Explain how the air balloon moves up and down.
Explain in terms of density

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Explanation:

The inside air that has been heated rises due to the difference of density between the hot and cold air particles. The heated air, the air that is inside the balloon, is less dense than the cool air, the air outside of the balloon. ... You can see that the bigger the volume gets, the smaller the density is.

air exerts forces on falling objects near earth’s surface.

explanation:

the term 'free falls' refer to object that are falling towards the earth, in a situation in which gravity is the only force acting on the object.

for an object in free fall, the acceleration is constant (g, the gravitational acceleration), and so the velocity of the object constantly increases.

for objects near earth's surface, there is always another force acting on the object: the resistance due to the air. this force is opposed to the direction of fall of the object, and it is proportional to the velocity of the object, so as the object falls down, this force increases up to a point when it balances the force of gravity, and the object continues its motion at constant velocity (called terminal velocity). since in this case gravity is not the only force acting on the object, we are not in a situation of free fall.