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NCERT Solutions for Class 9th Science Chapter 6 .3 The Force of Friction: Often Overlooked but Always Present — In-text Questions

Book page 976 Updated on2026-09-08

Q1.
Will the force applied by you make the object move? Many a times, you might have experienced that on applying a force on an object, it did not move and you had to apply a larger force to move it. Why is it so?
Answer

Not necessarily. The object moves only if your push is larger than the force of friction.

Applied force Fapplied acts forwards
Force of friction Ffriction acts backwards, between the bottom of the object and the floor
Net force = Fapplied – Ffriction
If Fapplied ≤ Ffriction → net force is zero → the object stays at rest
If Fapplied > Ffriction → net force is forwards → the object accelerates forwards
Why it happens: friction arises between the bottom surface of the object and the floor and always acts opposite to the direction in which the object tends to move. Up to a limit, it simply matches whatever you apply, so the two forces stay balanced and nothing happens. Only once you exceed that limit does a net force appear, and the object starts to move.
Q2.
Does it mean that you need to continuously apply a force to keep it moving?
Answer

Yes — but only because friction is present. The force is needed to cancel friction, not to keep the motion going.

To keep constant velocity: Fapplied = Ffriction
→ net force = 0 → no acceleration → velocity stays the same
Stop applying the force: net force = Ffriction backwards → the object decelerates and stops
Why it happens: this is exactly what Newton's first law says. Motion itself needs no force to continue; only a change of velocity needs one. On a real floor friction keeps trying to change the velocity, so you must keep supplying an equal and opposite force to hold the net force at zero. On a perfectly frictionless floor no force at all would be needed once the object was moving.
Did you know? For ages people believed a force was needed to keep an object moving at all. In the 17th century Galileo Galilei argued, from thought experiments, that if all impediments to motion are removed a body would continue moving indefinitely.
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