Q1.
Does the force of friction act only if the objects are moving on solid surfaces? What about objects moving through liquids and gases?
Answer
No, friction is not limited to solid surfaces. Air, water and other liquids also exert a force of friction on the objects moving through them.
Anything moving through air or water has to push that air or water out of its way, and the air or water pushes back — opposite to the motion, exactly as friction does between two solid surfaces. You feel it yourself when you cycle fast into the wind, or when you try to run in waist-deep water at a pond.
Because this friction wastes fuel and limits speed, aeroplanes, ships, boats and high-speed trains are designed with specific shapes — narrow and tapering — so that the air or water flows past them smoothly and the force of friction is reduced.
Did you know? Fish and birds have the same tapering shape, and for the same reason. Their bodies have been shaped by nature for the very problem engineers solve when they design a train nose.