NCERT Solutions for Class 9th Science Chapter 6 .3 The Force of Friction: Often Overlooked but Always Present — Activity 6.2: Let us measure

Book page 996 Updated on2026-09-19

Q1.

Pull the spring balance with gradually increasing force and note down the reading on it when the block just starts moving. What does this reading indicate?

Answer

It indicates the magnitude of the force of friction acting on the block from that surface (an approximate value of the largest friction the surface can supply before sliding begins).

Forces on the block, horizontal: pull by spring (forwards), friction (backwards)
At the instant of "just starting to move" the velocity is not yet changing
→ acceleration ≈ 0 → net force ≈ 0
→ F_spring = F_friction
So the scale reading = force of friction, in newtons

Tip: pull steadily and horizontally. A jerky or slanted pull adds a vertical component and spoils the reading.

Q2.

If the velocity of the block is neither increasing nor decreasing, what can you say about the net force acting on the block?

Fig. 6.14, page 99 — redrawn sketch: a block being pulled by a spring balance.

Answer

The net force on the block is zero.

Velocity constant → change in velocity = 0
→ acceleration a = 0 m s^–2
F = ma = m × 0 = 0 N

Why it happens: this is Newton's first law read backwards. Constant velocity — including the special case of being at rest — is the signature of zero net force. The block still has forces on it (your pull, friction, its weight, the normal force), but they cancel in pairs, so nothing is left over to change its velocity.

Q3.

Does the reading of the spring balance indicate the magnitude of the force of friction acting on the wooden block?

Answer

Yes — approximately. Because the net force is zero, the spring's pull and friction must be equal in magnitude and opposite in direction, so the scale reading gives the frictional force.

Why "approximately": the block does not move with a perfectly steady velocity in a hand-held pull, the spring balance itself has a limited least count, and friction changes slightly the moment sliding begins. So the reading is a good estimate, not an exact value. It is still perfectly good for comparing surfaces, which is what the activity needs.

Q4.

Compare the readings of the spring balance for all surfaces. Are the readings different? Is the reading smallest for the surface on which the stack of coins travelled the largest distance? Is the reading largest for which the distance travelled was the smallest?

Answer

Yes to all three. The readings differ from surface to surface, and they line up exactly with Activity 6.1.

Surface	Distance in Activity 6.1	Spring balance reading in Activity 6.2	Force of friction
Polished marble / tile	Largest	Smallest	Smallest
Laminated table top	Medium	Medium	Medium
Wooden table top	Smallest	Largest	Largest

Why the two activities agree: Activity 6.1 measures friction indirectly — a bigger friction means a bigger deceleration (a = F_friction/m) and hence a shorter stopping distance. Activity 6.2 measures the same friction directly in newtons. Both must, and do, rank the surfaces in the same order. That agreement is what makes the conclusion trustworthy.

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