Just take gravity and throw it into the trash can, two object do not attract each other. What there is is up and down and different density. A rock is more dense then air so it falls down, simple. Helium is less dense then air so it goes up. Easy solutions tend to be the right one.

Just take gravity and throw it into the trash can, two object do not attract each other. What there is is up and down and different densitets. A rock is more dense then air so it falls down, simple. Helium is less dense then air so it goes up. Easy solutions tend to be the right one.