NCERT Solutions for Class 9th Science Chapter 6 .3 The Force of Friction: Often Overlooked but Always Present — Activity 6.1: Let us investigate

Book page 986 Updated on2026-09-19

Q1.

Do you find that after losing contact with the rubber band, the velocity of the stack of coins decreases gradually and it comes to rest after travelling some distance?

Answer

Yes. The stack slows down steadily and stops after covering some distance from C.

While in contact with the band: force by band > friction → net force forwards → stack accelerates
After losing contact: only friction acts, backwards → net force backwards → stack decelerates
Velocity falls from its highest value to 0 m s^–1, then the stack stops

Why it happens: the stretched rubber band gives the coins a forward push, taking their velocity from zero up to some value. The moment the band is no longer touching them, that push vanishes but friction between the coins and the table does not. Friction acts opposite to the motion, so it is now the only (and hence the net) horizontal force, and it removes velocity until the stack is at rest.

Q2.

Does the stack of coins travel a larger distance than it did on the wooden table top before coming to rest? Does its velocity decrease more slowly now?

Fig. 6.12 (b) and (c), page 98 — redrawn sketch of the Activity 6.1 set-up.

Answer

Yes to both. On the laminated top the stack goes further, and its velocity falls more gradually.

Why it happens: the rubber band is stretched to the same mark C every time, so the coins start with the same velocity in both trials. What changes is the surface. A laminated top is smoother than a wooden one, so the force of friction on the coins is smaller. Since a = F/m, a smaller retarding force gives a smaller deceleration, so the velocity drops more slowly and the coins travel a longer distance before reaching rest.

Tip: keeping A, B and C at exactly the same separations is what makes the comparison fair — it guarantees the same starting velocity in every trial.

Q3.

Does the stack of coins travel an even larger distance and its velocity decrease even more slowly?

Answer

Yes. On polished marble or tiles the distance is the largest of the three and the slowing is the gentlest.

Ranking of friction: wooden top > laminated top > polished marble/tile
Ranking of deceleration (a = F_friction ÷ m): wooden > laminated > marble
Ranking of distance travelled: marble > laminated > wooden

Why it happens: a polished marble or tiled floor is smoother still, so friction on the coins is smallest. The smaller the retarding force, the smaller the deceleration, and the longer the stack keeps moving before its velocity reaches zero.

Q4.

What conclusion do you draw from your observations?

Answer

The force of friction depends on the nature of the surfaces in contact — and the smaller the friction, the slower the velocity falls and the further the object travels before stopping.

Surface	Nature	Force of friction	Distance travelled from C
Wooden table top	Roughest of the three	Largest	Smallest
Laminated table top	Smoother	Smaller	Larger
Polished marble / tile	Smoothest	Smallest	Largest

Why it happens: the coins always start with the same velocity, so the only thing that can change the stopping distance is the retarding force. Different surfaces give different frictional forces, hence different decelerations and different distances. Extending the trend, a surface with zero friction would let the coins move on for ever — which is precisely the thought experiment on page 100 and the content of Newton's first law.

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