Laws of Motion – Study Notes (NEET Physics)
Purpose: Master the fundamentals of Newton’s three laws, common applications, and problem‑solving techniques.
Time: 10‑15 min review each day.
1. Simple Explanation
| Concept | What it means |
|---|---|
| Inertia | Tendency of an object to keep its state of motion (rest ↔ uniform motion). |
| Force | Any interaction that can change an object’s motion. Represented by F (vector). |
| Mass | Measure of an object’s inertia. |
| Acceleration | Rate of change of velocity. a = Δv / Δt. |
Newton’s laws relate these concepts:
| Law | Statement | Key Takeaway |
|---|---|---|
| 1st | An object remains at rest or in uniform motion unless acted upon by a net external force. | No net force → no change in motion. |
| 2nd | F = m a. The net force equals mass times acceleration. | Force is what changes motion; heavier objects need more force for the same acceleration. |
| 3rd | For every action, there is an equal and opposite reaction. | Interactions always come in pairs. |
2. Key Points
| Topic | Important Details |
|---|---|
| Vector Nature | Forces, accelerations, and velocities are vectors; direction matters. |
| Net Force (ΣF) | Sum of all forces (vector addition). 0 → static equilibrium. |
| Mass vs Weight | Mass (kg) is invariant; weight = mg (force due to gravity). |
| Inertial vs Non‑Inertial Frames | Newton’s laws hold in inertial frames (no acceleration of the reference frame). |
| Friction | 1) Static friction (μs N) prevents motion up to a max value. 2) Kinetic friction (μk N) opposes motion once sliding starts. |
| Tension, Normal, Drag | Common forces encountered in NEET problems. |
| Impulse | (J = \Delta p = F_{\text{avg}}\Delta t). Useful for collisions. |
| Work & Energy | Work = F·d; kinetic energy (K = \tfrac12 mv^2). Useful for converting between force and velocity. |
3. Worked Example
Problem
A 5 kg block is pulled across a horizontal surface with a constant horizontal force of 40 N. The coefficient of kinetic friction is μk = 0.2. Find:
- The block’s acceleration.
- Its velocity after 3 s, starting from rest.
Step 1 – Draw a Free‑Body Diagram (FBD)
↑ N (normal)
|
F (pull) → ──────►
|
↓ mg (weight)
- Normal force (N) balances weight: N = mg = 5 kg × 9.8 m/s² = 49 N.
- Kinetic friction (f_k): ( f_k = μ_k N = 0.2 \times 49 = 9.8 , \text{N} ).
- Net horizontal force (ΣF): ( F_{\text{net}} = 40,\text{N} - 9.8,\text{N} = 30.2,\text{N} ).
Step 2 – Apply Newton’s 2nd Law
( F_{\text{net}} = m a )
( a = \frac{F_{\text{net}}}{m} = \frac{30.2}{5} = 6.04 , \text{m/s}^2 ).
Step 3 – Find Velocity after 3 s
Starting from rest (v₀ = 0):
( v = v_0 + a t = 0 + 6.04 \times 3 = 18.12 , \text{m/s} ).
Answers
- Acceleration ≈ 6.0 m/s².
- Velocity after 3 s ≈ 18.1 m/s.
4. Quick Revision Summary
| Topic | What to remember |
|---|---|
| Newton’s 1st Law | No net force → no change in velocity. |
| Newton’s 2nd Law | ( F_{\text{net}} = m a ); direction of F = direction of a. |
| Newton’s 3rd Law | Action ↔ Reaction; forces appear in equal & opposite pairs. |
| Friction | μs N (static, ≤ μs N) and μk N (kinetic). |
| Impulse | ( J = \Delta p ); useful in collision problems. |
| Conservation of Energy | ( \tfrac12 mv^2 + mgh = \text{constant} ) (in absence of non‑conservative forces). |
| Common Mistakes | 1) Treating friction as a fixed number; 2) Forgetting vector directions; 3) Ignoring the normal force when inclined planes are involved. |
Final Tips for NEET
- Practice FBDs – they’re the backbone of any dynamics problem.
- Use SI units consistently (kg, m, s).
- Check units of force (N = kg·m/s²) and acceleration (m/s²).
- Keep a “force‑balance” sheet: list all forces, sign conventions, and net force.
- Solve at least 5–10 problems per law each week to build fluency.
Happy studying!