Why do aeroplanes having wings fly at low altitudes?

In a test experiment pn a model aeroplane in a wind tunnel, the flow speeds on the upper and lower surfaces of the wing are 70 m s^-1 and 63 m s^-1 respectively. What is the lift on the wing if its area is 2.5 m^2 ? Take the density of air to be 1.3 kg m^-3 .

A fighter plane flying horizontally at an altitude of .1.5 km with speed 720 km/h passes directly overhead an anti-aircraft gun. At what angle from the vertical should the gun be fired for the shell with muzzle speed 600 m s^-1 to hit the plane? At what minimum altitude should the pilot fly the plane to avoid being hit? (Take g = 10 m s^-2) .

Explain why food does not cook speedily at high altitudes.

Why do we apply paint on iron articles?

An aeroplane when flying at a height of 3125mfrom the ground passes vertically below another plane at an instant when the angles of elevation of the two planes from the same point on the ground are 30° and 60° respectively. Find the distance between the two planes at the instant.

Why do soaps not work in hard water?

There are two cosecutive kilometre posts on a straight road.From an aeroplane flying vertically above the road in between the two km.Posts the angles of depression of the two km.Posts are seen to be 45^@ and 60^@ At what height is the aeroplane flying at the moment of observation?