I think it's when,
lets say an areoplane is moving at diferent speeds the particles are colliding with each other therefore making more space for the aircraft to increase it's speed so the drag from behind (friction) will have to increase too.
lets say an areoplane is moving at diferent speeds the particles are colliding with each other therefore making more space for the aircraft to increase it's speed so the drag from behind (friction) will have to increase too.