Foundations — Free body diagrams — systematic drawing technique

1.2.4 · D1 · Physics › Newton's Laws & Dynamics › Free body diagrams — systematic drawing technique

Parent page par jo kuch bhi hai, woh chhote chhote ideas ki ek pile par tika hua hai. Neeche, har ek idea ko bilkul scratch se banaya gaya hai: seedhe words → ek picture → kyun yeh topic iske bina nahi chal sakta. Ye is order mein hain ki har naya idea sirf pehle se bane ideas use karta hai.

1. Ek "body" aur usse dot mein sikaodna

Kuch bhi draw karne se pehle, tumhe ek cheez circle karni hogi aur usse pakka rakhna hoga. Agar tum poori table-plus-block ko circle karo, toh table ke forces internal aur invisible ho jaate hain. Agar tum sirf block ko circle karo, toh table environment ban jaati hai aur block par uski push ek aisi force ban jaati hai jo tumhe draw karni hi padegi.

Picture: hum pretend karte hain ki object ki shape matter nahi karti aur usse ek single point — ek dot — mein squash kar dete hain. Yeh legal kyun hai? Kyunki sliding/translating motion (spinning nahi) ke liye, object ka har part milke chalata hai, toh hum sirf yeh care karte hain ki object kahan hai, uska outline nahi.

Topic ko iske zaroorat kyun hai: poori recipe hai "dot par forces." Bina ek pakke chosen dot ke, "par" ka koi matlab nahi.

2. Force — ek push ya pull, arrow ki tarah draw kiya gaya

Ek arrow jo dono size aur direction carry karta hai usse vector kehte hain. Tum ek force ko akele ek number se describe nahi kar sakte — "10" ka koi matlab nahi jab tak tum "10 units, pointing left" na kaho. Yahi exact reason hai ki hum plain numbers ki jagah arrows use karte hain.

Topic ko iske zaroorat kyun hai: FBD hai hi force-arrows ka collection. Dekho Resolving Vectors into Components ke liye ki ek arrow kaise do numbers banta hai.

3. Mass $m$ — kisi object mein kitni "stubbornness" hai

Picture: ek shopping trolley khali vs. bhari hui. Bhari wali ko chalana mushkil aur rokna bhi mushkil. Yahi "mushkil-pana" mass hai.

Topic ko iske zaroorat kyun hai: mass force-arrows aur resulting motion ke beech ka bridge hai. Chhoti force + bada mass = bahut chhoti acceleration.

4. Acceleration $\vec{a}$ — motion ka change, motion khud nahi

Yeh sabse tricky zero-point hai, isliye thoda slow down karo. Velocity hai "abhi tum kitni tezi se aur kis taraf ja rahe ho." Acceleration hai "woh velocity kitni tezi se badal rahi hai." 100 km/h ki steady speed par cruise karta hua car ki bahut badi velocity hai lekin zero acceleration. Traffic light se abhi nikla hua car ki choti velocity hai lekin badi acceleration.

Topic ko iske zaroorat kyun hai: ${\vec{F}}_{net}=m\vec{a}$ ki right-hand side acceleration hai. Yahi reason bhi hai ki parent kyun warn karta hai ki "$m\vec{a}$" ko kabhi arrow ki tarah mat draw karo — yeh effect hai, arrows add karne ke baad calculate hota hai, yeh pushes mein se ek nahi hai. Dekho Newton's Second Law.

5. Gravity aur weight $W=mg$ — hamesha-on downward pull

Yahan do nayi letters hain:

• $m$ = mass (§3 mein build kiya).
• $g$ = Earth ki gravity ki strength per kilogram, lagbhag $9.8{\text{m/s}}^2$. Mass ko $g$ se multiply karo aur tumhe newtons mein pull milti hai.

Picture: object se seedha Earth ke centre ki taraf ek invisible rope. Yeh kaam karne ke liye kuch bhi touch nahi karta — yeh distance par act karta hai (ek long-range force). Isliye recipe isko pehle draw karti hai: yeh har single problem mein present hai aur precisely isliye bhool jaana aasaan hai kyunki koi cheez object ko touch nahi kar rahi remind karaane ke liye.

6. Chaar contact forces: $N$, $f$, $T$, $F$

Forces sirf wahan appear hoti hain jahan body kisi cheez ko touch karti hai (ya gravity ke zariye). Toh apne dot ki boundary ke around walk karo; har touch ek force hai. Parent jo use karta hai usme chaar flavours hain.

Topic ko chaaon ki zaroorat kyun hai: yeh parent ke mnemonic ka poora "N-F-T-A" checklist hai. Dekho Normal Force and Friction aur Tension in Strings and Pulleys.

7. Newton's Third Law — reaction FBD par kyun nahi hoti

Picture: tum ek wall par lean karte ho (tum wall ko right push karte ho), wall tumhe left push karti hai. Dono arrows exist karte hain — lekin ek wall par hai, doosra tumpar. Tumhara FBD hai "tumpar forces," toh sirf wall ka tumpar push wahan belong karta hai. Tumhara wall par push wall ke FBD par rehta hai.

Topic ko iske zaroorat kyun hai: yeh #1 tarika hai jisse beginners ek diagram ko over-crowd karte hain. Pair ka do bodies mein split hona jaanna har FBD ko honest rakhta hai. Dekho Newton's Third Law.

8. Arrows ko axes mein todna: $\sum F_x$, $\sum F_y$

Ek baar arrows draw ho jaayein, unhe "by eye" add karna guesswork hai. Toh hum do guide-lines (axes) banate hain: ek horizontal ($x$), ek vertical ($y$). Phir har arrow ko do ordinary numbers se describe kiya jaata hai: woh $x$ ke along kitna pull karta hai, aur $y$ ke along kitna.

Symbol $\sum$ (Greek capital "sigma") ka matlab simply sab add karo hai. Toh $\sum F_x$ = "har force ke $x$-parts add karo."

Topic ko iske zaroorat kyun hai: aise hi drawing algebra banti hai. Zyada padho Resolving Vectors into Components mein.

Prerequisite map

Isse top-down padho: ek body choose karna aur force-arrows samajhna tumhe weight aur contact forces place karne deta hai; third law tumhe galat body par reaction forces draw karne se rokta hai; mass aur acceleration right-hand side par wait karte hain; axes mein split karna finally picture ko solvable equations mein turn kar deta hai — destination hai the FBD recipe.

Equipment checklist

Right side cover karo aur khud ko test karo. Har cheez instant "haan, main ise picture kar sakta hoon" honi chahiye.