The key formula ==> H = 1/2 G T²
10.4 = 1/2 (9.8) T²
Divide each side by 4.9 : T² = 10.4 / 4.9
T = √ (10.4/4.9) = 1.46 seconds .
The mass of the ball doesn't make any difference. If not for air resistance,
a feather and a school bus, dropped from the same height at the same time,
would fall together, and hit the ground at the same time. We've known this
for roughly 600 years now, and I suspect you learned it in the same high
school class where your question came from.
I think the correct answer would be the first option. The elements that hydrogen fuel cells need to produce electricity are oxygen and hydrogen. The hydrogen is broken into protons and electrons with the aid of a catalyst which them reacts with oxygen producing water, heat and electricity.
Density is mass divided by volume making the answer 0.5 g/cm^3
It would take 7 or 7.5 hours
