NDA (I) 2018 General Ability Test · Question 80 of 150

Correct — B, It is same everywhere. Mass is the quantity of matter in a body and a measure of its inertia — how strongly it resists being accelerated. That quantity travels with the body. Carry a one-kilogram block to the top of a mountain, down a mine, to the Moon or into orbit and it is still one kilogram, because none of its matter has been removed. What changes is its weight, the gravitational pull on it, which is mass multiplied by the local value of g; on the Moon that pull is about one-sixth of its value on the earth's surface, and in free fall it registers as nothing at all. The stem asks about mass, so the constant quantity is the right answer.

• (a)It changes from one place to another — This is true of weight, not of mass, and it is the single most common confusion in this topic. Weight varies because g varies — with latitude, with altitude and from one celestial body to another — while the matter in the body stays the same.
• (c)It depends on its shape — Beat a lump of metal into a sheet or roll it into a ball and you have rearranged it, not added to or taken from it. Shape changes volume, surface area and how the body behaves in a fluid, but not its mass.
• (d)It does not depend on its temperature — This statement is very nearly true for ordinary purposes, which is what makes it the sharpest distractor in the set. Heating a body does not change the number of atoms in it, so a school laboratory would find no change on a balance. But the examiner is asking which statement is THE correct one about mass, and the defining, unconditional property is that it is the same everywhere. Option (d) is a negative side-remark about one particular variable rather than a statement of what mass is, and the key takes (b).

Mass and weight are different physical quantities with different units. Mass is measured in kilograms, is a scalar, and is the same at every point in the universe. Weight is a force, measured in newtons, is a vector directed towards the centre of the attracting body, and equals mass times the local acceleration due to gravity. A beam balance compares an unknown mass with known masses, so gravity cancels out on the two pans and it reads mass correctly anywhere. A spring balance measures the pull on the body, so it reads weight, and it gives the right mass only where the local g matches the g it was calibrated for.

One honest tension is worth naming here, because a careful candidate will notice it. Option (d) is not false in any way an NDA candidate could be expected to detect — at the level of school physics, mass is indeed independent of temperature. What separates the options is that (b) states the defining property of mass, while (d) merely denies one dependence among many. The official key marks (b), and the way to handle this family of questions is to look for the option that says what the quantity IS rather than the option that lists something it is not. It is also worth keeping the shape option in mind for a different reason: shape matters a great deal for weight as measured in air, since a large light object displaces more air and feels a bigger upthrust, but that is a buoyancy effect on the reading, not a change in mass.

• Mass is the quantity of matter in a body and a measure of its inertia; its SI unit is the kilogram.
• Weight is the gravitational force on a body, equal to mass times g, and its SI unit is the newton.
• Mass is the same everywhere; weight changes with location because g changes.
• The value of g on the Moon's surface is about one-sixth of its value at the earth's surface.
• A beam balance reads mass anywhere; a spring balance reads weight and is only reliable where g matches its calibration.

• Reading 'mass' but answering for weight — the reason option (a) is so tempting.
• Thinking a spring balance measures mass; it measures the force of gravity on the body.
• Assuming an astronaut floating in orbit has lost mass rather than merely lost apparent weight.

NDA sets this either as a direct property question like this one, or as a numerical asking what a spring balance or a spring's extension would read on another body.

• practice — not a real PYQ

  A body of mass 6 kg is taken from the earth to the Moon, where g is one-sixth of its earth value. On the Moon its mass will be

  • (a)1 kg
  • (b)6 kg
  • (c)36 kg
  • (d)zero

  Answer(b) 6 kg — mass does not change with location; only the weight falls to one-sixth.

• practice — not a real PYQ

  Which instrument will give the same reading for a given body at sea level and on a high mountain?

  • (a)Spring balance
  • (b)Beam balance
  • (c)Both of them
  • (d)Neither of them

  Answer(b) Beam balance — it compares two masses in the same gravitational field, so g cancels out.