Which of the following might be measured in
square meters?
a. the length of a pendulum string
b. the volume of an atom
c. the mass of a book
d. the area of a desk top

The correct answer to this question is D. The area of a desk top.

Explanation:

Square meters, or m^2, are the units that represent area, or the product of two lengths.  This is because the length and the width both have the units meters, so when multiplied to calculate area, the resulting units are square meters or m2.  Thus, the answer for this question is D.

The length of a pendulum string would be measured in meters.

The volume of an atom would be measured in m^3 (volume is the product of the length, width, and height, giving it a cubic power).  However, the volume would probably be measured in a much smaller unit because atoms are very small.

The mass of a book would likely be measured in grams or kilograms.

A. The length of a pendulum string . . . No. Square meters is not a unit of length.  That would be multiples or fractions of a meter.

B. The volume of an atom . . . No. Square meters is not a unit of volume.  That would be multiples or fractions of a cubic meter.

C. The mass of a book  . . . No. Square meters is not a unit of mass.  That would be multiples or fractions of a kilogram.

D. The area of a desk top . . . YES !  Square meters is a unit of area.

the minimum amperage that the current in this wire would have to have so that its weight is completely supported by the magnetic force due to earth's field, assuming that no other forces except gravity act on it