Question

A jet plane flies in air because
A. The thrust of the jet compensates the force of gravity.
B. The weight of air whose volume is equal to the volume of the plane is more than the weight of the plane.
C. The flow of air and the wings cause an upward force which compensates for the force of gravity.
D. The gravity does not act on objects moving with high speed.

Answer 1

Archimedes principle states that any body partially or fully submerged in a liquid or gas at the state of rest is acted upon by an upward buoyant force the magnitude of which is equal to the fluid displaced by the body. This weight is also called thrust.In this question two forces are acting on the jet i.e, the force of gravity and the buoyant force or the thrust. The force of gravity acts in the downward direction but the thrust acts upwards. Hence, the two forces would produce a resulting effect in their directions if the magnitude of force is more.

Complete step by step answer:
The jet flying in air would follow Archimedes principle which states that a body in a fluid would experience an upthrust. So, the jet will also experience the upward force. Also, the force of gravity acts on the jet. Let ${F_G}$ represent the force due to gravity and ${F_T}$ represents the upward thrust.
Three cases arise
(1) ${F_G} > {F_T}$
Here the magnitude of the thrust is less and hence the plane does not fly.
(2) ${F_G} = {F_T}$
This is the equilibrium position of the plane when it is flying. Both the forces are balanced and help the jet to fly.
(3) ${F_G} < {F_T}$
Here the magnitude of force of gravity is less so the plane takes off.Hence, we can say that the thrust compensates the force of gravity.

Hence, option A is correct.

Note: Archimedes principle is valid for fluids-liquids and gases both. Very often weight is misunderstood as mass of the object. These are two different terms and must be noted carefully. Mass is the inherent property of an object which tells how much matter it contains while weight is the normal reaction acting on the body. Mass is constant while the weight may vary.