standard-units-of-length MCQs for UPSC Prelims

25 practice questions on standard-units-of-length from the Measurement of Length and Motion section of the UPSC Prelims syllabus. 25 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.

7 Easy 13 Medium 5 Hard

Sample questions

Q1
easy

Why is it useful to estimate a length before measuring it?

  1. A A measured value far from the estimate warns you that something has gone wrong
  2. B Estimating removes the need to measure at all
  3. C Estimating tells you what material the object is made of
  4. D An estimate is always more accurate than a measurement
Show answer and explanation

Correct answer: A - A measured value far from the estimate warns you that something has gone wrong

An estimate sets a rough expectation, so a measurement far away from it points to a mistake such as a misread scale. Estimating does not replace measuring, and it is less accurate, not more. It says nothing about what an object is made of.

Q2
easy

How many centimetres make up $1\ \text{m}$?

  1. A $100\ \text{cm}$
  2. B $1000\ \text{cm}$
  3. C $10\ \text{cm}$
  4. D $60\ \text{cm}$
Show answer and explanation

Correct answer: A - $100\ \text{cm}$

One metre is divided into $100$ equal centimetres. $10$ is the number of millimetres in one centimetre. $1000\ \text{cm}$ would be $10\ \text{m}$. The number $60$ belongs to time, not to length.

Q3
hard

Ishaan must record the width of a hair-thin wire, the length of a cricket pitch and the distance run in a marathon. In that order, the most suitable units are

  1. A millimetre, kilometre, metre
  2. B centimetre, kilometre, metre
  3. C millimetre, metre, kilometre
  4. D metre, centimetre, kilometre
Show answer and explanation

Correct answer: C - millimetre, metre, kilometre

A hair-thin wire is well under a centimetre wide, so millimetres suit it. A cricket pitch is about $20\ \text{m}$ long, so metres suit it. A marathon is about $42\ \text{km}$, so kilometres suit it. Every other choice puts at least one of these three lengths in a unit far too big or far too small.

Q4
medium

A ribbon is $4\ \text{m}$ and $25\ \text{cm}$ long. What is its length in centimetres?

  1. A $425\ \text{cm}$
  2. B $4025\ \text{cm}$
  3. C $4.25\ \text{cm}$
  4. D $29\ \text{cm}$
Show answer and explanation

Correct answer: A - $425\ \text{cm}$

Four metres is $400\ \text{cm}$, and adding the extra $25\ \text{cm}$ gives $425\ \text{cm}$. Writing $25$ after $40$ hundreds gives the wrong value $4025\ \text{cm}$. Adding $4$ and $25$ treats metres and centimetres as the same unit. $4.25$ is the length in metres, not in centimetres.

Q5
medium

For which of these lengths is the metre the most suitable unit?

  1. A The width of a pencil lead
  2. B The length of a train journey from Mumbai to Pune
  3. C The height of a classroom door
  4. D The thickness of a sheet of paper
Show answer and explanation

Correct answer: C - The height of a classroom door

A door is about $2\ \text{m}$ tall, so metres fit it neatly. A pencil lead is about a millimetre wide, and a sheet of paper is thinner still, so both call for millimetres. A train journey of more than a hundred kilometres calls for kilometres.

Q6
hard

Without any scale in hand, which is the best way to estimate the length of a school corridor?

  1. A Guess a number that sounds large enough for a corridor
  2. B Walk along it counting steps, then multiply by the known length of one step
  3. C Measure one wall of a classroom and call the corridor the same length
  4. D Count the floor tiles and call that number the length in metres
Show answer and explanation

Correct answer: B - Walk along it counting steps, then multiply by the known length of one step

Pacing gives a count, and multiplying that count by a step length already known in centimetres turns it into a real estimate. A pure guess uses no comparison at all. A tile is not a metre, so a tile count is not a length unless the tile size is known. A classroom wall is a different length from a corridor.

Q7
medium

Which everyday length is closest to $1\ \text{m}$?

  1. A The length of a new pencil
  2. B The height of a two-storey building
  3. C The width of a fingernail
  4. D The height of a kitchen slab above the floor
Show answer and explanation

Correct answer: D - The height of a kitchen slab above the floor

A kitchen slab stands roughly a metre above the floor. A new pencil is about $18\ \text{cm}$ and a fingernail about $1\ \text{cm}$, so both are far shorter. A two-storey building is about $6\ \text{m}$ tall, which is far more than a metre.

Q8
medium

Riya estimates the length of her desk as about $1\ \text{m}$. Her measurement comes out as $1\ \text{m}$ and $5\ \text{cm}$. What should she conclude?

  1. A She should change the measurement to exactly $1\ \text{m}$
  2. B Her estimate was close, so the measurement looks trustworthy
  3. C Her estimate was wrong and should be thrown away
  4. D The desk must have been measured with a broken scale
Show answer and explanation

Correct answer: B - Her estimate was close, so the measurement looks trustworthy

A measured value of $1\ \text{m}$ and $5\ \text{cm}$ sits very close to an estimate of about $1\ \text{m}$, which is what a good estimate looks like. An estimate is never expected to match exactly. Nothing here points to a broken scale. Changing a measured value to match a guess destroys the measurement.

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