Lesson–4 : Laws of Motion

Stage–3 (Part–IV) : Multi-Concept Problems, Traps & Exam Mastery

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🔹 PART–Q : MULTI-CONCEPT NUMERICAL PROBLEMS

1. A body of mass 10 kg is initially at rest. A constant force of 20 N acts on it for 5 s. Find (a) final velocity, (b) change in momentum.

Solution:

Acceleration, a = F/m = 20/10 = 2 m/s²
Final velocity, v = u + at = 0 + 2×5 = 10 m/s

Change in momentum = m(v − u) = 10 × 10 = 100 kg m s⁻¹

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2. A man of mass 60 kg stands in a lift. Calculate his apparent weight when the lift:

• (a) moves upward with acceleration 2 m/s²
• (b) moves downward with acceleration 2 m/s²

Solution:

(a) N = m(g + a) = 60(10 + 2) = 720 N

(b) N = m(g − a) = 60(10 − 2) = 480 N

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🔹 PART–R : TRAP-BASED QUESTIONS (VERY IMPORTANT)

1. A body is moving with constant velocity. Is force acting on it? Explain.

Yes. Forces may be acting on the body, but they are balanced. Hence, net force is zero and velocity remains constant.

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2. Can action and reaction ever act on the same body?

No. Action and reaction forces always act on different bodies. Hence, they can never cancel each other.

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3. If net force is zero, can a body have acceleration?

No. According to Newton’s Second Law, acceleration is directly proportional to net force. Zero force implies zero acceleration.

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🔹 PART–S : BOARD EXAM LONG ANSWER PRACTICE (8 MARK STYLE)

Question:
State and explain Newton’s three laws of motion. Explain their significance with suitable examples.

Answer Outline:

• First law – Law of inertia (examples from daily life)
• Second law – Relation between force, mass and acceleration
• Third law – Action and reaction forces
• Applications in walking, swimming, vehicles, rockets

Proper explanation + examples = full marks.

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🔹 PART–T : COMMON STUDENT ERRORS (AVOID THESE)

• Assuming force is needed to maintain motion
• Forgetting Free Body Diagram
• Using wrong sign for acceleration in lift problems
• Mixing up momentum and velocity
• Writing action–reaction on same body

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🔹 PART–U : HOW TOPPERS APPROACH THIS CHAPTER

• Visualize forces before writing equations
• Write known, required, formula clearly
• Use F = ma confidently after FBD
• Revise lift & impulse problems multiple times

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🔹 PART–V : FINAL QUICK CHECKLIST (BEFORE EXAM)

• Can you state all three laws exactly?
• Can you draw correct Free Body Diagram?
• Can you solve lift problems without confusion?
• Do you remember impulse–momentum relation?

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Stage–3 (Part–IV) Completed ✔
STAGE–3 FULLY COMPLETED ✔✔✔✔