Solution

Which conditions are necessary to feel zero gravity? (click on the right answer)

You would feel weightless...

a) ... in free fall or if the sum of the gravitational forces which act on this place equal zero (example: at the earth's center).

b) ... as soon as you left the earth's atmosphere because gravitation does not exist in vacuum.

c) ... if you were on the moon because earth attraction has no effect there.

d) ... if you rose up in high altitudes with an hot-air balloon because you get lighter with increasing altitude until you weigh nothing at all in the end.