State Newton's third law of motion
Scenario
Jab tum zameen par chalte ho, toh tum darasal zameen ko peeche ki taraf push karte ho, par badle mein tum khud aage badh jate ho.
Zameen ko peeche dhakelne se tumhe aage ki taraf push kisne diya?
Mental model
Forces hamesha jode (pairs) mein aate hain. Tum duniya ko push karte ho, duniya tumhe exactly utna hi push back karti hai.
Newton ka third law batata hai ki force ka ek side nahi hota. Har action ka ek reaction hota hai. Agar object A object B par force lagata hai, toh usi waqt object B bhi object A par utna hi strong force ulte direction mein lagata hai.
Explanation
Newton ka Third Law nature ki symmetry ke baare mein baat karta hai. Iska statement hai: 'To every action, there is an equal and opposite reaction and they act on two different bodies.' Jab bhi nature mein koi force produce hota hai, toh woh akela nahi ho sakta. Forces humesha pairs (jode) mein exist karte hain.
Is law mein 'action' aur 'reaction' keval do forces ke naam hain. Agar hum ek force ko action kehte hain, toh uske jawaab mein hone wale doosre force ko reaction keh dete hain. Tum kisi bhi ek ko action aur doosre ko reaction bula sakte ho, usse farak nahi padta. Main baat yeh hai ki un dono ka magnitude (taqat) bilkul ek barabar hota hai, aur unki direction hamesha theek opposite hoti hai.
Ek aur sabse important point jahan students confuse hote hain, woh yeh hai ki action aur reaction forces humesha DO ALAG-ALAG objects par lagte hain. Isliye woh ek dusre ko cancel karke net force zero nahi banate. Jaise jab bandook se goli nikalti hai, toh bandook goli par aage force lagati hai (action), aur goli theek us waqt bandook par peeche force lagati hai (reaction, jise hum recoil kehte hain). Force goli par laga aur reaction gun par.
Dono forces equal hone ke bawajood zaruri nahi ki unka asar (acceleration) same ho. `F = ma` ke mutabiq, jiska mass zyada hoga (jaise gun), uski speed kam change hogi. Aur jiska mass bahut kam hoga (jaise bullet), woh theek usi force se bhayankar acceleration le legi.
Worked example
Problem: Explain why a swimmer pushes water backward in order to move forward.
- Swimmer apne hatho se paani ko peeche ki taraf dhakelta hai, yeh humara 'action' force hai.
- Newton's third law ke mutabiq, paani bhi ek barabar 'reaction' force swimmer par lagata hai opposite direction mein (aage).
Answer: By pushing the water backwards (action), the swimmer receives an equal and opposite force from the water forwards (reaction), which allows him to move ahead.
Problem: A heavy rifle fires a bullet. The force on the bullet is exactly equal to the recoil force on the rifle. Why does the bullet accelerate much more than the rifle?
- Dono par force equal hai (Third law).
- Second law se pata chalta hai ki acceleration inversely mass ke upar depend karta hai (`a = F/m`).
- Bullet ka mass bahut kam hai, toh uska acceleration bahut zyada hoga. Rifle bhari hai, toh uska acceleration kam hoga.
Answer: Although the action and reaction forces are equal in magnitude, they act on objects with very different masses. According to Newton's second law (`a = F/m`), the bullet gets a huge acceleration due to its small mass, while the heavy rifle gets a much smaller backward acceleration.
Key points
- Equal and Opposite: Action and reaction forces are always equal in magnitude but opposite in direction.
- Act on Different Objects: They never act on the same object, which is why they do not simply cancel each other out to zero.
Common mistakes
- Thinking action and reaction cancel each other: Kyuki forces equal aur opposite hain, students sochte hain net force zero ho jayega. Woh cancel out tabhi hote agar woh ek hi cheez par lag rahe hote. Par action ek cheez par aur reaction dusri cheez par lag raha hai.
Recall questions
- Do action and reaction forces act on the same body?
- If a bird pushes air downwards to fly, what is the reaction force?
- Why don't action and reaction forces produce equal accelerations in their respective objects?
Questions & answers
State Newton's third law of motion.
To every action, there is an equal and opposite reaction and they act on two different bodies.
Approach: Always ensure you mention that the forces act on two different bodies. Without this detail, the statement is incomplete.
How does a rocket accelerate in space?
By ejecting gas backwards (action), which produces a forward push (reaction).
Approach: Apply the third law to the gas and rocket.
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