Lesson–4 : Laws of Motion

Stage–3 (Part–V) : Model Questions, Exam Writing & Final Consolidation

🔹 PART–W : MODEL BOARD-EXAM QUESTIONS (VERY IMPORTANT)

1. Define momentum. State its SI unit. Derive the relation between force and acceleration.

Answer:

Momentum is defined as the product of mass and velocity of a body.

Mathematically, p = mv
SI unit of momentum is kg m s⁻¹.

According to Newton’s Second Law, force is proportional to the rate of change of momentum.

F ∝ (mv − mu)/t = ma
Hence, F = ma.

2. State Newton’s three laws of motion and explain any one law with a suitable example.

Answer:

First Law – Law of inertia
Second Law – Relation between force, mass, and acceleration
Third Law – Action and reaction

Example: When a bus starts suddenly, a passenger falls backward due to inertia of rest.

🔹 PART–X : NUMERICALS (MODEL PAPER LEVEL)

1. A force of 25 N acts on a body of mass 5 kg for 4 s. Find (a) acceleration, (b) change in velocity, (c) impulse.

Solution:

a = F/m = 25/5 = 5 m/s²
Change in velocity, Δv = at = 5 × 4 = 20 m/s
Impulse = F × t = 25 × 4 = 100 N s

2. A man of mass 50 kg stands in a lift. Find his apparent weight when the lift is:

(a) moving upward with acceleration 3 m/s²
(b) moving downward with acceleration 3 m/s²

Solution:

(a) N = m(g + a) = 50(10 + 3) = 650 N
(b) N = m(g − a) = 50(10 − 3) = 350 N

🔹 PART–Y : VERY IMPORTANT THEORY QUESTIONS

1. Why is Newton’s Second Law more fundamental than the First Law?

Newton’s Second Law gives the quantitative definition of force and explains how much force is required to change motion, whereas the First Law is only qualitative.

2. Explain the concept of apparent weight.

Apparent weight is the force exerted by the surface on a body. It changes when the body is accelerating, as in the case of a lift.

🔹 PART–Z : HOW TO WRITE PERFECT ANSWERS (EXAMINER VIEW)

Start with definition (1 mark)
Write correct formula (1 mark)
Substitute values with units (2 marks)
Final answer clearly boxed (1 mark)
Neat presentation = bonus impression

🔹 PART–AA : LAST-MINUTE REVISION POINTS

Mass measures inertia
Net force causes acceleration
Impulse = change in momentum
Action and reaction act on different bodies
Apparent weight varies in lift motion

🔹 PART–AB : CONFIDENCE CHECK (SELF-TEST)

Can you derive F = ma without seeing notes?
Can you solve lift problems in one attempt?
Can you explain inertia with daily-life examples?

Stage–3 (Part–V) Completed ✔
🎯 STAGE–3 (PART I–V) FULLY COMPLETED ✔✔✔✔✔

MIND GROW Magazine