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NCERT Solutions for Class 9th Science Chapter 6 .4 Newton's First Law of Motion — Pause and Ponder

Book page 1016 Updated on2026-09-08

Q3.
An object is moving with a constant velocity. Is there a net force acting upon it?
Answer

No. The net force on it is zero.

Constant velocity → no change in magnitude or direction of velocity
→ acceleration a = 0 m s–2
Net F = ma = m × 0 m s–2 = 0 N
Why it happens: a net force is what changes velocity, not what maintains it. Individual forces may well be acting — a car moving at a steady 60 km h–1 has engine thrust, friction, weight and the normal force on it — but they cancel one another exactly, leaving a zero net force.
Tip: "constant velocity" also covers being at rest (v = 0 held constant). Both cases mean zero acceleration and hence zero net force.
Q4.
Suppose, no net force is acting on an object. Which of the following situations are possible? (i) Object remains at rest if at rest. (ii) Object keeps moving with a constant velocity if already moving. (iii) Object is moving with a constant acceleration.
Answer

(i) and (ii) are possible. (iii) is not possible.

SituationPossible?Reason
(i) Remains at rest if at restYesZero net force means zero acceleration; a body at rest has no reason to start moving
(ii) Keeps moving with constant velocityYesZero acceleration means the velocity cannot change in magnitude or direction
(iii) Moving with constant accelerationNoAny acceleration a ≠ 0 needs a net force F = ma ≠ 0 — a contradiction
Why (iii) fails: acceleration and net force are locked together by F = ma. If a is constant and non-zero, then F = ma is constant and non-zero too. You cannot have acceleration without a net force to cause it.
Q5.
In the real world, it is difficult to find a situation where no forces are acting on an object. But by applying additional forces, a condition can be achieved where the net force on the object is zero. Explain with the help of an example.
Answer

You cannot remove forces, but you can balance them — add forces so that they cancel in pairs and the net force becomes zero.

Example — a book lying on a table.

Downward: weight of the book, F = mg
Upward: normal force N from the table
Net force = N – mg = 0 N, since N = mg
→ a = 0 m s–2 → the book stays at rest

Example — pushing a heavy box at a steady speed.

Forwards: your applied push, say 40 N
Backwards: force of friction, 40 N
Vertically: weight mg down, normal force N up, N = mg
Net force = 0 N in both directions → the box slides with constant velocity
Why it happens: Newton's first law is about the net force, not about individual forces. Four separate forces act on the pushed box, yet the object behaves exactly as if none acted, because they cancel in two opposite pairs. Other everyday examples: a tug of war where both teams pull equally hard, and a ball floating on water where the buoyant force balances the weight.
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