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NCERT Solutions for Class 9th Science Chapter 6 .5 Newton's Second Law of Motion — Pause and Ponder

Book page 1066 Updated on2026-09-08

Q6.
A toy car of mass 100 g is moving with a constant velocity of 0.5 m s–1. What is the net force acting on the toy car?
Answer

The net force is zero.

m = 100 g = 0.1 kg, velocity is constant at 0.5 m s–1
Change in velocity = 0 → a = 0 m s–2

F = ma
F = 0.1 kg × 0 m s–2
F = 0 N
Why the mass and the speed do not matter: Newton's second law connects force to acceleration, not to velocity. However fast the car is moving, if that velocity is not changing there is no acceleration, so there can be no net force. Forces such as friction may still act on the car, but something else must be balancing them exactly.
Tip: the numbers 100 g and 0.5 m s–1 are there to tempt you into a calculation. Read the word "constant" first — it settles the answer before any arithmetic.
Q7.
Two children of different masses are sitting on identical swings. To impart identical initial acceleration, for which child would you require to apply a larger force? Explain why.
Answer

You need the larger force for the heavier child — the one with the greater mass.

F = ma
Acceleration a is to be the same for both children
So F is directly proportional to m

Lighter child, m1: F1 = m1a
Heavier child, m2 (> m1): F2 = m2a > F1
Why it happens: mass measures how strongly an object resists a change in its motion — its inertia. To produce the same change of velocity in the same time, you must overcome more inertia in the heavier child, and that needs a bigger push.
Check it yourself: for a = 2 m s–2, a 20 kg child needs F = 20 kg × 2 m s–2 = 40 N, while a 30 kg child needs F = 30 kg × 2 m s–2 = 60 N — half as much again.
Q8.
How are glass items packed for transportation using a bubble wrap or hay protected from damage?
Answer

The bubble wrap or hay stretches out the stopping time of every jolt, and a longer stopping time means a smaller force on the glass.

During a bump, the glass has to lose its velocity: from u to 0
a = (v – u) ÷ t = (0 – u) ÷ t
F = ma = –mu ÷ t

The mass m and the velocity change u are fixed by the jolt
So larger t → smaller |a| → smaller |F|
Why it happens: a hard crate would stop the glass in a few milliseconds; the compressible bubbles or springy hay let it come to rest over a much longer time as they squash. The same change of velocity spread over a longer time means a much smaller acceleration, and by F = ma a much smaller force on the glass — small enough to stay below the force that would crack it. The padding also spreads the force over a larger area of the glass.
Did you know? Exactly the same principle protects a cricket fielder who pulls his hands back while catching, a car passenger saved by an airbag, and a high jumper landing on a foam mat.
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