All objects on earth, including men, insects, buses, trees, buildings, lakes and atmospheric air are struck to earth’s gravity because the latter operates a force towards the centre of the earth from the centres of gravity of each of the objects in the earth’s gravitational field. Since the gravitational attraction among the objects is negligible in comparison to the gravitational pull the earth exerts on each object in its field, the mutually relative positions of these objects remain unchanged. Thus, the earth provides an inertial frame of reference for the objects it holds in its field. The mutual and relative positions of the objects would change only when one or more of these objects make a resultant displacement overcoming the gravitational pull.
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An insect is no exception for this inertial influence. It needs air to fly and the air it flies in is also moving with the same speed as the bus. If the fly does not make any forward or backward movement it remains in the same location because the fly is materially (and inertially) connected to the vehicle through the air. Imagine stationary vehicle with its air removed by evacuation before a (super) fly is suspended in the bus with no forward or backward movement. If the vehicle, then suddenly, commences a forward movement, the fly would really hit the back of the vehicle because now there is no air that goes along with the vehicle holding the fly, materially and inertially stuck to the vehicle’s frame of reference.