spin mein twist kar sakta hai.
Picture: figure s03 mein contact point par chhota magenta arrow.
Intuition Friction slope ke
upar kyun point karta hai
Friction ke bina object sirf slide karta, kabhi spin nahi karta. Friction rim par woh grip hai jo sliding resist karta hai — toh yeh backward (slope ke upar) point karta hai. Wahi backward grip, rim par act karte hue, exactly wahi hai jo object ko forward spin mein twist karta hai. Dekho Friction in Rolling .
τ (tau) = torque = f × R
Simple words: torque, Greek letter τ ("tau") se likha jaata hai, yeh hai ki koi force kitna "twist" deliver karta hai. Centre se R distance par apply ki gayi force τ = f ⋅ R strength se twist karti hai. Woh distance R lever arm hai. Yahan friction ka twist §5 ke positive (rolls-forward) sense mein hai, toh τ > 0 .
Yeh tool kyun? Kuch spin karaane ke liye, sirf force nahi chahiye — force off-centre apply honi chahiye. Gravity centre se act karti hai (koi twist nahi), lekin friction rim par act karta hai (centre se R distance), toh sirf friction torque contribute karta hai. Dekho Torque and Angular Acceleration .
I = moment of inertia
Simple words: kisi object ko spin mein laana kitna mushkil hai (jaisa ki m hai — ise move mein laana kitna mushkil hai, uske opposite). Mass jo centre se door hai, use same mass centre ke paas se zyada mushkil se spin karaaya jaata hai.
Picture: figure s04 dekho — same mass, lekin ring (mass at rim) disc (mass near axis) se bahut zyada mushkil se spin hoti hai.
Topic ko kyun chahiye: I decide karta hai ki energy ka kitna hissa spin mein jaata hai. Bada I → spin ke liye zyada energy chori → aage ki motion ke liye kam bacha → chhota a .
Intuition "Door" kyun itna matter karta hai
Rim par mass ko har turn mein bada circle travel karna padta hai, toh ise whirl karne ke liye bahut push chahiye. Axis ke paas wala mass har turn mein barely move karta hai, toh use spin karna aasaan hai. Isi wajah se ring (sab mass rim par) humare saare shapes mein spin karne ki sabse mushkil hai.
β ke numbers ka poora derivation dekhne ke liye Moment of Inertia dekho.
I mass aur size par depend karta hai, jo shapes compare karne ke liye clumsy hai. Toh hum unhe abhi, use karne se pehle strip out karte hain:
β = "kitna spread out" number
Simple words: ek akela number jo bataata hai ki mass average par axis se kitni door baith hai (as a fraction of R ). Chhota β = mass centre ke paas bunched = spin karna aasaan. Bada β = mass rim par = spin karna mushkil.
Picture: figure s04 mein disc ka β = 1/2 hai, ring ka β = 1 .
Shape
mass kahan baith hai
β
Solid sphere
centre ki taraf densely packed
2/5 = 0.40
Solid cylinder / disc
disc mein evenly
1/2 = 0.50
Hollow sphere
ek thin shell par
2/3 ≈ 0.67
Ring / hoop
sab rim par
1
Ab hamare paas har ingredient hai, including β . Dekho kaise do Newton laws combine hote hain, aur dekho m dono sides par kaise appear karta hai.
Definition Glue — rolling without slipping
Simple words: object exactly utna hi turn karta hai jitna travel karta hai — koi skidding nahi. Ek poora turn ise ek circumference aage move karta hai.
Picture: figure s03 mein contact point momentarily slope par frozen hai; object us par pivot karta hai.
Equations: v = ω R aur (dono ka rate of change) a = α R .
Topic ko kyun chahiye: yeh akela bridge hai jo Law 1 aur Law 2 combine karne deta hai. Dekho Rolling Without Slipping .
m cancel hota hai
Law 2 se I = β m R 2 (§9 mein define kiya) aur α = a / R ke saath:
f = R I α = R β m R 2 ( a / R ) = β ma
Use karo Law 1 mein:
m g sin θ − β ma = ma
Har term mein m hai. m se divide karo:
g sin θ = a ( 1 + β ) ⇒ a = 1 + β g s i n θ
Yeh lo — m literally dono sides par appear hua aur cancel ho gaya. R bhi gayab ho gaya. Sirf shape (β ) aur slope (θ ) bacha.
Common mistake "Friction yahan energy waste karta hai."
Yeh sahi kyun lagta hai: friction usually heat banata hai. Fix: rolling without slipping mein contact point instantaneously rest mein hota hai, toh static friction zero work karta hai — woh sirf energy ko spin mein redirect karta hai. Dekho Friction in Rolling .
Intuition Yeh aakhri symbol kyun tha jo tumhe chahiye tha
Poori comparison ek number β par aa jaati hai. Ise a = 1 + β g sin θ mein plug karo aur tumhara jawaab ready hai — chhota β jeet ta hai. Mass aur radius isliye gayab ho gaye kyunki woh gravity ke pull aur inertia dono ke andar the, exactly cancel karte hue.
Gravity along slope mg sin theta
Sign convention down-slope positive
Torque tau equals f times R
Rotation law tau equals I alpha
Shape factor beta equals I over m R squared
Rolling condition a equals alpha R
Acceleration a equals g sin theta over one plus beta
Upar se neeche padho: raw measurements (θ , m , g , R , f , I ) plus ek sign convention, do laws feed karte hain, shape factor β inertia ko package karta hai, rolling condition sab kuch glue karta hai, aur parent formula nikal aata hai.
Aage parent note par jaane se pehle right side cover karo aur khud ko test karo.
θ kya measure karta hai, aur iski poori range kya hai?Slope ka tilt; ek physical incline sirf 0 ∘ se 9 0 ∘ tak span karta hai, jiske across sin θ 0 se 1 tak rise karta hai.
Slope ke neeche pull ke liye hum sin θ kyun use karte hain (cos θ nahi)? Sine seedhe-neeche gravity ka along-slope component deta hai; yeh badhta hai jab ramp tilt hoti hai.
Straight-line quantities ke liye humne positive kaunsi direction choose ki, aur isse force signs kaise fix hote hain? Down-slope + hai; gravity m g sin θ + enter karta hai, friction f (up-slope) − enter karta hai.
Humne positive spin sense kaunsa choose kiya? Woh sense jo object ko slope ke neeche roll karaata hai (top aage move kare); us taraf ka twist + τ hai.
Incline ke along translation law state karo. m g sin θ − f = ma .
m ke do kaam kya hain, aur woh cancel kahan hota hai?Gravity ka pull (m g ) AUR inertia; yeh m g sin θ − β ma = ma ke har term mein appear karta hai, toh m se divide karne par remove ho jaata hai.
v aur a mein farq?v = centre ki current velocity (down-slope positive); a = woh velocity kitni tez badh rahi hai.
ω aur α kis ke spinning twins hain?ω , v ka twin hai (spin rate); α , a ka twin hai (spin-up rate).
Torque ka symbol kya hai, aur yeh f aur R se kaise banta hai? τ (tau); τ = f R , force times lever arm.
Torque kaunsi akeli force provide karti hai, aur gravity kyun nahi? Friction f — yeh rim par act karta hai (lever arm R ); gravity centre se act karti hai, toh koi twist nahi.
I simple words mein kya hai?Spin kiye jaane ki resistance; axis se door mass ise bada banata hai.
β define karo aur yeh useful kyun hai.β = I / m R 2 , ek pure number jo sirf shape capture karta hai, m aur R cancel karke.
Rolling-without-slipping equations state karo. v = ω R aur a = α R .
Kya static friction rolling without slipping mein work karta hai? Nahi — contact point momentarily rest mein hota hai, toh zero work hota hai.