Tairna ya Doobna?
Bada sa lakdi ka lattha tairta hai, chhota sa sikka doob jaata hai. Kyun? Jawaab hai: density! ⚖️
🧭 Is Post Mein
📖 Kahani: Balti Wala Mukabla
Ravivaar ki subah thi. Dadi aangan mein chawal dho rahi thin. Priya ne dekha: chawal ke daane neeche baith gaye, par unke halke chhilke (bhoosi) paani ke upar tairne lage. Dadi ne haath se chhilke nikaal kar phenk diye.
Paas mein Golu ne paani ki balti mein khel shuru kar diya: “Chalo, dekhte hain kya doobta hai!” Usne nimbu daala, woh tairne laga. Lakdi ka chammach bhi tair gaya. Phir steel ka chammach daala, woh dhamm se tale mein! Ek chhota sa sikka bhi doob gaya.
Rohan ne bade bhaav se kaha: “Simple hai yaar! Jo halki cheez hai woh tairti hai, jo bhaari hai woh doobti hai.”
Priya muskurayi: “Achha? Toh nadi mein itna bada, bhaari lakdi ka lattha kaise tairta hai? Aur yeh chhota sa, halka sikka kaise doob gaya?” Rohan chup! 🤔
Toh asli sawaal yeh hai: koi cheez tairegi ya doobegi, yeh kis baat se tay hota hai? Sirf “bhaari” ya “halka” hona kaafi nahi hai. Aaj ek nayi cheez seekhenge jiska naam hai density. 🔍
🎯 Is Post Mein Kya Seekhenge
- Kuch cheezein paani mein tairti (float) aur kuch doobti (sink) kyun hain
- “Bhaari doobta hai, halka tairta hai” wali soch adhoori kyun hai
- Density kya hai: ek tay volume mein kitna mass bhara hai
- Formula: Density = Mass ÷ Volume, aur iski units (kg/m³, g/cm³, g/mL)
- Paani ki density lagbhag 1 g/mL, aur relative density ka matlab
- Paani se kam density = tairna, zyada density = doobna (aur iske kuch apvaad)
🧱 Basics: Pehle Yeh Aana Chahiye
1. Matter (padarth): jiska mass ho aur jo jagah gherta ho. Pathar, paani, hawa, sab matter hain.
2. Mass: kisi cheez mein kitna padarth (matter) bhara hai. Unit: gram (g), kilogram (kg).
3. Volume: koi cheez kitni jagah gherti hai. Unit: cm³, m³, millilitre (mL), litre (L). Yaad rakho: 1 mL = 1 cm³ aur 1 L = 1000 mL.
4. Bhaag (division) aur decimal: density nikaalne ke liye mass ko volume se bhaag dena hota hai, aur answer aksar 0.91 ya 2.7 jaise decimal mein aata hai.
💡 Concept: Tairna-Doobna aur Density
Step 1: Roz ka observation. Chawal dhote waqt chhilke tairte hain, chawal doobte hain. Paani mein tel daalo toh tel upar tairta hai. Lakdi ka buraada (sawdust) tairta hai, ret (sand) neeche baith jaati hai. Yaani kuch cheezein float karti hain, kuch sink.
Step 2: “Bhaari-halka” ka chakkar. Aam taur par log kehte hain: jo tairta hai woh paani se halka, jo doobta hai woh bhaari. Par ek bada lattha ek chhote sikke se kahin zyada bhaari hai, phir bhi lattha tairta hai! Toh sirf poora wazan dekhna galat hai.
Step 3: Sahi tulna kaise? Ek jaisi size (barabar volume) ka lakdi ka tukda aur lohe ka tukda haath mein lo. Loha bahut bhaari lagta hai. Jab hum kehte hain “loha lakdi se bhaari hai”, toh asal mein hum keh rahe hain ki barabar jagah mein lohe mein zyada mass bhara hai. Isi khaas gun (property) ko density kehte hain.
Step 4: Bheed wali bus. Ek bus mein 60 log thoonse hue hain, doosri usi size ki bus mein sirf 8 log. Pehli bus “dense” hai, doosri “kam dense”. Ghane jungle mein ped paas-paas hote hain, use dense forest kehte hain. Density bhi yahi batati hai: ek tay jagah mein kitna saamaan thoosa hua hai.
Step 5: Science wali definition. Kisi padarth ke ek unit volume mein jitna mass hota hai, use us padarth ki density kehte hain.
Step 6: Units. Mass ki SI unit kg aur volume ki m³ hai, isliye density ki SI unit kg/m³ (kilogram per cubic metre) hai. Chhoti cheezon aur liquids ke liye g/cm³ ya g/mL zyada aasaan hain. Kyunki 1 mL = 1 cm³, isliye 1 g/mL = 1 g/cm³. Aur 1 g/cm³ = 1000 kg/m³.
Step 7: Paani hamara “naapne ka dand”. Kamre ke taapmaan par 1 mL paani ka mass lagbhag 1 g hota hai. Yaani paani ki density lagbhag 1 g/mL (ya 1000 kg/m³). 100 mL paani ≈ 100 g, 1 L paani ≈ 1 kg.
Step 8: Tairne ka niyam (pehla nazariya). Thos (solid) cheez ki density paani se kam (1 g/cm³ se kam) ho toh woh aam taur par tairti hai. Paani se zyada ho toh doobti hai. Lakdi (lagbhag 0.6) tairti hai, loha (lagbhag 7.9) doobta hai.
Step 9: Shape aur size se density nahi badalti. Lakdi ke tukde ko aadha kaato: mass bhi aadha, volume bhi aadha, toh density wahi rahegi. Density padarth ki pehchaan hai. Haan, taapmaan aur dabaav (pressure) se yeh badal sakti hai, khaaskar gases ki. (Isko Post 8 mein vistaar se padhenge.)
Ek line mein: tairna-doobna “kitna bhaari” se nahi, balki “kitna ghana” (density) se zyada tay hota hai.
🖼️ Diagrams: Picture se Samjho
🔐 Deep Concept: Relative Density aur “Sirf Density Hi Sab Kuch Nahi”
1. Relative density (saapeksh ghanatva): kisi padarth ki density ko paani ki density (usi taapmaan par) se bhaag do. Jo number milta hai, woh batata hai ki padarth paani se kitne guna ghana hai.
Jaise: ek aluminium ke tukde ka mass 27 g aur volume 10 cm³ hai. Density = 27 ÷ 10 = 2.7 g/cm³. Paani ki density 1 g/cm³ hai, toh aluminium ki relative density = 2.7 ÷ 1 = 2.7. Yaani aluminium paani se 2.7 guna ghana hai. Dhyaan do: relative density ki koi unit nahi hoti, kyunki g/cm³ ÷ g/cm³ kat jaata hai.
2. Paani mein tairne ka shortcut: relative density 1 se kam → tairega; 1 se zyada → doobega.
3. Units ka rishta: 1 g/cm³ = 1000 kg/m³. Kyun? 1 m³ = 10,00,000 cm³ (100 × 100 × 100). Toh 1 g/cm³ wali cheez ke 1 m³ ka mass = 10,00,000 g = 1000 kg. Isliye paani ki density = 1000 kg/m³ aur aluminium ki = 2700 kg/m³. Ulta: 1 kg/m³ = 1 g/L.
4. Density hi akela factor nahi hai. Kisi cheez ka tairna sirf uske padarth ki density par nahi, balki poori cheez ki ausat (average) density par bhi nirbhar karta hai, jisme andar bhari hawa bhi gini jaati hai. Lohe ki keel doob jaati hai, par lohe/steel ka bada jahaz tairta hai, kyunki jahaz khokhla (hollow) hai aur uske andar bahut saari hawa hai. Bamboo bhi khokhla hone ki wajah se aasaani se tairta hai.
5. Liquid bhi badlo toh nateeja badal sakta hai. Tairna-doobna cheez aur liquid, dono ki density ki tulna hai. Samudra ka khaara paani (lagbhag 1.03 g/cm³) saade paani se thoda zyada ghana hai, isliye samudra mein tairna thoda aasaan lagta hai. Isi tarah ek cheez tel mein doob sakti hai par paani mein tair sakti hai.
✏️ Examples: Density Nikaalo, Phir Batao
Dhyaan do: lakdi ke gutke ka mass (60 g) pathar (26 g) se zyada hai, phir bhi lakdi tairti hai! Faisla density karti hai, kul wazan nahi.
⚖️ Difference Table: Mass vs Volume vs Density
| Baat | Mass | Volume | Density |
|---|---|---|---|
| Matlab | Kitna padarth bhara hai | Kitni jagah gheri hai | Ek unit volume mein kitna mass |
| SI unit | kg | m³ | kg/m³ |
| Doosri units | g | cm³, mL, L | g/cm³, g/mL |
| Tukda aadha karo toh | Aadha | Aadha | Wahi rehti hai |
| Kya batata hai | Cheez kitni bhaari hai | Cheez kitni badi hai | Padarth kitna “ghana” hai, tairega ya doobega |
| Kaise naapein | Balance (taraazu) | Measuring cylinder ya l × w × h | Mass ÷ Volume (calculate karke) |
🚫 Misconceptions: Log aisa sochte hain… lekin
❌ Galat: “Bhaari cheezein doobti hain, halki tairti hain.”
✅ Sach: bhaari lattha tairta hai, halka sikka doobta hai. Faisla density (barabar volume mein mass) se hota hai, kul wazan se nahi.
❌ Galat: “Badi cheez ki density zyada hoti hai.”
✅ Sach: density size par nirbhar nahi. Lohe ki chhoti keel aur lohe ki badi rod, dono ki density lagbhag 7.9 g/cm³ hai.
❌ Galat: “1 kg rui (cotton) 1 kg lohe se halki hai.”
✅ Sach: dono ka mass barabar, 1 kg! Bas rui bahut zyada jagah gherti hai, kyunki uski density kam hai.
❌ Galat: “Lohe se bani har cheez doobegi.”
✅ Sach: khokhle lohe ke jahaz ki ausat density (hawa milakar) paani se kam hoti hai, isliye woh tairta hai.
❌ Galat: “Relative density ki unit g/cm³ hai.”
✅ Sach: relative density sirf ek number hai, iski koi unit nahi hoti.
🔤 Key Terms (Exam mein English shabd hi likhna)
❓ FAQs
1. Density aasaan shabdon mein kya hai?
Kisi padarth ki ek tay jagah (jaise 1 cm³) mein kitna mass bhara hai, wahi uski density hai. Zyada mass thoosa = zyada density.
2. Paani ki density kitni hai?
Kamre ke taapmaan par lagbhag 1 g/mL, yaani 1 g/cm³ ya 1000 kg/m³. Isliye 1 litre paani lagbhag 1 kg ka hota hai.
3. Tel paani par kyun tairta hai?
Tel ki density (lagbhag 0.91 g/mL) paani (lagbhag 1 g/mL) se kam hai, isliye kam dense tel upar parat bana leta hai.
4. Kya kisi cheez ko kaatne se uski density badal jaati hai?
Nahi. Kaatne par mass aur volume dono ek hi anupaat mein ghatte hain, isliye Mass ÷ Volume wahi rehta hai.
5. Kya density se hamesha pakka pata chalta hai ki cheez tairegi ya nahi?
Thos, bhari hui (solid) cheezon ke liye kaafi had tak haan. Lekin khokhli cheezon (jahaz, bamboo, band khaali bottle) mein andar ki hawa ausat density ghata deti hai, aur liquid badalne se bhi nateeja badal sakta hai. Isliye NCERT bhi kehta hai ki density akela factor nahi hai.
🧪 Activity: Pehle Anumaan, Phir Jaanch — Kya Tairega, Kya Doobega?
Saamaan: paani se bhari ek badi transparent baalti ya tub, aur ghar/school ki kuch cheezein: sikka, lohe ki chaabi, steel ka chammach, lakdi ki pencil, lakdi ka chammach, patta, kankad, barf ka tukda, mombatti ka tukda, aaloo, seb (apple), thoda tel, thode chawal ke daane aur unke chhilke, ek chammach ret aur ek chammach lakdi ka buraada.
Kya dikhega: lakdi ki cheezein, patta, barf, mombatti, seb, tel, chhilke aur buraada tairenge. Sikka, chaabi, steel ka chammach, kankad, aaloo, chawal aur ret doobenge. Bada seb chhote sikke se bhaari hai, phir bhi tairta hai!
Nishkarsh: tairna-doobna sirf “bhaari-halka” se tay nahi hota. Jin padarthon ki density paani se kam hai woh tairte hain, aur jinki zyada hai woh doobte hain. Isi gun ka naam density hai.
Yeh activity NCERT ki Class 8 Science book Curiosity (Chapter 9) ke “Why Do Objects Float or Sink in Water?” wale hisse par aadharit hai, aur yahan apne shabdon mein samjhayi gayi hai. (NCERT ne is hisse mein alag se numbered activity nahi di hai, isliye yeh activity usi baat ko karke dikhati hai.)
🏠 Mini Experiments (Ghar pe)
Kya hua? Neeche shahad (lagbhag 1.4), beech mein paani (1.0), upar tel (0.91). Sikka (bahut ghana) tale tak gaya, lakdi (lagbhag 0.6) sabse upar tel par tairi. Har cheez apni density ke hisaab se apni “manzil” par ruk gayi!
Gile aate ki ek chhoti goli banao aur paani mein daalo: woh doob jaayegi. Ab utne hi aate se ek patli, chaudi katori (naav jaisi) banao aur dheere se paani par rakho. Woh tair sakti hai! Aata wahi, mass wahi, bas shape badli. Katori ke andar hawa aa gayi, isliye poori katori ki ausat density paani se kam ho gayi. (Na tairi toh deewarein aur patli karo.)
Lagbhag ek hi size ka ek lakdi ka gutka aur ek pathar dhoondho. Dono haathon mein uthao. Kaun bhaari laga? Ab dono ko paani mein daalo. Barabar volume par jo zyada bhaari laga, wahi zyada dense hai aur wahi dooba.
🧠 Think Like a Scientist
Sawaal: Rohan kehta hai, “Jo bhaari hai woh doobta hai.” Ek scientist ki tarah iski jaanch kaise karoge?
Scientist ka tareeka dekho
1. Observe (dekho): balti mein lakdi ka bada chammach tairta hai aur chhota sikka doobta hai.
2. Guess (anumaan): agar Rohan sahi hai, toh jo cheez taraazu par zyada bhaari hogi woh zaroor doobegi.
3. Test (jaanch): rasoi ke digital taraazu par ek bada seb (lagbhag 150 g) aur ek steel ka chhota chammach (lagbhag 20 g) tolo. Phir dono ko paani mein daalo.
4. Result: zyada bhaari seb tairta hai, kam bhaari chammach doobta hai.
5. Conclusion: Rohan ka niyam galat hai. Kul wazan se faisla nahi hota. Faisla hota hai ki har cm³ mein kitna mass hai, yaani density. Seb ki density paani se kam hai, steel ki bahut zyada. 🔬
🌍 Real Life Mein Tairna-Doobna
- 🍚 Chawal dhona: halke chhilke aur kachra upar tair jaate hain, bhaari daane neeche, isliye unhe aasaani se alag kar lete hain.
- 🌾 Beej chhantna: kisaan beejon ko paani mein daalte hain. Khokhle ya kharab beej tair jaate hain, achhe bhare hue beej doob jaate hain.
- 🛶 Bamboo ki naav aur bede: purane zamaane se log khokhle, halke bamboo aur lakdi ke latthon se bede (raft) banakar nadi paar karte aaye hain.
- 🦺 Life jacket: isme bahut kam density wala foam bhara hota hai, jo insaan ko paani mein upar rakhta hai.
- 🧊 Barf ka tukda: nimbu-paani mein barf upar tairti hai. Kyun? Padho Science ki Neev: Barf paani pe kyun tairti hai?
- 🛢️ Samudra mein tel ka failna: jahaz se tel gire toh woh paani par parat bana kar fail jaata hai, kyunki tel kam dense hai. Isse samudri jeevon ko nuksaan hota hai.
- 🍛 Daal ka tadka: ghee-tel aksar daal ke upar tairta dikhta hai.
🔁 Recap: Poora Post Ek Nazar Mein
- Kuch cheezein paani mein tairti (float), kuch doobti (sink) hain: chhilke vs chawal, buraada vs ret, tel vs paani.
- “Bhaari doobta, halka tairta” adhoora hai. Asli gun density hai.
- Density = Mass ÷ Volume, yaani ek unit volume mein mass.
- SI unit kg/m³; aasaan units g/cm³ aur g/mL. 1 g/cm³ = 1000 kg/m³.
- Paani ki density lagbhag 1 g/mL. 1 L paani ≈ 1 kg.
- Density shape aur size par nirbhar nahi, par taapmaan aur dabaav se badal sakti hai.
- Relative density = padarth ki density ÷ paani ki density; koi unit nahi. 1 se kam = tairega, 1 se zyada = doobega.
- Density akela factor nahi: khokhli cheezon ki andar ki hawa aur liquid ka prakaar bhi asar daalte hain.
✍️ Fill in the Blanks
- Kisi padarth ke ek unit volume mein jitna mass hota hai, use uski ______ kehte hain.
- Density ki SI unit ______ hai.
- Kamre ke taapmaan par paani ki density lagbhag ______ g/mL hoti hai.
- Jin thos cheezon ki density paani se kam hoti hai, woh paani mein aam taur par ______ hain.
- Relative density ki ______ unit nahi hoti.
- 1 g/cm³ = ______ kg/m³.
Answers
1) density (ghanatva) · 2) kg/m³ · 3) 1 · 4) tairti · 5) koi · 6) 1000
✅ True / False
- Bhaari cheez paani mein hamesha doobti hai.
- Density = Mass ÷ Volume.
- Lakdi ke gutke ko do barabar tukdon mein kaatne se uski density aadhi ho jaati hai.
- 1 litre tel ka mass 1 litre paani ke mass se kam hota hai.
- Aluminium ki relative density 2.7 hai, yaani woh paani se 2.7 guna ghana hai.
- Steel ka jahaz isliye tairta hai kyunki steel ki density paani se kam hai.
Answers (kyun ke saath)
1) False: bada bhaari lattha tairta hai. Faisla density se hota hai, kul wazan se nahi.
2) True: ek unit volume mein mass hi density hai.
3) False: mass aur volume dono aadhe hote hain, density wahi rehti hai.
4) True: tel ki density (lagbhag 0.91 g/mL) paani se kam hai, toh 1 L tel lagbhag 910 g aur 1 L paani lagbhag 1000 g.
5) True: 2.7 ÷ 1 = 2.7, bina unit ka number.
6) False: steel ki density (lagbhag 7.9 g/cm³) paani se bahut zyada hai. Jahaz khokhla hai, andar ki hawa milakar uski ausat density paani se kam hoti hai.
🔘 MCQs
Q1. Density ka sahi formula kaunsa hai? (a) Mass ÷ Volume (b) Mass × Volume (c) Volume ÷ Mass (d) Mass + Volume
✅ (a) Mass ÷ Volume: ek unit volume mein kitna mass, yahi density hai.
(b) guna karne se “ek unit volume mein mass” nahi milta.
(c) yeh ulta hai; isse “ek gram kitni jagah leta hai” milega.
(d) mass aur volume alag-alag cheezein hain, inhe joda nahi ja sakta.
Q2. Density ki SI unit kya hai? (a) g/cm³ (b) kg (c) kg/m³ (d) m³/kg
✅ (c) kg/m³: mass ki SI unit kg aur volume ki m³ hai.
(a) g/cm³ density ki unit hai, par SI unit nahi.
(b) kg sirf mass ki unit hai.
(d) yeh ulti unit hai (volume ÷ mass).
Q3. Ek cheez ka mass 30 g aur volume 40 cm³ hai. Uski density aur paani mein nateeja kya hoga? (a) 1.33 g/cm³, doobegi (b) 0.75 g/cm³, tairegi (c) 0.75 g/cm³, doobegi (d) 1200 g/cm³, doobegi
✅ (b): 30 ÷ 40 = 0.75 g/cm³, jo 1 se kam hai, toh tairegi.
(a) 1.33 tab aata jab volume ÷ mass (40 ÷ 30) karte; yeh galat formula hai.
(c) density sahi hai par 0.75 < 1 hone se cheez doobegi nahi.
(d) 1200 = 30 × 40, guna karna galat hai.
Q4. In mein se kaunsi cheez paani mein doobegi? (a) Cork (0.24 g/cm³) (b) Barf (0.92 g/cm³) (c) Aluminium (2.7 g/cm³) (d) Lakdi (0.6 g/cm³)
✅ (c) Aluminium: 2.7 paani ki density 1 se zyada hai.
(a) cork 0.24, bahut kam density, tairega.
(b) barf 0.92, 1 se kam, tairegi.
(d) lakdi 0.6, 1 se kam, tairegi.
Q5. Relative density ki unit kya hai? (a) g/cm³ (b) kg/m³ (c) Koi unit nahi (d) g/mL
✅ (c) Koi unit nahi: yeh do densities ka bhaag hai, units kat jaati hain.
(a), (b) aur (d) density ki units hain, relative density ki nahi.
Q6. Lakdi ke ek gutke ko beech se kaat kar do barabar tukde kiye. Har tukde ki density: (a) aadhi ho jaayegi (b) dugni ho jaayegi (c) wahi rahegi (d) zero ho jaayegi
✅ (c) wahi rahegi: mass aur volume dono aadhe hue, bhaag wahi.
(a) sirf mass aadha nahi hua, volume bhi aadha hua.
(b) density badhne ka koi kaaran nahi.
(d) tukde mein mass aur volume dono hain, density zero nahi ho sakti.
Q7. 1 g/cm³ kitne kg/m³ ke barabar hai? (a) 1 kg/m³ (b) 10 kg/m³ (c) 100 kg/m³ (d) 1000 kg/m³
✅ (d) 1000 kg/m³: 1 m³ = 10,00,000 cm³, toh 1 g/cm³ wali cheez ka 1 m³ = 10,00,000 g = 1000 kg.
(a) 1 kg/m³ toh 1 g/L ke barabar hai, bahut chhota.
(b) aur (c) cm³ aur m³ ka sahi rishta (10 lakh guna) nahi lagaya.
Q8. Steel ka bada jahaz paani par kyun tairta hai? (a) Steel ki density paani se kam hai (b) Jahaz khokhla hai, andar ki hawa milakar uski ausat density paani se kam hai (c) Samudra ka paani steel se zyada ghana hai (d) Jahaz bahut bada hai, isliye halka hai
✅ (b): hawa bhari khokhli banawat ausat density ghata deti hai.
(a) steel ki density lagbhag 7.9 g/cm³ hai, paani se bahut zyada.
(c) samudra ka paani lagbhag 1.03 g/cm³ hai, steel se bahut kam ghana.
(d) bada hone se koi cheez halki nahi ho jaati; jahaz toh bahut bhaari hota hai.
🔗 Assertion–Reason
Options: (a) A aur R dono sahi, aur R, A ko samjhata hai · (b) A aur R dono sahi, par R, A ko nahi samjhata · (c) A sahi, R galat · (d) A galat, R sahi
1. A: Lakdi ka bada, bhaari lattha nadi mein tairta hai. R: Lakdi ki density paani ki density se kam hai.
✅ (a): lattha bhaari hai par uski density paani se kam hai, isi wajah se tairta hai. R, A ka sahi kaaran hai.
2. A: Tel paani ke upar parat banata hai. R: Tel ka mass hamesha paani ke mass se kam hota hai.
✅ (c): A sahi hai. R galat hai: mass matra par nirbhar karta hai; 2 litre tel (lagbhag 1820 g) 1 litre paani (lagbhag 1000 g) se bhaari hai. Asli kaaran hai tel ki density kam hona.
3. A: Relative density ki koi unit nahi hoti. R: Lohe ki density paani ki density se zyada hai.
✅ (b): dono baatein sahi hain, par lohe ki density zyada hone se yeh nahi samajh aata ki relative density bina unit ki kyun hai. (Asli kaaran: do densities ke bhaag mein units kat jaati hain.)
📝 Very Short Answer (1 mark)
1. Density kise kehte hain?
Kisi padarth ke ek unit volume mein maujood mass ko.
2. Density ki SI unit likho.
kg/m³ (kilogram per cubic metre).
3. Paani ki density kitni hoti hai (g/mL mein)?
Lagbhag 1 g/mL.
4. Relative density ka formula likho.
Relative density = padarth ki density ÷ paani ki density (usi taapmaan par).
5. Tel aur paani mein kaun zyada dense hai?
Paani.
📝 Short Answer (2–3 marks)
1. Bhaari lattha tairta hai aur halka sikka doobta hai. Kyun?
Tairna-doobna kul wazan se nahi, density se tay hota hai. Lakdi ki density (lagbhag 0.6 g/cm³) paani se kam hai, isliye bada lattha bhi tairta hai. Sikke ki dhaatu ki density paani se kai guna zyada hai, isliye chhota sikka bhi doob jaata hai.
2. Ek pathar ka mass 150 g aur volume 50 cm³ hai. Uski density nikaalo aur batao woh paani mein tairega ya doobega.
Density = Mass ÷ Volume = 150 g ÷ 50 cm³ = 3 g/cm³. Yeh paani (1 g/cm³) se zyada hai, isliye pathar doobega.
3. Relative density kya hai? Aluminium (density 2.7 g/cm³) ka udaharan do.
Kisi padarth ki density ko paani ki density se bhaag dene par jo number milta hai, use relative density kehte hain. Aluminium: 2.7 ÷ 1 = 2.7. Yaani aluminium paani se 2.7 guna ghana hai. Iski koi unit nahi hoti.
4. Kisi cheez ko kaatne par uski density kyun nahi badalti?
Density = Mass ÷ Volume. Kaatne par mass aur volume dono ek hi anupaat mein ghatte hain (jaise dono aadhe). Isliye unka bhaag, yaani density, wahi rehta hai. Density padarth ka gun hai, size ya shape ka nahi.
📝 Long Answer (5 marks)
1. Density kya hai? Iski definition, formula, units, paani ki density aur tairne-doobne ka niyam do udaharanon ke saath samjhao.
Definition: kisi padarth ke ek unit volume mein jitna mass hota hai, use uski density kehte hain.
Formula: Density = Mass ÷ Volume.
Units: SI unit kg/m³; doosri aasaan units g/cm³ aur g/mL. 1 g/cm³ = 1000 kg/m³.
Paani: kamre ke taapmaan par paani ki density lagbhag 1 g/mL hai, yaani 1 mL paani ≈ 1 g.
Niyam: thos cheez ki density paani se kam ho toh aam taur par tairti hai, zyada ho toh doobti hai.
Udaharan 1: lakdi ka gutka, 60 g aur 100 cm³ → 0.6 g/cm³ → tairega.
Udaharan 2: lohe ka tukda, 79 g aur 10 cm³ → 7.9 g/cm³ → doobega.
Density shape-size se nahi badalti, par taapmaan aur dabaav se badal sakti hai.
2. “Density hi akela factor nahi hai jo tairna-doobna tay karta hai.” Is baat ko udaharanon ke saath samjhao. Relative density ka bhi zikr karo.
Relative density: padarth ki density ÷ paani ki density. Paani mein: 1 se kam → tairna, 1 se zyada → doobna. Yeh bina unit ka number hai.
Lekin aur baatein bhi maayne rakhti hain:
(i) Shape aur andar ki hawa: steel ki keel doobti hai, par steel ka khokhla jahaz tairta hai, kyunki andar ki hawa milakar poore jahaz ki ausat density paani se kam ho jaati hai. Aate ki goli doobti hai, par usi aate ki patli katori tair sakti hai.
(ii) Liquid ka prakaar: nateeja cheez aur liquid dono ki density ki tulna se tay hota hai. Samudra ka khaara paani (lagbhag 1.03) saade paani se ghana hai, isliye usme tairna aasaan hai. Lakdi tel aur paani dono par tairti hai, par koi cheez tel mein doob kar paani par tair bhi sakti hai.
(iii) Khokhle beej/bamboo: andar hawa hone se tairte hain.
Isliye density sabse zaroori sanket hai, par poori kahani ke liye shape, hawa aur liquid ko bhi dekhna padta hai.
📰 Case-Based Question
Haryana ke ek gaon mein Kavita ke nanaji buaai (sowing) se pehle gehun ke beej chhaant rahe the. Unhone ek baalti paani mein beej daale aur haath se ghumaya. Kuch beej upar tair aaye. Nanaji ne bataya, “Yeh khokhle hain ya inhe keede ne kha liya hai.” Unhone tairte beej nikaal kar alag kar diye aur neeche baithe hue bhare-poore beej hi khet mein boye. Kavita ne socha, “Toh achhe beej bhaari hone se doobe?”
Q1. Kharab beej paani mein kyun tair aaye?
Khokhle ya keede khaye beejon mein padarth kam aur hawa wali khaali jagah zyada hoti hai. Isliye unki ausat density paani se kam ho jaati hai aur woh tairte hain.
Q2. Yeh tareeka padarth ke kis gun (property) par aadharit hai?
Density par: paani se zyada density wale bhare beej doobte hain, kam density wale khokhle beej tairte hain.
Q3. Ek beej ka mass 0.03 g aur volume 0.025 cm³ hai. Kya yeh achha beej hai?
Density = 0.03 ÷ 0.025 = 1.2 g/cm³. Yeh paani (1 g/cm³) se zyada hai, isliye beej doobega, yaani yeh bhara hua, achha beej hai.
Q4. Kavita ki baat “achhe beej bhaari hone se doobe” ko science ki bhasha mein sudhaaro.
Achhe beej isliye doobe kyunki unki density paani se zyada hai, yaani unke har cm³ mein zyada mass bhara hai. Sirf “bhaari” kehna adhoora hai: ek bada halka cheez ka tukda bhi chhote beej se bhaari ho sakta hai aur phir bhi tair sakta hai.
🚀 HOTS
1. 1 kg rui aur 1 kg loha: kiska mass zyada hai, kaun zyada jagah gherega, aur kiski density zyada hai?
Mass dono ka barabar (1 kg) hai. Rui bahut zyada jagah (volume) gheregi. Density = Mass ÷ Volume, toh barabar mass par jiska volume zyada, uski density kam. Isliye lohe ki density zyada hai, rui ki kam.
2. Do gutke A aur B ek hi size ke hain, dono ka volume 100 cm³. A ka mass 80 g aur B ka 120 g hai. Paani mein kaun tairega?
A: 80 ÷ 100 = 0.8 g/cm³ → 1 se kam → tairega. B: 120 ÷ 100 = 1.2 g/cm³ → 1 se zyada → doobega. Barabar size par jiska mass zyada, uski density zyada.
3. Ek band, khaali plastic bottle tairti hai. Usme poora paani bhar kar dhakkan band kar do, phir bhi woh mushkil se doobti hai. Kyun? (Plastic ki density lagbhag 1.0–1.4 g/cm³ maano.)
Khaali bottle mein hawa bhari thi, toh poori bottle ki ausat density paani se bahut kam thi, isliye tairi. Paani bharne par andar ka hissa paani jaisa ho gaya; ab ausat density lagbhag paani jitni hi hai, bas plastic ki wajah se thodi si zyada. Isliye woh na zor se tairti, na tezi se doobti: dheere doobti hai ya lagbhag beech mein latki rehti hai. Yeh dikhata hai ki andar ki hawa ausat density ko kaise badalti hai.
🏆 Olympiad Corner
1. Ek lakdi ke gutke ka mass 500 g aur volume 625 cm³ hai. Paani mein tairte waqt uska lagbhag kitna hissa paani ke andar rahega?
Density = 500 ÷ 625 = 0.8 g/cm³, relative density 0.8. Tairti cheez ka andar dooba hissa uski relative density ke barabar hota hai, toh lagbhag 80% (4/5) hissa paani ke andar aur 20% bahar rahega.
2. Har ek 1 litre: paani (1 g/mL), sarson ka tel (lagbhag 0.91 g/mL), shahad (lagbhag 1.4 g/mL), petrol (lagbhag 0.74 g/mL). Kiska mass sabse zyada hoga?
Mass = Density × Volume. 1000 mL ke liye: paani ≈ 1000 g, tel ≈ 910 g, shahad ≈ 1400 g, petrol ≈ 740 g. Sabse zyada mass shahad ka.
3. Lohe ki density lagbhag 7.9 g/cm³ hai. Ise kg/m³ mein badlo.
1 g/cm³ = 1000 kg/m³, toh 7.9 g/cm³ = 7.9 × 1000 = 7900 kg/m³.
Har option pe tap karo, turant pata chalega sahi ya galat.
Q1. Ek wooden block: mass 120 gram, volume 200 cm³. Density kitni, aur water mein float karega ya sink?
(a) 1.67 gram/cm³, sink karega
❌ Galat: Volume ÷ mass kiya: 200 ÷ 120 ≈ 1.67. Sahi: mass ÷ volume = 120 ÷ 200 = 0.6, float.
(b) 0.6 gram/cm³, sink karega
❌ Galat: Rule reverse lagaya. 0.6 water ki density 1 se kam hai: float.
(c) 24000 gram/cm³, sink karega
❌ Galat: Mass × volume kiya: 120 × 200 = 24000. Density = 120 ÷ 200 = 0.6.
(d) 0.6 gram/cm³, float karega
✅ Sahi! Density = mass ÷ volume = 120 ÷ 200 = 0.6 gram/cm³. Water ki density 1 se kam: float.
Q2. Ek liquid ki density 0.8 gram/cm³ hai (water: 1 gram/cm³). Uski relative density kya hai?
(a) 0.8, koi unit nahi
✅ Sahi! Relative density = liquid ki density ÷ water ki density = 0.8 ÷ 1 = 0.8. Unit kat jaati hai: koi unit nahi.
(b) 0.8 gram/cm³
❌ Galat: Relative density pe unit laga di. gram/cm³ ÷ gram/cm³: unit kat jaati hai.
(c) 1.25, koi unit nahi
❌ Galat: Divide reverse kiya: 1 ÷ 0.8 = 1.25. Sahi: 0.8 ÷ 1 = 0.8.
(d) 800, koi unit nahi
❌ Galat: Kg per cubic metre wala 1000 se multiply kiya: 0.8 × 1000 = 800. Relative density = 0.8 ÷ 1 = 0.8.
Q3. Ek wooden log 50 kg ka hai aur ek iron nail sirf 5 gram ka. Log float karta hai, nail sink. Kyun?
(a) Light cheezein sink karti hain
❌ Galat: Heavy-light ka rule reverse lagaya, aur woh rule hi adhoora hai. Density dekho.
(b) Float ya sink density se tay hota hai, total mass se nahi
✅ Sahi! Wood ki density water se kam, iron ki zyada. Total mass nahi, har cm³ ka mass dekho.
(c) Log ka mass nail se kam hai
❌ Galat: Mass galat compare kiya: 50 kg log nail se bahut heavy hai. Phir bhi float: density kam hai.
(d) Nail ka shape usse sink karata hai
❌ Galat: Shape ko reason maana. Iron ki density water se zyada hai; shape se density nahi badalti.
Q4. Ek liquid ki density 1.2 gram/cm³ hai. Kg per cubic metre mein kitni?
(a) 1.2 kg per cubic metre
❌ Galat: Unit badli par number nahi badla (1.2). 1.2 × 1000 = 1200.
(b) 12 kg per cubic metre
❌ Galat: 10 se multiply kiya: 1.2 × 10 = 12. Sahi: 1.2 × 1000 = 1200.
(c) 1200 kg per cubic metre
✅ Sahi! 1 gram/cm³ matlab 1000 kg per cubic metre. To 1.2 × 1000 = 1200.
(d) 0.0012 kg per cubic metre
❌ Galat: 1000 se divide kiya: 1.2 ÷ 1000 = 0.0012. Sahi: 1.2 × 1000 = 1200.
Q5. Dough (aata) ki chhoti solid ball water mein sink karti hai, par utne hi dough se bani patli, chaudi boat (katori, naav jaisi) float karti hai. Kyun?
(a) Boat banane se dough ki density kam ho gayi
❌ Galat: Shape se material ki density badli maani. Dough ki density same; average density air se kam hui.
(b) Boat ka mass ball se kam ho gaya
❌ Galat: Mass kam maana. Utna hi dough hai, mass same; andar ki air se average density kam hui.
(c) Boat float karti hai kyunki water ki density badh gayi
❌ Galat: Water ki density badli maani. Water wahi hai; boat ki average density kam hui.
(d) Boat ke andar air hai, isliye uski average density water se kam
✅ Sahi! Dough ki density wahi hai. Boat hollow hai: dough + andar ki air milake average density 1 se kam.
📚 Aur Padho (External Reference)
- NCERT Class 8 Science Curiosity, Chapter 9 (official PDF)
- DIKSHA: Sarkar ka free learning platform, NCERT ke videos ke liye
💖 CharuMam Motivation
Aaj humne dekha ki bada, bhaari lattha bhi tair jaata hai, aur chhota sa sikka bhi doob jaata hai. Faisla bahar ke size ya wazan se nahi, andar ki “density” se hota hai.
Beta, zindagi mein bhi aisa hi hai. Kabhi koi bada problem aaye, ya log kahein ki “tum chhote ho, tumse nahi hoga”, toh yaad rakhna: tumhe upar kya rakhega, yeh andar ki baat tay karti hai. Mann mein umeed, mehnat aur halkapan (tension kam) ki “hawa” bhar lo, phir bhaari se bhaari paristhiti mein bhi tum lattha ki tarah tair jaoge. 🛶
— CharuMam
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