Suspend a metre scale on a knife-edge through its centre of gravity. Place the unknown mass at one end; balance with a known mass on the other side. By the principle of moments, m_unknown × d_unknown = m_known × d_known. Repeat with different distances and average the result.
-- NCERT Physics Lab Manual Class 11, Part 8, p. 128Experiment 04 Metre Scale Moments
8 MCQs9-step worked example
Source: NCERT Experimental SkillsOfficial key: NTA-verifiedLast updated: 25 Sep 2026
Experiment 04 Metre Scale Moments, explained for NEET
The moment that costs marks here is assuming the metre scale balances at the 50.0 cm graduation. It balances at its centre of gravity, and a real wooden scale — varnished unevenly, drilled, slightly worn at one end — sits a millimetre or two off centre. The NCERT Physics Laboratory Manual has you find that balancing point first, with nothing hanging on the scale, precisely because it is not assumed.
Once the knife edge sits at the centre of gravity, the weight of the scale acts straight down through the pivot. Its moment arm is zero, so it drops out of the equation, and the balance condition reduces to the principle of moments applied to the two hanging loads only:
m·d₁ = M·d₂
with m the unknown mass at distance d₁ from the knife edge and M the known mass at d₂ on the other side. Distances are measured from the knife edge, never from the zero of the scale.
The second arrangement in the manual deliberately moves the knife edge away from the centre of gravity and hangs only the unknown body. Now the scale's own weight supplies the restoring moment, and it must be written in. Students who carry the first arrangement's equation into the second one silently set a real moment to zero.
For NEET this appears as a short two-step numerical from the measurement-and-error family: read the two arm lengths off a diagram, substitute, report. Both arms are read on a metre scale graduated in millimetres, so three significant figures is what the data supports — a calculator reading 74.99999 g is still 75.0 g.
Watch-out: an arm shortens by the thickness of the thread loop if you read to the graduation rather than to the actual point of suspension, and a heavy body on a short arm magnifies that slip.
Can you answer these Experiment 04 Metre Scale Moments MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
For a metre scale resting on a knife edge and carrying loads, the principle of moments states that at balance:
Show answer and why every option is right or wrong
Answer: C. Option C is the principle of moments as stated for this experiment in the NCERT Physics Lab Manual Class 11, Part 8, page 128 — rotational equilibrium means the two opposing moment sums are equal, not that each moment vanishes.
Why A is wrong: A is wrong because that is the condition for translational equilibrium; a scale can have zero net force on it and still turn about the knife edge.
Why B is wrong: B is wrong because only the scale's own weight passes through the knife edge in the first arrangement, and the hanging loads certainly do not.
Why D is wrong: D is wrong because each hanging load exerts a genuinely non-zero moment; it is the two opposing sums that cancel.
MCQ 2Easy RecallPractice
In this experiment, the moment of a hanging weight about the knife edge is calculated as:
Show answer and why every option is right or wrong
Answer: A. Option A is the definition of a moment used throughout the balancing procedure in the NCERT Physics Lab Manual Class 11, Part 8, page 128.
Why B is wrong: B is wrong because dividing by distance would make a load hung far out less effective than one hung close in, the opposite of what the balancing trials show.
Why C is wrong: C is wrong because the lever arm is the distance from the knife edge to the point of suspension, not a fixed property of the scale.
Why D is wrong: D is wrong because the moment is linear in distance; a squared distance appears nowhere in this experiment.
MCQ 3Easy RecallPractice
Before any load is hung, the bare metre scale is balanced on the wedge in order to locate:
Show answer and why every option is right or wrong
Answer: D. Option D: the balancing point of the unloaded scale is its centre of gravity, and the whole purpose of finding it experimentally is that the scale's weight acts there — the first step of the procedure in the NCERT Physics Lab Manual Class 11, Part 8, page 128.
Why A is wrong: A is wrong because the centre of gravity of a real metre scale need not fall on the 50.0 cm graduation; assuming that it does is exactly the error the preliminary balancing trial removes.
Why B is wrong: B is wrong because the zero mark is an end graduation and has nothing to do with where the scale balances.
Why C is wrong: C is wrong because the smallest resolvable division is fixed by how the scale is engraved and is not found by balancing it.
MCQ 4Direct ApplicationPractice
A metre scale is balanced on a knife edge placed at its centre of gravity. A body of unknown mass hangs 20.0 cm from the knife edge on one side, and a known mass of 50.0 g hangs 40.0 cm from the knife edge on the other side, the scale being horizontal. The unknown mass is:
Show answer and why every option is right or wrong
Answer: B. Option B: balancing moments about the knife edge, m × 20.0 = 50.0 × 40.0, so m = 1.00 × 10² g — the relation applied in the NCERT Physics Lab Manual Class 11, Part 8, page 128.
Why A is wrong: A is wrong because it comes from inverting the arm ratio, 50.0 × 20.0 / 40.0, which puts the longer arm under the known load instead of over it.
Why C is wrong: C is wrong because it simply copies the known mass, ignoring that the two arms are unequal.
Why D is wrong: D is wrong because it applies the ratio twice over; only one factor of d₂/d₁ belongs in the balance condition.
MCQ 5Concept TrapPractice
With the knife edge set exactly at the centre of gravity of the metre scale, the weight of the scale itself is absent from the balance equation. The reason is that:
Show answer and why every option is right or wrong
Answer: D. Option D: a force acting through the pivot has zero perpendicular distance and therefore zero moment, which is why the knife edge is set at the centre of gravity for the first arrangement in the NCERT Physics Lab Manual Class 11, Part 8, page 128.
Why A is wrong: A is wrong because the scale's mass is real, and it is used as the known mass in the second arrangement of this same experiment.
Why B is wrong: B is wrong because the normal reaction also acts at the knife edge, so it too has zero moment there; no cancellation of moments is taking place.
Why C is wrong: C is wrong because the scale's weight is not distributed between the loads at all, and no such sharing appears in the balance condition.
MCQ 6Direct ApplicationPractice
In the second arrangement, the knife edge is shifted away from the centre of gravity and only the unknown body is hung, 16.0 cm from the knife edge. The metre scale, of mass 1.00 × 10² g, now balances with its centre of gravity 4.0 cm from the knife edge on the opposite side. The mass of the body is:
Show answer and why every option is right or wrong
Answer: A. Option A: the scale's own weight now supplies the balancing moment, so m × 16.0 = 1.00 × 10² × 4.0, giving m = 2.5 × 10¹ g — the second method described in the NCERT Physics Lab Manual Class 11, Part 8, page 128.
Why B is wrong: B is wrong because it inverts the arms, using 1.00 × 10² × 16.0 / 4.0 and placing the scale's weight on the short arm.
Why C is wrong: C is wrong because it multiplies the two distances together, which is not a moment balance at all.
Why D is wrong: D is wrong because it squares the arm ratio, 1.00 × 10² × (4.0/16.0)² = 6.25 g; moments are linear in the arm length.
MCQ 7Direct ApplicationPractice
Balance is obtained with a known mass of 20.0 g at 35.0 cm from the knife edge and the unknown body at 25.0 cm on the other side, the knife edge being at the centre of gravity. Both arm lengths are read on a metre scale graduated in millimetres. The mass of the body should be reported as:
Show answer and why every option is right or wrong
Answer: C. Option C: m = 20.0 × 35.0 / 25.0 = 28.0 g, and with every input quoted to three significant figures the result is reported to three, following the reporting convention of the NCERT Physics Lab Manual Class 11, Part 8, page 128.
Why A is wrong: A is wrong because it uses 20.0 × 25.0 / 35.0, swapping the arms so that the known mass sits on the longer one.
Why B is wrong: B is wrong because a fourth significant figure claims a precision the millimetre graduations of the metre scale cannot support.
Why D is wrong: D is wrong because the arithmetic slips by a factor of ten; 35.0 / 25.0 is 1.40, not 0.140.
MCQ 8CalculationPractice
An unloaded metre scale of mass 1.00 × 10² g balances at its 49.0 cm mark. A student nevertheless sets the knife edge at the 50.0 cm mark, hangs the unknown body at the 30.0 cm mark and a known 40.0 g mass at the 90.0 cm mark, and finds the scale horizontal. Taking the scale's own weight properly into account, the mass of the body is:
Show answer and why every option is right or wrong
Answer: B. Option B: the centre of gravity lies 1.0 cm from the knife edge on the same side as the body, so m × 20.0 + 1.00 × 10² × 1.0 = 40.0 × 40.0, giving m = 75.0 g, the off-centre case treated in the NCERT Physics Lab Manual Class 11, Part 8, page 128.
Why A is wrong: A is wrong because it puts the scale's moment on the far side, as though the centre of gravity lay beyond the known mass.
Why C is wrong: C is wrong because it is the value obtained by ignoring the scale's weight altogether, which is only legitimate when the knife edge sits at the centre of gravity.
Why D is wrong: D is wrong because it overcorrects, using a 2.0 cm offset when the centre of gravity is only 1.0 cm from the knife edge.
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How do you solve a Experiment 04 Metre Scale Moments question? A worked example
- 1
Given.
A metre scale balances, unloaded, on a knife edge placed at its centre of gravity. A body of unknown mass is suspended 24.0 cm from the knife edge on the left; a known mass M = 50.0 g is suspended 36.0 cm from the knife edge on the right. The scale is horizontal at balance.
- 2
Required.
The mass m of the body.
- 3
Concept.
With the knife edge at the centre of gravity, the scale's weight acts through the pivot and contributes no moment. Only the two hanging loads produce moments, and at balance the anticlockwise sum equals the clockwise sum (NCERT Physics Lab Manual Class 11, Part 8, page 128).
- 4
Formula.
m·g·d₁ = M·g·d₂, so m = M d₂ / d₁. The g cancels, which is why this experiment yields a mass without ever needing a value for g.
- 5
Substitution.
m = (50.0 g × 36.0 cm) / 24.0 cm.
- 6
Calculation.
50.0 × 36.0 = 1.80 × 10³; 1.80 × 10³ / 24.0 = 75.0. The knife-edge position is the defined origin from which both arms are measured and is exact by construction, as is the cancelled factor g; neither contributes to the significant-figure count. The three measured inputs each carry three significant figures.
- 7
Final answer.
m = 75.0 g.
- 8
Common trap.
Reading d₁ and d₂ as graduation numbers instead of as distances from the knife edge. If the knife edge sits at the 50.0 cm mark, a load at the 26.0 cm mark has an arm of 24.0 cm, not 26.0 cm. Subtract every time — and subtract from the measured balancing point, not from 50.0 cm.
- 9
Similar NEET-style question.
The same scale and the same 50.0 g known mass are used, but the known mass is moved to 30.0 cm from the knife edge and balance is restored by sliding the body to 20.0 cm. What mass does the body now register? (Answer: 75.0 g — unchanged, because only the ratio d₂/d₁ enters, and 30.0 / 20.0 = 36.0 / 24.0.)
What to remember before solving Experiment 04 Metre Scale Moments questions
More in Experimental Skills: 6 exam traps and mistakes · 3 formulas · 1 question pattern from its other lessons.
Experiment 04 Metre Scale Moments questions from past NEET papers
No question in our NEET 2020–2025 set targets this topic directly.
Sources
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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