Question

Dynamic of lift of an aeroplane is based on:
A. Torricelli Theorem
B. Bernoulli’s Theorem
C. Conservation of angular momentum
D. Principle of continuity

Answer 1

Hint: Torricelli theorem tells about speed of outflow of fluid from the side of an open tank. Equation of continuity tells about the conservation of mass in a non-compressible fluid and angular momentum is mostly important in rotational motion phenomena. Bernoulli theorem is used to explain the dynamic lift of a plane.

Formula used: According to Bernoulli equation:
$P+\left( \dfrac{1}{2} \right)\rho {{v}^{2}}+\rho gh=const.$, where, P is pressure at a point, v is speed at that particular point, g is the acceleration gravity, h is height and $\rho $ is the density of the fluid.

Complete step by step answer:
Bernoulli’s theorem states that increase in speed of fluid will lead to decrease in pressure of the fluid.
At any point in the flight, four forces act on an aeroplane. They are drag, thrust, weight and lift.
For flying the aeroplane needs lift to overcome the gravitational weight.
When an aeroplane starts to move forward on the ground, the wings are placed in such a manner that the wind is divided between above the wing wind and below the wing wind.
We can use Bernoulli theorem to explain the lift. The upper surface of the wing is curved, due to which the air above the wing gets high speed and gets stretched. This creates a pressure difference situation above and below the wings.
Above the wings the pressure becomes lower due to higher speed according to the Bernoulli theorem. As we know that air moves from low pressure to high pressure. Therefore, the high-pressure wind will push the winds in an upward direction and hence will give lift to the aeroplane.

So, the correct answer is “Option B”.

Additional Information: Bernoulli’s theorem is also used to explain flying of birds and aerofoils.
It also explains why in many ball games, the spinning ball in air deviated from its expected parabolic path.
Bernoulli’s theorem is helpful in explaining working of steam locomotives.

Note: Students might get confused that the lift is because of angular momentum conservation as dynamic lift of aeroplane is comparable with rotating ball lift but it must be kept in mind that even the rotating ball lift is also explained by Bernoulli’s theorem.