Consider the following statements: I. The weight of an object is the same everywhere on the surface of the Earth, but its inertial mass could be different. II. The inertial mass and gravitational mass of an object are proportional and the proportionality constant is the same everywhere. III. The inertial mass and weight of an object are the same at different places on Earth. Which of the statements given above is/are correct?
- A.I only
- B.II only
- C.I and II
- D.II and III
Correct answer
Explanation
The correct answer is B, II only. Mass is the same everywhere, but weight changes with g, so only statement II holds.
Inertial mass measures how much a body resists a change in motion (F = ma); gravitational mass measures how strongly gravity pulls it. Experiments from Galileo and Newton to Eötvös show the two are exactly proportional, with the same constant for every body and every place, which is why all bodies fall with the same acceleration; with suitable units they are equal. Weight, however, is mg, and g is larger at the poles than at the equator and falls with height, so weight differs from place to place.
Statement I is wrong on both counts: weight varies over the Earth's surface, while mass does not change.
Statement III is wrong because weight is not the same at different places; only mass is.
So A, C and D, which include I or III, are all wrong.
Exam tip: mass is constant and measured in kg; weight = mg is a force in newtons and is greatest at the poles.
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