Lesson–4 : Laws of Motion

Stage–3 (Part–VI) : Concept Clarity, Tricky Questions & Final Edge

🔹 PART–AC : VERY COMMON CONFUSING CONCEPTS (CLEARLY EXPLAINED)

1. Is force required to keep a body in motion?

No. According to Newton’s First Law, a body continues to move with uniform velocity if no unbalanced external force acts on it. Force is required only to change the state of motion.

2. Can a body have velocity without force?

Yes. A body can move with constant velocity even when net force is zero.

3. Can a body have force without acceleration?

Yes. If forces are balanced, net force becomes zero and acceleration is zero even though forces are acting.

🔹 PART–AD : VERY IMPORTANT WHY-QUESTIONS (BOARD FAVOURITES)

1. Why does a person feel heavier in a lift moving upward?

When the lift accelerates upward, the normal reaction increases. Hence, the apparent weight of the person becomes greater than true weight.

2. Why do athletes land on sand beds?

Sand increases the time of contact, reducing the force of impact according to impulse–momentum principle.

3. Why does a gun recoil when a bullet is fired?

According to Newton’s Third Law, the forward motion of the bullet produces an equal and opposite reaction on the gun, causing recoil.

🔹 PART–AE : MULTI-STEP REASONING QUESTIONS

1. A heavy truck and a light car move with same speed. Which has more momentum? Explain.

Momentum depends on mass and velocity. Since truck has greater mass, it has greater momentum.

2. Two bodies of different masses experience the same force. Which body accelerates more?

From F = ma, acceleration is inversely proportional to mass. Hence, the lighter body accelerates more.

🔹 PART–AF : CONCEPT + NUMERICAL COMBINATION QUESTIONS

1. A force of 60 N acts on a body of mass 12 kg for 3 s. Find (a) acceleration, (b) change in velocity, (c) impulse.

Solution:

a = F/m = 60/12 = 5 m/s²
Δv = at = 5 × 3 = 15 m/s
Impulse = F × t = 60 × 3 = 180 N s

2. A man of mass 70 kg is standing in a lift at rest. Find his apparent weight.

Solution:

Since lift is at rest, acceleration = 0
Apparent weight = mg = 70 × 10 = 700 N

🔹 PART–AG : FREQUENT EXAM TRAPS (AVOID THESE)

Assuming force is needed to maintain motion
Thinking acceleration is zero at all times
Mixing up mass and weight
Writing action–reaction forces on same body
Ignoring Free Body Diagram

🔹 PART–AH : HOW TO SCORE FULL MARKS (FINAL INTERMEDIATE STRATEGY)

Use textbook language for definitions
Always draw a rough Free Body Diagram
Show all steps clearly in numericals
Write units for every physical quantity
Underline final answers

🔹 PART–AI : SELF-EVALUATION CHECKLIST

Can you explain all three laws with examples?
Can you solve lift problems without confusion?
Can you explain impulse with daily-life examples?
Can you identify action–reaction correctly?

Stage–3 (Part–VI) Completed ✔
🎯 STAGE–3 (PART I–VI) COMPLETELY FINISHED ✔✔✔✔✔✔

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