Question 11
According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the roadside, it will probably not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move.
The student's justification is incorrect since action-reaction force always apply to different bodies and hence cannot cancel each other. When two equal and opposite forces act on the same body, they cancel each other.
As, the truck has a very high mass, the static friction between the road and the truck is high. When pushing the truck with a small force, the frictional force cancels out the applied force and the truck does not move. Hence, the truck will only move if a force greater than the friction force is applied to it.
Chapter 8: Force and Laws of Motion — Quick Reference
Quick Revision Points
- Balanced forces: net force = 0, no acceleration. Unbalanced: net force ≠ 0, causes acceleration
- First Law (Inertia): object stays at rest/uniform motion unless external force acts
- Momentum p = mv (kg·m/s); Second Law: F = ma = Δp/Δt
- 1 N = force giving 1 kg an acceleration of 1 m/s²
- Third Law: every action has equal and opposite reaction (on different bodies)
- Conservation of momentum: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂ (no external force)
- Read the detailed chapter notes for complete coverage of all NCERT topics.
- Practice all NCERT in-text and back exercise questions — they are frequently asked in exams.
- Focus on comparison tables, diagrams, and definitions — these are high-scoring areas.
- For numericals (if applicable), practice at least 20 problems of varying difficulty.
- Refer to the practice question bank (200+ questions) for thorough preparation.
- Detailed Notes: ch08-force-and-laws-of-motion.html
- Practice Questions: 100+ questions with answers in 05-practice-questions/
- Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
- Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html
Chapter 7: Motion — Quick Reference
Quick Revision Points
- Distance (scalar, total path) vs Displacement (vector, shortest path)
- Speed = distance/time (scalar); Velocity = displacement/time (vector)
- Acceleration a = (v−u)/t; unit: m/s²
- Equations: v = u + at; s = ut + ½at²; v² = u² + 2as
- d-t graph slope = speed; v-t graph slope = acceleration; area under v-t = distance
- 1 km/h = 5/18 m/s; Free fall: u = 0, a = g = 9.8 m/s²
- Uniform circular motion: constant speed, changing velocity (direction changes)
- Read the detailed chapter notes for complete coverage of all NCERT topics.
- Practice all NCERT in-text and back exercise questions — they are frequently asked in exams.
- Focus on comparison tables, diagrams, and definitions — these are high-scoring areas.
- For numericals (if applicable), practice at least 20 problems of varying difficulty.
- Refer to the practice question bank (200+ questions) for thorough preparation.
- Detailed Notes: ch07-motion.html
- Practice Questions: 100+ questions with answers in 05-practice-questions/
- Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
- Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html