For which of the following situations can the equation F = mg not be used to calculate the force due to gravity?

a) The pull of gravity on an apple on the surface of the Earth.
b) The pull of gravity on an airplane flying at an altitude of 8 km above the surface of the Earth.
c) The pull of the Earths gravity on the Moon.
d) The equation can be used for all of the above

All of them seem correct... I think its choice D but no sure

D is not the correct answer. The acceleration due to gravity at or near the earth's surface (g) is derived from Newton's Law of gravitation, which gives the force of gravity between two bodies:


F = \frac {GM_1M_2} {r^2}


For a body located at the earth's surface the values for G, M_1, and r can be replaced by the gravitational constant, the earth's mass, and the radius of the earth to yield:


F = ( \frac {GM}{r^2} )M_2 = \frac {(6.67 x 10^{-11} \frac {m^3}{Kg-s^2})(5.97 x 10^{24} Kg)}{(6.378 x 10^6m)^2} M_2 = (9.8 \frac m {s^2})M_2

So we set the value for 'g' equal to 9.8 m/s^2 to make the math easier, but realize it only applies to objects at or near the earth's surface.