Newton's third law — action-reaction, common misconceptions: Question bank

1.2.3 · D5 · Physics › Newton's Laws & Dynamics › Newton's third law — action-reaction, common misconceptions

Shuru karne se pehle, parent note ke chaar S's yaad rakho — Swap the labels, Same the size, Split the bodies, Simultaneous in time. Yahan ka almost har trap in chaar words mein se kisi ek se toot jaata hai.

Notation & definitions jo tumhe pehle chahiye

Is page par use hone wala har symbol yahan define kiya gaya hai, kisi bhi question mein aane se pehle.

Recall

$d P / d t = 0$ se third law kyun milta hai (equivalence, derived) Assumptions: ek isolated two-body system — sirf $A$ aur $B$ , aur only forces jo woh feel karte hain woh ek doosre se hain (koi external nahi). Toh experiment kehta hai total momentum $P$ kabhi nahi badalti. Constant ka matlab zero rate of change hai: $\frac{d P}{d t} = \frac{d p _{A}}{d t} + \frac{d p _{B}}{d t} = 0.$ Second law se har rate us body par net force hai, aur $A$ par sirf $B$ se force hai: $\frac{d p _{A}}{d t} = F_{B \to A}$ aur $\frac{d p _{B}}{d t} = F_{A \to B}$ . Substitute karo: $F_{B \to A} + F_{A \to B} = 0$ , yani $F_{A \to B} = - F_{B \to A}$ . Dekho Conservation of Momentum.

True ya false — justify karo

Nature mein har force ka ek reaction partner hota hai.

True. Ek force do bodies ke beech ek interaction ka aadha hissa hai, toh hamesha ek doosra body hoga jo wapas push kare; ek akeli force exist nahi kar sakti.

Table par rakhi book ka weight normal force se cancel hota hai, isliye weight aur normal force ek 3rd-law pair hain.

False. Dono same body par act karti hain (book par) aur alag type ki forces hain (gravity vs contact) — woh second law equilibrium se cancel hoti hain, third se nahi.

Action aur reaction ka magnitude hamesha equal hota hai even jab do objects ki masses bahut alag hon.

True. Magnitude equality third law se guaranteed hai aur mass par depend nahi karti; sirf resulting accelerations alag hote hain

a = F / m

ke through.

Agar horse cart ko pull kare aur cart equally wapas pull kare, toh system kabhi accelerate nahi kar sakta.

False. Woh dono forces alag bodies par act karti hain, isliye woh kabhi cancel nahi hoti; cart+horse system accelerate hota hai kyunki ground se external forward friction hoti hai.

Ek single free-body diagram mein ek 3rd-law pair saath dikh sakta hai.

False. Dono forces do alag bodies par act karti hain, isliye woh do alag free-body diagrams mein jaati hain — dono ko ek diagram mein dekhna classic error hai.

Jab tum taali bajate ho, tumhara right hand zyada force feel karta hai kyunki tum use zyada swing karte ho.

False. Motion chahe jo bhi ho, dono haath har instant mein ek doosre par equal-magnitude forces lagate hain; "zyada swing karna" speed change karta hai, pair ki equality ko nahi.

Ek isolated pair ke liye momentum conservation logically third law ke equivalent hai.

True.

d P / d t = 0

F_{A \to B} = - F_{B \to A}

milta hai, aur ulta bhi — ek doosre ko imply karte hain, jaisa upar derive kiya. Dekho Conservation of Momentum.

Gravity ka reaction partner ek aur gravitational force hota hai.

True. Agar Earth book ko neeche pull kare, toh book Earth ko equal magnitude se upar pull karti hai — same type (gravity), labels swapped.

Third law sirf gravity aur contact ke liye kaam karta hai, charges ke beech electric ya magnetic forces ke liye nahi.

False. Do charges bhi equal-opposite forces se attract/repel karte hain; static charges ke liye pair collinear hai, toh momentum exactly conserve hota hai jaise gravity mein (subtle exceptions sirf fast-moving charges ke liye aate hain, jahan field khud momentum carry karta hai).

Ek rocket ko accelerate karne ke liye air ya ground ki zaroorat hoti hai jis par push kar sake.

False. Reaction partner ejected exhaust gas hai, surroundings nahi; rocket vacuum mein bhi kaam karta hai. Dekho Rocket Propulsion & Variable Mass.

Error dhundho

"Earth apple ko 1 N se neeche pull karta hai, aur apple Earth ko bahut chhoti force se upar pull karta hai kyunki Earth bahut badi hai."

Magnitudes exactly equal hain (dono 1 N); Earth ki badi mass uska acceleration negligible banati hai, force ko nahi.

"Road se friction engine ki force ka reaction hai, isliye woh car par ek 3rd-law pair banate hain."

Woh same body par act karti hain aur unrelated force types hain; "road tyre ko aage push kare" ka asli partner hai "tyre road ko peeche push kare" — road par. Dekho Normal Force and Friction.

"Tug-of-war mein jeetnewali team rope ko haarnewali team se zyada pull karti hai."

Rope dono taraf equal tension transmit karta hai; winner jeet ta hai kyunki woh ground ko zyada push karta hai, ek external force, rope pair ko out-pulling karke nahi.

"Jab tum wall par lean karte ho, pehle tum push karte ho, phir kuch der baad wall wapas push karti hai."

Koi time delay nahi — pair simultaneous hota hai; jis instant tum push karo, wall pehle se hi tumhe push kar rahi hoti hai.

"Ek swimmer aage isliye move karta hai kyunki paani ka reaction force swimmer ke weight ke equal size ka hota hai."

Weight ek vertical gravitational force hai jiska horizontal swim pair se koi lena-dena nahi; swimmer paani ko peeche push karta hai, paani swimmer ko aage push karta hai — same type, horizontal, weight se unrelated.

"Weight (Earth book ko neeche pull kare) aur normal force (table book ko upar push kare) ek 3rd-law pair hain kyunki woh equal aur opposite hain."

Galat — dono same body par act karti hain (book par) aur alag force types hain, isliye yeh pair test fail karta hai. Asli pairs hain book↔Earth (dono gravity) aur book↔table (dono normal).

Why questions

Do equal-and-opposite forces cancel kyun fail kar sakti hain?

Cancellation ke liye dono forces same object par chahiye taaki woh us object ke net force mein add ho sakein; ek 3rd-law pair do objects mein split ho jaata hai, isliye kisi ka bhi net force dono nahi dekhta. Dekho Free Body Diagrams.

Ek mosquito windscreen par kyun splatter hota hai jabki truck unharmed rehta hai, agar forces equal hain?

Same force, lekin

a = F / m

— mosquito ki tiny mass use enormous acceleration aur stress deti hai, truck ki badi mass barely react karti hai. Dekho Newton's Second Law.

Do skaters mein se jo lighter hota hai woh door kyun jaata hai zyada speed se?

Equal force equal contact time tak matlab equal impulse

F t

, toh har ek equal momentum

m Δ v

gain karta hai; isliye

m_{A} v_{A} = m_{B} v_{B}

aur chhoti mass ko badi speed leni padti hai.

Third law hi woh reason kyun hai ki tum apne bootstraps khinch kar khud ko nahi utha sakte?

Internal action-reaction pairs equal aur opposite hote hain aur same system ke andar act karte hain, isliye woh zero net external force contribute karte hain — sirf ek bahari body poore system ko accelerate kar sakta hai. Dekho Center of Mass Motion.

Hum kehte hain ek pair same line par act karta hai, sirf opposite directions mein kyun nahi?

Agar do equal-opposite forces parallel lines par offset hote, toh woh system ko spin karenge (turning effect) even bina kisi outside push ke — lekin ek isolated system khud se spinning shuru nahi kar sakta, isliye pair ek hi line share karna chahiye.

Walking ke liye friction kyun chahiye even jab tumhari legs hi force supply karti hain?

Tumhara paaon ground ko peeche push karta hai (action); ground tumhe aage push karta hai (reaction) — friction ke bina ground woh forward reaction nahi de sakta aur tum slip kar jaoge. Dekho Normal Force and Friction.

Edge cases

Agar do objects contact mein hain lekin ek doosre ke relative move nahi kar rahe, tab bhi koi action-reaction pair hai?

Haan — pair interaction par depend karta hai, motion par nahi; static contact bhi equal-opposite contact forces exchange karta hai.

Ek girti hui ball se Earth pull hone par reaction partner kya hai?

Ball Earth ko equal magnitude se upar pull karti hai; Earth ka acceleration real hai lekin astronomically small, isliye hum use kabhi notice nahi karte.

Do objects empty space se gravity ke through interact karte hain bina contact ke — kya woh third law maante hain?

Haan — unke beech Newtonian gravity ek equal-opposite collinear pair hai, aur exactly isi liye ek isolated two-body system momentum conserve karta hai.

Do charged particles rest par ek doosre ko empty space se third-law pair se attract/repel karte hain?

Haan — static charges ke liye electric forces equal, opposite, aur collinear hain, toh momentum conserve hota hai bilkul gravity ki tarah; sirf rapidly changing fields apna khud ka momentum carry karte hain aur simple picture ko complicated banate hain.

Jab ek mass infinity tak jaaye, kya third law toot jaata hai?

Nahi — force magnitudes equal rehti hain; sirf infinite mass ka acceleration zero ho jaata hai, toh woh "immovable" lagta hai jabki pair khud unchanged rehta hai.

Agar do objects ke beech contact force momentarily zero ho (woh abhi alag ho rahe hain), kya tab bhi pair hai?

Zero-magnitude force trivially ek zero partner rakhta hai; pairing statement

F_{A \to B} = - F_{B \to A}

dono sides zero ke saath bhi hold karta hai, isliye separation ke instant par kuch bhi violate nahi hota.

Kya third law ek single rigid body ke parts ke beech bhi apply hota hai jo internally ek doosre par push karte hain?

Haan — internal pairs equal aur opposite hote hain aur body ke total mein cancel karte hain, aur precisely isi liye internal forces center of mass ko accelerate nahi kar sakti. Dekho Center of Mass Motion.

Recall One-line self-test

Agar tum bata sako kyun upar ka har "False" galat hai bina "yes/no" kahe, toh tum ne pair internalize kar liya hai. Woh ek sentence jo 90% traps resolve karta hai ::: "Ek 3rd-law pair do alag bodies par act karta hai, isliye woh kabhi cancel nahi hota aur kabhi ek free-body diagram mein nahi aata."