Foundations — Friction — static (maximum), kinetic, rolling

1.2.6 · D1 · Physics › Newton's Laws & Dynamics › Friction — static (maximum), kinetic, rolling

Pehle aap parent note Friction padh sako, us mein har symbol aapko ek picture ki tarah mean karna chahiye, na ki sirf ek letter. Ye page unhe ek ek karke build karta hai, har ek usse pehle wale pe resting hai.

1. Force, aur wo arrow jo use draw karta hai

Figure dekho: box ko teen arrows milte hain. Ek lamba orange arrow right ko point karta hai = ek strong rightward push. Ek chhota arrow = ek weak push. Picture hi force hai. Jab bhi parent note kehta hai "push with $40\text{N}$", ek certain length ka arrow imagine karo.

Force ki unit newton hai, likha jaata hai $\text{N}$. Ek newton lagbhag aapke haath mein rakh ke ek chhote seb ke weight ke barabar hai.

2. Mass $m$, gravity $g$, aur weight $mg$

Inhe ek saath rakhne par aapko weight milta hai:

Is topic ko ye kyun chahiye? Kyunki friction is baat ke proportional hai ki surfaces kitne hard press ho rahi hain ek doosre mein, aur ek flat floor par ye weight hi press karta hai. No weight → no press → no friction.

3. Normal force $N$ — floor ka peeche push karna

Ye wo symbol hai jis par parent note sabse zyada lean karta hai. Socho box floor par still baitha hai. Gravity use $mg$ se neeche pull karti hai. Phir bhi ye floor mein nahi dhans jaata. Kuch toh hoga jo use upar push kar raha hai, exactly gravity ko cancel karte hue.

Figure mein, violet arrow $N$ seedha floor se upar point karta hai, surface ke $90^{\circ }$ par. Ek flat floor par jahan aur kuch vertically push nahi ho raha, $N$ ko exactly weight balance karna chahiye: $N=mg(\text{flat floor, no vertical push})$

4. Friction force $f$ aur uski direction

Figure dekho. Aap box ko right ki taraf force $F$ se push karo. Box right slide karne ki tendency hai, toh friction $f$ (magenta arrow) left ki taraf point karta hai — hamesha sliding tendency ke opposite. Normal force $N$ abhi bhi upar point karti hai; weight $mg$ abhi bhi neeche point karta hai.

Parent note $f$ ko teen names mein split karta hai situation ke hisaab se:

• $f_s$ — static friction: surfaces stuck together, abhi slide nahi hui.
• $f_k$ — kinetic friction: surfaces sliding.
• $f_r$ — rolling friction: ek wheel rolling.

Subscript bas ek label hai jo kis situation mein hai ye batata hai.

5. Proportionality $\propto$ aur coefficient $\mu$

Parent likhta hai $f\propto N$. Symbol $\propto$ ko padho "grows in step with."

Figure straight-line picture dikhata hai: friction $f$ ko normal force $N$ ke against plot karo, aur aapko ek straight line milti hai origin se jis ki steepness $\mu$ hai. Steep line = grippy surfaces = bada $\mu$. Shallow line = slippery = chhota $\mu$.

Parent is same number ke teen flavours use karta hai:

• ${\mu }_s$ — static coefficient (ceiling $f_s^{max}={\mu }_{s}N$ set karta hai).
• ${\mu }_k$ — kinetic coefficient (sliding friction $f_k={\mu }_{k}N$ set karta hai), hamesha ${\mu }_k<{\mu }_s$.
• ${\mu }_r$ — rolling coefficient, bahut chhota.

6. Inequality $\le$ — kyun static friction special hai

7. Angles, aur tools $\tan$ aur $\theta$

Parent note ka incline section ek angle $\theta$ (Greek "theta") aur function $\tan$ use karta hai.

Topic ko specifically $\tan$ kyun chahiye? Angle of repose par, sliding force $mg\sin \theta$ exactly max friction ${\mu }_{s}mg\cos \theta$ ke barabar ho jaati hai. Ek ko doosre se divide karo aur $mg$ cancel ho jaata hai, chhod ke $\frac{\sin \theta }{\cos \theta }=\tan \theta ={\mu }_s$. Toh $\tan$ exactly wo tool hai jo jawaab deta hai "kis tilt par ye slip karta hai?" — ye driving aur pressing dono effects ko ek ratio mein package karta hai. (Incline geometry ka deep dive Inclined Plane Problems mein hai.)

8. Newton's Second Law — master rule jo sab ko tie karta hai

Yahi reason hai ki friction matters: ye unme se ek arrow hai jo hum add up karte hain $F_{net}$ find karne ke liye. Newton's Second Law aur bookkeeping tool Free Body Diagrams dekho saare arrows ek saath draw karne ke liye.

Prerequisite map

Equipment checklist

Khud ko test karo — sirf out loud answer karne ke baad reveal karo.