Distinguish mass and weight
Scenario
Aapna weighing machine jo 'weight' batata hai (jaise `60 kg`), wo actually weight nahi, aapka mass hai.
Physics ke hisaab se mass aur weight mein aisa kya bada difference hai?
Mental model
Mass is 'how much stuff', Weight is 'how hard Earth is pulling that stuff'.
Mass yeh batata hai ki aapke andar kitna matter hai, jo kahin nahi badalta. Weight ek force hai, ek kichav (pull) hai jo planet aap par laga raha hai. Planet badlega, weight badlega.
Explanation
Rozmarra ki zindagi mein hum 'mass' aur 'weight' ko same maante hain, par physics mein yeh dono bilkul alag hain. Mass ek object mein maujood matter ki quantity hai. Yeh measure karta hai ki usme kitna inertia hai. Kisi bhi cheez ka mass kabhi zero nahi ho sakta aur yeh universe mein kahin bhi le jao, bilkul same rehta hai. Mass ka SI unit kilogram (`kg`) hai.
Weight, dusri taraf, woh force hai jisse earth (ya koi aur planet) uss object ko apne center ki taraf kheenchta hai. Force hone ki wajah se, weight ko calculate karne ka formula `W = m * g` hota hai (mass multiplied by gravity). Kyunki `g` ki value jagah-jagah badalti hai (earth ke poles vs equator ya phir moon par), isliye weight bhi badalta rehta hai.
Kyunki weight actually ek force hai, iska SI unit Newton (`N`) hai, `kg` nahi. Weighing scale earth ki gravity ke hisab se calibrated hota hai, isliye wo humein humara mass read karke dikha deta hai, par internally wo actually weight measure kar raha hota hai.
Worked example
Problem: An object has a mass of `10 kg` on Earth. What will be its mass and weight on the Earth? (Take `g = 9.8 m/s^2`)
- Mass har jagah same hota hai, toh wo `10 kg` hi rahega.
- Weight nikalne ke liye mass ko gravity se multiply karenge.
Answer: Mass is `10 kg`, Weight is `98 N`.
Problem: Can the weight of an object be zero? Can its mass be zero?
- Weight `W = m * g` par depend karta hai. Agar `g` ki value zero ho jaye (jaise space mein center of the earth par), toh weight zero ho sakta hai.
- Par kisi material object ka matter kabhi gayab nahi hota, isliye mass zero nahi ho sakta.
Answer: Yes, weight can be zero where gravity is zero. But mass can never be zero.
Key points
- Mass is Constant: Mass is a scalar quantity and remains constant everywhere in the universe.
- Weight is Variable Force: Weight is a vector quantity (a force directed downwards) and varies with the acceleration due to gravity (`g`).
Common mistakes
- Writing unit of weight as kg: Daily life language ke confusion ki wajah se weight ka unit `kg` likh diya jata hai, jabki physics mein weight Newton (`N`) mein measure hota hai.
Recall questions
- What is the SI unit of weight?
- Which quantity remains constant throughout the universe, mass or weight?
- What is the SI unit of mass and weight?
Questions & answers
What are the differences between the mass of an object and its weight?
Mass is the quantity of matter and remains constant everywhere (Unit: kg). Weight is the force of gravity acting on it and varies with location (Unit: Newton).
Approach: Draw a clear two-column table and contrast definition, SI unit, constancy, and whether it can be zero.
Can the weight of an object be zero?
Yes, in deep space or during free fall where g = 0.
Approach: Weight is mg, so if gravity is zero, weight is zero, but mass is not.
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