HP TGT Physics Chapter 2: Force & Laws of Motion — Friction, Recoil & Lift Concepts

HP TGT Non-Medical exam mein Mechanics ka sabse bada hissa Newton's Laws of Motion hai. Sirf niyam ratne se kaam nahi chalega, unka application (anuprayog) samajhna zaroori hai.

Kya aap jante hain ki Rocket kis siddhanth par udta hai? Ya Lift mein humein apna wazan kam ya zyada kyun lagta hai?

Aaj hum inhi concepts ko Exam Point of View se cover karenge.

1. Newton's First Law & Inertia (Jadatva)

Statement: Koi vastu tab tak apni avastha (rest/motion) change nahi karti jab tak uspar bahari bal (External Force) na lage.
Inertia (Jadatva): Mass jitna zyada, Inertia utna zyada.

Types of Inertia (Exam Special):

Inertia of Rest: Bus achanak chalne par yatri piche girte hain.
Inertia of Motion: Chalti bus rukne par yatri aage girte hain.
Inertia of Direction: Bus ke mudne par yatri bahar ki taraf jhukte hain.

2. Newton's Second Law (The Real Law)

Yeh law humein Force ka formula deta hai.

Statement: Rate of change of momentum is directly proportional to applied force.
Formula:
$F = \frac{dp}{dt} = ma$
(Jahan $p$ momentum hai).
Application: Cricket player ball pakadte waqt hath piche khinchta hai taaki Time ( $t$ ) badh jaye aur Force ( $F$ ) kam lage.

3. Newton's Third Law (Action-Reaction)

Statement: Every action has an equal and opposite reaction.
Key Point: Action aur Reaction hamesha alag-alag bodies par lagte hain. Isliye wo ek doosre ko cancel nahi karte.
Example: Swimming, Walking, Rocket Propulsion.

4. Conservation of Momentum & Recoil of Gun

Jab koi bahari force na ho ( $F_{ext} = 0$ ), toh Total Momentum constant rehta hai.

m_1u_1 + m_2u_2 = m_1v_1 + m_2v_2

Recoil Velocity of Gun (Bandook ka piche hatna):

Agar Bandook ka mass $M$ hai aur Bullet ka mass $m$ hai, aur bullet ki velocity $v$ hai, toh Gun ki recoil velocity ( $V$ ) hogi:

V = -\frac{mv}{M}

(Negative sign shows backward direction).

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5. Impulse (Aaveg)

Jab bahut bada force bahut kam samay ke liye lage.

Formula: $Impulse (J) = Force \times Time = Change \text{ } in \text{ } Momentum (\Delta p)$
Example: Bat dwara ball ko hit karna.

6. Friction (Gharshan) - Most Important Missing Topic

Friction wo force hai jo relative motion ka virodh karta hai.

Types of Friction:

Static Friction ( $f_s$ ): Jab body ruki hui ho. (Self-adjusting force).
Limiting Friction ( $f_l$ ): Static friction ki maximum value.
Kinetic Friction ( $f_k$ ): Jab body chal rahi ho. ( $f_k < f_l$ )

Coefficient of Friction ( $\mu$ ):

f = \mu N

(Jahan $N$ = Normal Reaction force).

Banking of Roads:

Pahadon par sadkein kinare se uthi hui (banked) hoti hain taaki gaadi fisle nahi.

\tan\theta = \frac{v^2}{rg}

7. Apparent Weight in a Lift (Lift mein Bhaar)

Yeh topic TGT Commission ka favorite hai. Maan lo aadmi ka mass $m$ hai.

Condition	Acceleration	Apparent Weight (W′)	Feeling
Lift at Rest / Constant Velocity	$a = 0$	$W' = mg$	Real Weight
Lift Accelerating Up (Upar)	$a$ (Up)	$W' = m(g + a)$	Bhari (Heavier)
Lift Accelerating Down (Niche)	$a$ (Down)	$W' = m(g - a)$	Halka (Lighter)
Free Fall (Rassi toot jaye)	$a = g$	$W' = m(g - g) = 0$	Weightlessness

8. Important Formulas Table

Concept	Formula
Force	$F = ma$
Momentum	$p = mv$
Impulse	$J = F \times t$
Recoil Velocity	$V = -\frac{mv}{M}$
Friction	$f = \mu N$
Lift (Up)	$R = m(g+a)$

9. HP TGT Special MCQs (Exam Level)

Q1. A rocket works on the principle of conservation of:

A) Mass

B) Energy

C) Linear Momentum

D) Angular Momentum

Ans: C (Third Law + Conservation of Momentum).

Q2. If the string of a lift breaks, the weight of the person inside becomes:

A) $mg$

B) $2mg$

C) Zero

D) Infinite

Ans: C (Free fall condition).

Q3. To reduce friction, we use:

A) Lubricants

B) Ball Bearings

C) Polishing

D) All of the above

Ans: D

Q4. A bullet of mass 10g is fired from a gun of mass 2kg with a velocity of 200 m/s. Calculate recoil velocity of gun.

A) 1 m/s

B) -1 m/s

C) 2 m/s

D) 0.5 m/s

Ans: B (Formula: $V = -\frac{0.01 \times 200}{2} = -1$ ).

Q5. Inertia depends on:

A) Velocity

B) Mass

C) Area

D) Volume

Ans: B (More mass = More Inertia).

10. Circular Dynamics: Centripetal & Centrifugal Force

Jab koi cheez gol ghoomti hai, toh Newton ke laws kaise lagte hain?

Centripetal Force (Abhikendra Bal): Center ki taraf lagne wala force. Iske bina circular motion impossible hai.
- Formula:
  $F_c = \frac{mv^2}{r}$
- Example: Chandrama (Moon) ka Prithvi ke chaaron taraf ghoomna (Gravitational force provides centripetal force).
Centrifugal Force (Apkendra Bal): Yeh ek Pseudo Force (Abhasi Bal) hai jo bahar ki taraf lagta hai.
- Example: Cream separator (Doodh se malai nikalna) ya Washing Machine ka dryer.

Banking of Roads (Sadak ka kinara uthana):

Ghumavdar mod (Turn) par gaadi fisle nahi, isliye sadak ka bahari kinara utha diya jata hai.

Safe Velocity Formula:
$v = \sqrt{rg \tan\theta}$
(Yahan friction ko neglect kiya gaya hai).

11. Connected Motion (Pulley & String Problems)

HP TGT mein aksar chitra (diagram) wale sawal aate hain jahan ek Pulley se do mass latke hote hain (Atwood Machine).

Maan lo do block hain: $m_1$ (bhari) aur $m_2$ (halka).

Acceleration ( $a$ ): Poore system ka acceleration kya hoga?
$a = \frac{(m_1 - m_2)g}{m_1 + m_2}$
Tension ( $T$ ): Rassi mein kitna tanav (tension) hoga?
$T = \frac{2m_1m_2g}{m_1 + m_2}$

Trick: Net Pulling Force / Total Mass = Acceleration.

12. Types of Equilibrium (Santulan)

Force lagne ke baad body kaise behave karti hai?

Stable Equilibrium: Body ko hilane par wo wapas apni jagah aa jaye. (Ex: Katori mein rakhi goli).
Unstable Equilibrium: Body ko hilane par wo door chali jaye. (Ex: Ulti katori par rakhi goli).
Neutral Equilibrium: Body nayi jagah par bhi santulan mein rahe. (Ex: Plane table par rakhi ball).

13. Collision (Takkar) - Conceptual

Jab do bodies takrati hain:

Elastic Collision: Momentum aur Kinetic Energy dono conserve rehte hain. (Ex: Ideal gas molecules ki takkar).
Inelastic Collision: Sirf Momentum conserve rehta hai, Kinetic Energy loss hoti hai (Heat/Sound mein). (Ex: Sadak par gaadiyon ki takkar).
Perfectly Inelastic: Takkar ke baad bodies chipak jati hain. (Ex: Keechad ka diwar par chipakna).

14. Angle of Friction & Angle of Repose

Friction ko gehrai se samjhein:

Angle of Friction ( $\theta$ ): $\tan \theta = \mu$ (Coefficient of friction).
Angle of Repose ( $\alpha$ ): Wo maximum angle jispar inclined plane (dhalan) par rakhi cheez fisalna shuru karde.
- Relation: Angle of Repose = Angle of Friction ( $\alpha = \theta$ ).

15. The Horse and Cart Problem (Ghoda-Gaadi)

Newton ka 3rd Law samjhane ke liye yeh classic example hai.

Sawal: Agar Action aur Reaction equal hain, toh ghoda gaadi ko kaise khinch pata hai? Wo toh cancel hone chahiye?
Jawab:
1. Ghoda apne pairo se zameen ko piche dhakelta hai (Action).
2. Zameen ghode ko aage dhakelti hai (Reaction).
3. Agar Zameen ka Reaction force > Gaadi ka Friction force, tabhi gaadi aage badhegi.
- Key Concept: Action aur Reaction alag-alag bodies par lagte hain, isliye cancel nahi hote.

Conclusion

Physics mein Laws of Motion base banata hai. Friction aur Lift wale questions par pakad mazboot karein kyunki numericals yahin se aate hain.

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HP TGT Physics Chapter 2: Force & Laws of Motion — Friction, Recoil & Lift Concepts