Newton's Three Laws of Motion

• First law (inertia) — body stays at rest or uniform motion unless acted upon by unbalanced force.
• Second law — F = ma; force equals mass times acceleration.
• Third law — every action has equal and opposite reaction (forces on different bodies).
• Applications: seat belts, recoiling gun, walking, rocket propulsion.

1. First law — inertia; no change in state without net external force.
2. Inertia types — inertia of rest, motion, direction.
3. Second law — F_net = ma; 1 N = 1 kg × 1 m/s².
4. Third law — F_AB = −F_BA; action and reaction act on different objects.
5. Balanced forces — net F = 0 → no acceleration (may be at rest or constant velocity).
6. Diagram (text) — person pushing wall: wall pushes person back with equal force.
7. NCERT Ch. 9 — hockey puck on ice vs rough ground; cricketer catching ball.
8. Real world — airbags increase time to stop → reduce force on passenger (F = Δp/Δt).

Finding acceleration when a 5 kg block is pushed with 20 N net force

Given: m = 5 kg, F_net = 20 N
Step 1 — Newton's 2nd law: F = ma → a = F/m
Step 2 — a = 20/5 = 4 m/s² in direction of net force
Step 3 — If friction 5 N opposes motion, applied force must be 25 N for same acceleration.
Step 4 — Third law: block pushes floor back with 20 N horizontal force.

• Thinking action–reaction forces cancel on the same body (they act on different bodies).
• Misconception: heavier objects always fall faster (Galileo: same g in vacuum).
• Confusing mass (kg) with weight (N).
• Assuming constant velocity needs a force (needs zero net force).
• Applying F = ma when forces are not balanced.

• State Newton's three laws in your own words.
• Why do we jerk a tablecloth while keeping dishes on it?
• A 10 N force gives 2 m/s² acceleration. Find mass.
• Give one example of Newton's third law.

Open the Practice tab for graded questions on Newton's Three Laws of Motion.