Why does cold air make hot air move in physics?

Answer 1

Actually, both cold air & hot air move.

Imagine 3 laser beams are focused to converge on a single point in a typical room with normal air temperature. When the lasers are switched on, the beams converge and energize the air molecules that are in the convergence zone. As the photons in the light collide with the air molecules, the air molecules absorb energy, vibrate/oscillate, & collide with other nearby air molecules very rapidly. This highly kinetic energy state is what we call heat (i.e. energy in the form of heat).

Because the heated molecules are colliding with other molecules, the distance between the heated molecules increases. This results in a lower density region (where heated air molecules are) within a higher density zone (where the cooler air molecules are).

Gravity pulls on each of the molecules equally, however the denser cooler air is pulled downward to displace the warmer less dense air.

To visualize this easier, imagine a drop of oil is injected into the center of a cup of water and the oil is less dense than the water. Eventually, gravity will pull the more dense water molecules below the oil molecules. As the more dense water displaces the less dense oil, the displaced oil moves upwards to the surface.

Original Poster's Answer:

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Technically it is the hot air that makes the cold air move however that is the wrong terminology. Hot air rises in fluids due to convection currents. When air is heated it expands (as particles gain more kinetic energy) and therefore becomes less dense. It will then rise forcing the more dense air down. The more dense air then gets heated and becomes less dense and rises...thus the cycle continues. A real life scenario of this is why it is hotter in a sauna the higher up you sit.