Solar radiation pressure — Hinglish notes

3.2.35 · Physics › Orbital Mechanics & Astrodynamics

1. Force kahaan se aati hai? (Scratch se derive karo)

HOW hum pressure build karte hain, step by step.

Step 1 — Energy flux. Sun power emit karta hai $L_{⊙} \approx 3.828 \times 1 0^{26} W$ . Distance $r$ par, $4 π r^{2}$ area wale ek sphere par spread hoke, intensity (power per area) hai $S (r) = \frac{L _{⊙}}{4 π r ^{2}} .$ Yeh step kyun? Energy conserved hoti hai, isliye wahi power Sun par centered har sphere se pass hoti hai — bada sphere, patla flux.

Earth par ( $r = 1 AU$ ), $S \approx 1361 W/m^{2}$ — yeh solar constant hai.

Step 2 — Energy flux se momentum flux tak. Light ka har joule $1/ c$ momentum carry karta hai. Toh momentum jo per area per second arrive karta hai woh hai $\frac{S}{c} .$ Yeh step kyun? Momentum flux = force per area = pressure. Humne sirf "energy per second per area" ko "momentum per second per area" mein convert kiya $p = E / c$ use karke.

Step 3 — Base pressure. $P_{rad} = \frac{S}{c}$ Yeh step kyun? Yeh perfectly absorbing surface par pressure hai jo Sun ki taraf face kar rahi ho. 1 AU par: $P_{rad} = \frac{1361}{3 \times 1 0 ^{8}} \approx 4.5 \times 1 0^{- 6} Pa .$

Tiny — lekin constant aur har jagah.

2. Absorbing vs reflecting (woh factor jo logon ko trip karta hai)

3. Spacecraft ka acceleration

Acceleration kyun, force kyun nahi? Orbits acceleration ki parwah karte hain ( $a = F / m$ ). Ek light spacecraft zyada push hota hai.

$m$ mass, $A$ cross-section, absorbing wale spacecraft ke liye (worst simple case): $a_{SRP} = \frac{P _{rad} ( 1 + η ) A}{m} = \frac{S ( r )}{c} \frac{( 1 + η ) A}{m} .$

$A / m$ ka ratio area-to-mass ratio hai — SRP sensitivity ke liye single most important design number.

4. Worked examples

5. Common mistakes (Steel-man then fix)

6. Active recall

Recall Khud test karo (answers chhupao)

Mirror ko black surface se double force kyun milti hai? → recoil momentum change ko double karta hai.
Solar constant kya hai? → $\approx 1361 W/m^{2}$ .
Kaun sa single spacecraft number SRP sensitivity govern karta hai? → area-to-mass ratio $A / m$ .
SRP/gravity ratio distance ke saath kyun nahi badlta? → dono $\propto 1/ r^{2}$ .

Recall Feynman: ek 12-saal ke bacche ko explain karo

Sunlight photons naam ki tiny bullets se bani hai. Har ek super light hai, lekin phir bhi jab yeh kisi cheez se takrati hai toh ek choti si kick deti hai — jaise lakhon rét ke kaṇ ek sail se takra rahe hon. Space mein koi air nahi hai jo peeche dhakele, toh ek gentle, kabhi na rukne wali push slowly ek spacecraft ko speed up karti hai. Ek shiny surface zyada push hoti hai kyunki photons bounce off hote hain (trampoline ki tarah) bajaye sirf spatting ke.

7. Connections

Orbital perturbations — SRP ek non-gravitational perturbing force hai.
Solar sails — SRP use karke propulsion.
Atmospheric drag — LEO mein competing non-conservative force.
Photon momentum and relativity — $E = p c$ origin.
Inverse-square law — kyun intensity $1/ r^{2}$ ki tarah fall karti hai.
Yarkovsky effect — asteroids par SRP ka thermal re-emission cousin.

Energy ke terms mein Photon momentum

p = E / c

1 AU par Solar constant

\approx 1361 W/m^{2}

Absorbing surface par base radiation pressure

P = S / c \approx 4.5 \times 1 0^{- 6} Pa

at 1 AU

Absorbing ke comparison mein perfectly reflecting surface ka factor

reflecting double hai,

(1 + η)

with

η = 1

Intensity

1/ r^{2}

ki tarah kyun fall karti hai

fixed power sphere area

4 π r^{2}

par spread hoti hai

SRP acceleration ke liye key spacecraft parameter

area-to-mass ratio

A / m

SRP/gravity ratio distance-independent kyun hai

dono

1/ r^{2}

ki tarah scale karte hain

Flat plate par force (simple model)

F = P_{r a d} A (1 + η) cos θ