A balloon filled with helium gas is set free and starts to move upward. Which of the following best explains why the helium balloon moves upward?
Choices:
[-] The density of helium is less than the density of air.;
[-] The air resistance lifts the balloon up.;
[-] There is no gravity acting on the helium balloons.;
[-] The wind blows the balloon upward.;The answer is:
The density of helium is less than the density of air.