We know that if we rubbed two solids, it generates heat. Also the shuttles coming into the atmosphere generates heat. So, likewise does a movement of a solid in a liquid generate heat?
Also, what will happen if a liquid with a liquid, a liquid with a gas, a gas with a gas.... rubbed?
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1 Answer
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In general the energy of the system must be conserved. Heat and sound result from shuttle reentry because the kinetic energy of the shuttle is being reduced and that energy must go somewhere. The molecular collisions of the air molecules with the shuttle and with one another result in an average increase in the speed of the molecules (heat).
Similarly, a rock decelerating in water will also result in sound and heat. The same applies to the interactions of all phases of matter.
Friction, at least in its common meaning, is concerned only with the rubbing together of solids.
