Hot air balloon

Question 1

Describe why hot air balloons rise.

Answer 1

Hot air is less dense than cold air, so it will displace a greater volume of normal air, increasing the buoyancy force.
Note:
Higher temperature=higher volume=less dense
Lower temperature=Low volume=More dense

Question 2

how can u make an object fly?

Answer 2

reduce its average density so it becomes lower than the density of air.