GOLDEN RULES
- Always draw FBD first before writing any equation.
- Action-Reaction check: Do the two forces act on DIFFERENT bodies? Yes → Third Law pair.
- Static friction self-adjusts: Compare F_applied with μ_s N first!
- Inclined plane: N = mg cos θ (NOT mg). Weight component along plane = mg sin θ.
- Centripetal force is NOT extra: It is provided by existing forces (friction/tension/gravity).
CRITICAL FORMULAS
| Situation | Formula |
|---|---|
| Second Law | F = ma = dp/dt |
| Momentum | p = mv ; conserved if F_ext = 0 |
| Impulse | J = F = |
| Lift (up) | W' = m(g + a) |
| Lift (down) | W' = m(g − a) |
| Lift (free fall) | W' = 0 |
| Atwood a | (m_{1}−m_{2})g / (m_{1}+m_{2}) |
| Atwood T | 2m_{1}m_{2}g / (m_{1}+m_{2}) |
| Static friction | 0 ≤ f_s ≤ μ_s N |
| Kinetic friction | f_k = μ_k N |
| Angle of repose | tan θ = μ_s |
| Level road max speed | v = √(μrg) |
| Banking (no friction) | tan θ = /(rg) |
DIMENSION TABLE
| Quantity | Dimension | SI Unit |
|---|---|---|
| Force | [M^{1}$$L^{1}$$T^{-2}] | N |
| Momentum | [M^{1}$$L^{1}$$T^{-1}] | kg·m/s |
| Impulse | [M^{1}$$L^{1}$$T^{-1}] | N·s |
| μ (friction coeff.) | [dimensionless] | — |
TOP 3 NEET TRAPS
- Trap 1: N and mg are NOT an action-reaction pair (same body).
- Trap 2: f_s ≠ μ_s N always. Check if F_applied < μ_s N first.
- Trap 3: On an incline, N = mg cos θ, not mg.