4.6.9 · D1 · Maths › Ordinary Differential Equations › Second-order linear ODEs — superposition principle, general
Ek second-order linear ODE ek machine hai jo ek function y ( x ) leta hai aur, uski slope aur uski bending use karke, check karta hai ki woh ek rule follow karta hai ya nahi. Poori theory ek sawaal ka jawaab dene ke liye exist karti hai: kyunki solutions ki scaled copies ek saath add hokar naye solutions banati hain, mujhe har solution describe karne ke liye kitne kam "building-block" solutions chahiye?
Is page pe kuch bhi assume nahi kiya gaya . Agar parent note the parent topic ne koi symbol bina bataye use kiya, toh hum use yahan build karte hain, ek aisi order mein jahan har idea sirf usse pehle waale ideas pe lean karta hai.
Hum inse milenge, ek ek karke:
Ek function y ( x ) — ek curve.
Derivative y ′ — uski steepness.
Second derivative y ′′ — steepness khud kaise badlti hai (bending).
Prime aur Leibniz notation — ek hi cheez ki do spellings.
Coefficients p ( x ) , q ( x ) aur forcing g ( x ) .
Linear aur homogeneous ka pictures mein asli matlab.
Constants c 1 , c 2 aur initial conditions .
Determinant ⋅ — do-line ki grid.
Integral sign ∫ aur exponential exp (Abel ke liye zaroori).
Chaliye har ek ko earn karte hain.
y ( x ) likhne ka matlab hai: ek input number x chuno, aur rule tumhe ek output number y wapas deta hai. Input ko left-to-right plot karo (horizontal x -axis) aur output ko up-down (vertical y -axis), aur tumhe ek curve milega.
x ko time samjho aur y ko spring pe mass ki height . Jaise time flow karta hai, mass ek wiggling curve trace karta hai. Woh curve hi function hai.
Intuition Topic ko yeh kyun chahiye
Poora chapter is baare mein hai ki kaun si curves steepness aur bending ke baare mein ek certain rule follow karti hain. Toh sabse pehla object ek curve y ( x ) hai jise hum dekh saken.
Curve ko dekho. Kisi bhi ek point pe, itna zoom in karo jab tak curve ek seedha ramp jaisa na dikhe. Kitni steeply woh climb karti hai? Woh number derivative hai.
y ′
y ′ ( x ) (padho "y -prime") woh slope hai jo curve ki point x pe hai : kitne units y upar jaata hai jab x ek unit right move karta hai. Uphill ⇒ y ′ > 0 . Downhill ⇒ y ′ < 0 . Flat top ya bottom ⇒ y ′ = 0 .
Intuition Topic ko yeh kyun chahiye
Ek spring wapas kheenchti hai is basis par ki mass kitni tez/kahan hai. "Kitni tez" exactly ek derivative hai. Har physical law jo rate mention karta hai woh secretly ek y ′ hai.
Ab wahi trick phir karo, lekin slope ke saath. Jaise tum curve pe chalte ho, slope khud badlti rahti hai. Slope kitni tez badal rahi hai? Yahi second derivative hai.
Definition Second derivative
y ′′
y ′′ (padho "y -double-prime") derivative ka derivative hai — steepness kitni tez badlti hai. Yeh curvature / bending measure karta hai:
y ′′ > 0 : curve upar ki taraf bend karti hai (smile, woh cup jo paani hold karta hai).
y ′′ < 0 : curve neeche ki taraf bend karti hai (frown, girata hua cup).
y ′′ = 0 : momentarily seedha.
Intuition "Second-order" kyun kehte hain
Ek ODE ka order sabse bada derivative hota hai joh usmein hota hai. Is chapter ki equations y ′′ tak pahunchti hain — yahi "second-order" naam hai. Physically y ′′ acceleration hai (velocity y ′ ke change ki rate), aur Newton ka F = ma hi wajah hai ki springs, circuits, aur pendulums sab is chapter mein aate hain.
Tum ek hi object ko do tarike se likhte dekhoge. Koi "zyada correct" nahi hai — woh ek hi steepness hain.
Definition Prime aur Leibniz notation
Prime
Leibniz
Padho aise
y ′
d x d y
y ki change ki rate per unit x
y ′′
d x 2 d 2 y
slope ke change ki rate
d ek chhota "difference" hai: d x d y limit se "tiny run tiny rise " hai jaise §2 mein. Prime notation sirf ise abbreviate karta hai.
d x 2 d 2 y matlab kuch square ho raha hai."
Kyun sahi lagta hai: usmein do chhote 2 hain. Fix: upar ka 2 count karta hai kitni baar differentiate kiya (d do baar apply hua), aur neeche ka 2 ( d x ) squared se aata hai. Kuch bhi square nahi ho raha. Yeh y ′′ hai, simple.
Parent ka standard form hai
y ′′ + p ( x ) y ′ + q ( x ) y = g ( x ) .
Woh letters kya hain?
Definition Har function ka role
p ( x ) slope y ′ ko multiply karta hai — socho isko friction / damping ke taur par jo is baat par depend karta hai ki tum kahan ho.
q ( x ) position y ko multiply karta hai — socho isko spring stiffness ke taur par jo is baat par depend karta hai ki tum kahan ho.
g ( x ) akele right side pe baithta hai — ek bahari push (forcing) jo system ke bahar se apply hoti hai.
Teeno sirf x ki known functions hain . Yeh kabhi y par depend nahi karte. Yahi exactly woh cheez hai jo equation ko linear banati hai (agla section).
Intuition Ise "monic" (leading
1 ke saath) kyun likhte hain
Natural physics equation hai a y ′′ + b y ′ + c y = h . Har term ko a se divide karne par leading coefficient 1 milta hai aur b / a , c / a , h / a p , q , g ban jaate hain. Hum yeh ek baar, shuru mein karte hain, taaki baad ke har formula (Wronskian, Abel) clean y ′′ ke saath padhe aur koi stray a na ho.
Yeh do adjectives decide karte hain ki poori theory apply bhi hogi ya nahi, isliye inhe dhyan se define karo.
y kaise enter ho sakta hai)
Equation linear hai agar y , y ′ , y ′′ har ek sirf first power mein aaye, kabhi ek doosre se multiply na ho , aur kabhi sin ( ⋅ ) ya ( ⋅ ) 2 jaisi functions ke andar wrap na ho .
✅ q ( x ) y — ek known function times y (theek hai).
❌ y y ′ — do unknowns multiply hue.
❌ sin ( y ) — unknown ek nonlinear function ke andar.
Intuition Linearity ka visual test
Linear matlab: output ko input se sirf scaling aur adding se banao. Agar input ko double karne par woh har term ko exactly double karta hai jise woh touch karta hai, aur inputs bina cross-terms ke add hote hain, toh yeh linear hai. y y ′ jaisi products yeh todti hain — y ko double karne par y y ′ quadruple ho jaata hai.
Definition Homogeneous (kya right side zero hai?)
Homogeneous matlab forcing band hai: g ( x ) ≡ 0 . Symbol ≡ matlab "har x ke liye equal", na ki sirf ek point par.
Homogeneous: y ′′ + p y ′ + q y = 0 — system akela chhod diya gaya.
Non-homogeneous: g ( x ) = 0 — koi cheez bahar se ise push kar rahi hai.
Mnemonic Homogeneous = "home alone"
Bahar se koi push nahi (g = 0 ) matlab system home alone hai, apne aap oscillate kar raha hai.
Definition Arbitrary constants
c 1 , c 2
Yeh numbers hain jinhe tum freely choose kar sakte ho — dials. Har second-order ODE exactly do dials khule chhodti hai, kyunki second derivative ko "undo" karna do baar integrate karne jaisa hai, aur har integration ek unknown constant add karta hai.
Definition Initial conditions
Do facts, usually ek starting point x 0 par:
y ( x 0 ) = a ( jahan se shuru ) , y ′ ( x 0 ) = b ( kitni tez shuru ) .
Yeh do facts do free dials ko do equations mein badal dete hain, c 1 , c 2 ko ek curve par pin karte hain.
Intuition Do dials, do facts
Position mass ko batati hai kahan se shuru karna hai; velocity batati hai kis direction mein aur kitni tez . Saath mein woh pure family mein se ek single trajectory select karte hain — exactly woh do numbers jo do constants ko chahiye.
Parent note mein Wronskian ek determinant hai. Yahan woh vertical-bar grid ka matlab hai, zero se.
2 × 2 determinant
Chaar numbers ke ek square ke liye,
a c b d = a d − b c .
Main diagonal multiply karo (a d ), doosri diagonal subtract karo (b c ).
Intuition Picture: ek area / ek collapse-detector
Columns ( c a ) aur ( d b ) ko origin se do arrows ki tarah padho. Determinant woh signed area hai jo parallelogram span karta hai jo woh banate hain.
Area = 0 : arrows genuinely alag directions mein point karte hain — independent .
Area = 0 : arrows ek line par hain — ek doosre ki scaled copy hai — dependent .
Yahi exactly wajah hai ki Wronskian independence test karta hai: woh poochta hai ki kya do solution-arrows ek hi line par collapse ho gayi hain.
Intuition Topic ko yeh kyun chahiye
"y 1 aur y 2 genuinely alag solutions hain, ya ek sirf doosre ki stretched copy hai?" yeh ek collapse ka sawaal hai — aur determinant collapse-detector hai. ( y 1 , y 1 ′ ) aur ( y 2 , y 2 ′ ) ko isme daalne par Wronskian milta hai W = y 1 y 2 ′ − y 2 y 1 ′ .
Abel ka theorem likhta hai W ( x ) = W ( x 0 ) exp ( − ∫ x 0 x p ( t ) d t ) . Do aakhri symbols.
∫ x 0 x p ( t ) d t
Elongated-S sign ∫ matlab accumulate / jodna . ∫ x 0 x p ( t ) d t woh signed area hai jo curve p ke neeche start point x 0 se x tak hai — p ka running total.
exp ( u ) = e u
e ≈ 2.718 ek fixed number hai. Function exp ( u ) = e u woh curve hai jo apni khud ki slope ke barabar hai (d u d e u = e u ). Yahan uski ek crucial property:
e u > 0 har real u ke liye (yeh KABHI zero nahi hota).
Intuition Yeh do Abel ka theorem kyun kaam karte hain
W = W ( x 0 ) × ( kuch jo kabhi zero nahi ) . Toh W tabhi zero ho sakta hai jab W ( x 0 ) zero tha. Yahi parent ka "all-or-nothing " hai: Wronskian ko ek convenient point par check karo aur tum use har jagah jaante ho. Kabhi-zero-na-hone-wala exponential yahi poori wajah hai.
function y of x - a curve
derivative y prime - slope
second derivative y double prime - bending
coefficients p q and forcing g
two constants and initial conditions
Wronskian - independence test
Superposition and general theory
Jab yeh boxes solid feel hone lagen, parent topic sirf assembly hai. Aage ki machinery ke liye dekho Characteristic equation — constant coefficient ODEs , Method of undetermined coefficients , Variation of parameters , Abel's theorem , aur deeper backing Existence and uniqueness theorems for ODEs mein. "Solutions form a vector space" ka claim Linear algebra — vector spaces and bases par tikaa hai, aur single-derivative warm-up hai First-order linear ODEs .
Test karo khud ko — reveal karne se pehle jawab out loud bolo.
y ( x ) kya draw karta hai?Ek curve: input x horizontal, output y vertical.
y ′ kya measure karta hai, ek picture ki tarah?Curve ki slope (steepness) ek point par.
y ′′ kya measure karta hai?Bending/curvature — slope kitni tez badlti hai; > 0 smile, < 0 frown.
Ise second-order ODE kyun kehte hain? Sabse bada derivative jo present hai woh doosra hai, y ′′ .
Kya d x 2 d 2 y aur y ′′ ek hi hain? Haan — second derivative ki do spellings; kuch bhi square nahi hota.
y ′′ + p y ′ + q y = g mein g kya hai?Bahari forcing (push); agar g ≡ 0 toh equation homogeneous hai.
Ek equation ko linear kya banata hai? y , y ′ , y ′′ sirf first power mein aate hain, unka aapas mein koi product nahi, koi sin y etc. nahi.
≡ 0 ka matlab kya hai?Har x ke liye zero ke barabar, sirf ek point par nahi.
Exactly do constants c 1 , c 2 kyun? Ek second derivative ko "do integrations" chahiye, har ek ek constant add karta hai.
Konse do facts constants fix karte hain? Initial conditions y ( x 0 ) = a aur y ′ ( x 0 ) = b .
a c b d kya equal hai?a d − b c .
Geometrically zero determinant ka matlab kya hai? Do column-arrows ek line par hain — dependent (parallelogram collapse ho jaata hai).
Abel ke theorem ke liye e u special kyun hai? Yeh kabhi zero nahi hota, isliye W ya toh poora zero hai ya kabhi zero nahi.
∫ x 0 x p d t kya represent karta hai?p ke neeche running signed area x 0 se x tak.