Define distance
Scenario
Aap apni society mein ghoomte hue park gaye, phir market, aur wapas ghar. Aapka pedometer (step counter) saare steps count karta raha chahe aap kisi bhi direction mein gaye ho.
Yeh total steps mila kar jo rasta aapne cover kiya, usko physics mein kya kehte hain?
Mental model
Distance ek car ka odometer hai, jo sirf ye dekhta hai ki taya (tyre) kitna ghooma, usse farq nahi padta ki aap kis disha (direction) mein gaye.
Distance simply total actual path length hai jo ek moving body ne cover kiya hai. Isme raste ke har mod (turn) aur chakkar (loop) ki lambaai jud jaati hai. Yeh hamesha add hoti rehti hai.
Explanation
Physics mein motion tab hota hai jab ek object waqt ke saath apni position badalta hai. Is motion ko describe karne ka sabse simple tareeqa 'distance' nikalna hai. Distance ka matlab hai ki shuruati point se aakhiri point tak object jis raste (path) par chala hai, uski total length kya hai.
Jaisa ki ek car ka odometer (jo meter reading dikhata hai) kaam karta hai, distance ko disha (direction) se koi matlab nahi hota. Chahe aap north jayein, right mud kar east jayein, ya ghoom kar wapas wahin aa jayein jahan se shuru kiya tha, distance lagatar badhta rahega. Isliye hum distance ko ek scalar quantity kehte hain, kyunki isme sirf magnitude (size ya value) hota hai, direction nahi.
Distance ki SI unit meter (`m`) hoti hai. Lamba rasta napne ke liye hum kilometer (`km`) ka use karte hain, jahan `1 km = 1000 m`. Sabse zaroori baat ye hai ki agar koi object move kar raha hai, toh uska distance kabhi bhi negative ya zero nahi ho sakta. Yeh hamesha ek positive value hoti hai jo waqt ke saath bas badhti hi jaati hai.
Jab aap kinematics ke aage ke concepts jaise speed padhenge, toh distance ek bohot important role play karega, kyunki speed yehi batata hai ki aapne specific time mein kitna distance cover kiya.
Worked example
Problem: A boy walks `3 km` East, turns around and walks `4 km` West. What is the total distance covered by the boy?
- Distance raste ki total lambai hoti hai. Direction ka koi farq nahi padta.
Answer: `7 km`
Problem: A car travels along a circular track of circumference `500 m` and completes `2` full rounds. Find the total distance travelled.
- Ek round mein car ne circumference ke barabar distance cover kiya.
- `2` rounds ka total distance nikalne ke liye simply `2` se multiply karte hain.
Answer: `1000 m`
Key points
- Path length: Distance is the total length of the actual path covered by an object.
- Scalar quantity: It is a scalar quantity, meaning it has only magnitude and no specific direction.
- Always positive: For a moving object, distance can never be zero or negative.
Common mistakes
- Confusing distance with net position change: Agar koi return journey karke wapas starting point par aata hai, toh log sochte hain ki usne koi movement hi nahi ki. Par distance hamesha add hota hai, wo kabhi wapas aane se cancel (subtract) nahi hota.
- Including direction signs: Kabhi kabhi students backward direction ko negative (`-`) sign de dete hain, par distance calculate karte waqt har segment ki length ko sirf positively add karna hota hai.
Recall questions
- What is the SI unit of distance?
- Is distance a scalar or a vector quantity?
- Can the distance travelled by a moving object ever be zero?
Questions & answers
A farmer moves along the boundary of a square field of side `10 m`. What is the distance covered in `1` complete round?
`40 m`
Approach: The distance is the perimeter of the square field. Add the lengths of all `4` sides: `10 + 10 + 10 + 10 = 40 m`.
A girl runs `5 km` to a store and returns back home via the same route. What is the total distance she ran?
`10 km`
Approach: Add the forward journey distance and return journey distance. `5 km + 5 km = 10 km`.
Continue learning
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Next: Distinguish distance from displacement
Next: Define average speed