Lesson–4 : Laws of Motion

Stage–3 (Part–VIII) : Integrated Thinking, Twisted Questions & Final Consolidation

🔹 PART–AQ : TWISTED CONCEPTUAL QUESTIONS (EXAMINER STYLE)

1. A body is moving in a straight line with constant speed. Can its velocity still change? Explain.

Yes. Velocity is a vector quantity. Even if speed remains constant, a change in direction will change velocity.

2. Two bodies of different masses are acted upon by the same force for the same time. Which gains more momentum? Explain.

Both bodies gain the same change in momentum because change in momentum = F × t, which is same for both.

3. Can a body have zero velocity but non-zero acceleration? Give an example.

Yes. At the highest point of vertical motion, velocity becomes zero but acceleration due to gravity is still acting.

🔹 PART–AR : LINKING LAWS OF MOTION TO DAILY LIFE

Seat belts → inertia + impulse
Airbags → increase time of collision
Rocket motion → Newton’s third law
Walking → friction + action–reaction
Jumping from boat → momentum conservation idea

Questions framed from daily life situations are very common in recent board exams.

🔹 PART–AS : MULTI-STEP REASONING (HOTS LEVEL)

1. Why is it easier to pull a roller than to slide a heavy box?

Rolling friction is much smaller than sliding friction. Hence, less force is required to pull a roller.

2. Why does a person pushing a wall not move backward?

Although the wall exerts an equal and opposite force, the friction between the person’s feet and the ground prevents backward motion.

🔹 PART–AT : COMBINATION NUMERICALS (THINK-FIRST TYPE)

1. A force of 80 N acts on a body of mass 16 kg for 5 s. Find (a) acceleration, (b) final velocity, (c) impulse.

Solution:

a = F/m = 80/16 = 5 m/s²
v = u + at = 0 + 5 × 5 = 25 m/s
Impulse = F × t = 80 × 5 = 400 N s

2. A man of mass 60 kg stands in a lift. If the lift is moving downward with uniform velocity, find the apparent weight.

Solution:

Uniform velocity ⇒ acceleration = 0
Apparent weight = mg = 60 × 10 = 600 N

🔹 PART–AU : DECISION-MAKING FLOW (HOW TO ATTACK ANY QUESTION)

Read the question slowly
Identify law involved (1st / 2nd / 3rd)
Check whether forces are balanced or unbalanced
Draw Free Body Diagram mentally or on paper
Apply F = ma only after clarity
Check direction and units

🔹 PART–AV : COMMON CONFUSIONS — FINAL CLEARANCE

Speed ≠ velocity
Zero force ≠ zero velocity
Action ≠ reaction on same body
Acceleration depends on net force, not individual forces

🔹 PART–AW : FINAL EXAM CONFIDENCE BOOST

If you can:
✔ Identify the correct law
✔ Draw or imagine FBD
✔ Apply F = ma confidently
✔ Use impulse logic where needed

Then NO question from Laws of Motion can trouble you.

Stage–3 (Part–VIII) Completed ✔
🏆 STAGE–3 (PART I–VIII) FULLY, FINALLY & PERFECTLY COMPLETED ✔✔✔✔✔✔✔✔

MIND GROW Magazine