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Dynamic lift is a type of force that acts on a body such as an airplane wing, a hydrofoil, or a spinning ball, by virtue of its motion through a fluid. It is different from other kinds of lift in fluids.
- In static lift or buoyancy, internal fluid in the body is lighter than the surrounding fluid and the body does not require movement.
- Static lift is used by boats, balloons, blimps, etc.
- In planing lift, the lower portion of the body is immersed in a liquid flow.
- Planing lift is used by surfboards, motorboats, sailboats, and windsurfers.
- Dynamic lift is responsible for the curved path of the spinning ball and the lift of an aircraft.
- For example, In the game of cricket, basketball, or golf we can easily notice that the spinning ball deviates from its parabolic trajectory throughout the duration of its motion in the air.
Read Also: Pressure of an ideal gas
Key Terms: Dynamic Lift, Magnus Effect, Magnus spin, Examples of Magnus Effect, Magnus Effect Applications, Magnus lift, Force.
Read More: Charles Law
Overview
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A force is exerted on an object flowing through a fluid. Lift is that component of force acting on the object in a perpendicular direction to the direction of the flow.
- The force is called an aerodynamic force if the surrounding fluid is air and the force is called hydrodynamic force if the surrounding is water or any other liquid.
- Considering an object which is moving through the fluid and due to that moving object from fluid, there is a normal force acting on the body.
- This phenomenon is known as dynamic lift.
- This phenomenon is popularly observed in airplanes.
- Whenever an airplane is flying in the air, due to its motion through the fluid, fluid here is the air in the atmosphere.
- Due to the motion through this fluid, there is a normal force that acts on the airplane in the vertically upward direction.
Check Important Difference Between Kinetic and Potential Energy
Magnus Effect
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Magnus force is a type of force that is applied on a swiftly spinning cylinder or a sphere moving through the direction of air or another fluid in a direction at an angle to the axis of spin following Bernoulli's relation.
- This force is responsible for the deviation of balls when hit or thrown with a spin on the ground.
- This effect is considered the Magnus effect.
- In simple words, we can say that Dynamic lift by virtue of the spinning of an object is known as the Magnus Effect.
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Explanation of Magnus Effect
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Magnus effect with the help of a real-world example.
Spinning of Ball
Case 1: The ball is not spinning.
When the ball moves in the air without spinning, the velocity of the ball above and below the ball will be the same. Hence, there will be no pressure difference (ΔP = 0). Therefore, there will be no dynamic lift applied to the ball.
Case 2: The ball is moving in the air while spinning.
When the ball spins in the air, it drags the air above hence the velocity above the ball is more as compared to the velocity below the ball. There will be a pressure difference. The pressure will be more below the ball. Therefore, because of the pressure difference applied on the ball, there is an upward force which is known as the dynamic lift.
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| Sphygmomanometer | Specific heat capacity of water |
Applications of Magnus Effect
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- The Magnus effect is applied in sports like cricket, football, baseball, and many more. This concept has majorly come into effect to understand the physics behind ball sports.
- It is used mainly in external ballistics. The combined sideways wind component of wind causes Magnus's force to act on the bullet.
- Rotor ships use Flettner rotors which are mast-like cylinders that are mounted vertically on the ship's deck. This helps in propulsion. A forward thrust is created as the wind starts to blow from the side due to the Magnus effect.
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Things to Remember
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- The Magnus effect is not a result of Bernoulli’s principle, but they are interrelated to each other.
- Dynamic lift due to the spinning of a body is the Magnus effect.
- In Bernoulli’s principle for a streamlined fluid flow, the sum of pressure, kinetic energy per unit volume, and potential energy per unit volume of fluid remain constant.
- Pascal’s law states that if the pressure applied to uniform liquids is limited, then the fluids will transmit the same pressure in all directions at the same rate.
- Viscosity is a property of a fluid that opposes the force that tends to cause the fluid to flow.
Read Also: Surface Energy
Sample Questions
Ques. A fully loaded army aircraft has a mass of 3.3 x 105 kg. Its total wing area is 500 m2. It is a level flight with a speed of 960 km/h. Calculate the pressure difference between the lower and upper surfaces of the wings. (3 Marks)
Ans. Weight of the army aircraft = Dynamic lift
mg = (P1 - P2) A
mg/A = ΔP
ΔP = 3.3 x 105 x 9.8/500 (value of g = 9.8 m/s2)
ΔP = 6.5 x 103 N/m2
Therefore, the Pressure difference between the lower and upper surfaces of the wings is 6.5 x 103 N/m2
Ques. The velocity of the wind over the surface of the wing of an airplane is 80 m/s and under the wing is 60m/s. If the area of the wing is 4 m2 then, calculate the dynamic lift experienced by the wing in the density of air. (3 Marks)
Ans. Given, v1 = 80m/s
v2 = 60m/s
A = 4m2
Density of air ρ = 1.3kg/m3
From Bernoulli's theorem, P1 + 1/2ρv12 = P2 + 1/2ρv22
P2 - P1 = 1/2 ρ(v12 - v22) ……… (1)
Lift on the wings = (P1 - P2)A
From (1),
= 1/2 ρ(v12 - v22)A
= 1/2 x 1.3 x (802 - 602) x 4
= 7280N
Therefore, the Dynamic lift experienced by the wing in the density of air is 7280N.
Ques. Define Bernoulli’s principle. (2 Marks)
Ans. Bernoulli’s principle states that “During a steady flow of a liquid, the energy at any point in a channel is the sum of velocity head(v), pressure head(p) and elevation head(z).” It takes the form of a conservation equation where the sum of three variables will remain constant as long as no losses or additions occur.
Energy = v + P + z = constant
Ques. In an experiment on a model airplane in a wind tunnel, the flow speeds on the upper and lower surfaces of the wing are 70 m/s and 63 m/s. What is the lift on the wing if its area is 2.5 m2? Take the density of air to be 1.3kg/m3. (5 Marks)
Ans. Given,
- v1 = 70m/s
- v2 = 63m/s
- A = 2.5 m/s2
- ρ = 1.3 kg/m3
According to Bernoulli’s theorem,
P1 + 1/2ρv12 = P2 + 1/2ρv22
P2 - P1 = 1/2 ρ(v12 - v22)
P1 = pressure on the upper surface of the wing
P2 = pressure on the lower surface of the wing
Dynamic lift on the wing,
(P1 - P2) = 1/2 ρ(v12 - v22)A
= 1/2 x 1.3[(70)2 - (63)2] x 2.5
= 1512.87
= 1.51 x 103 N
Therefore, the lift on the wing of the airplane is 1.51 x 103 N.
Ques. Water is flowing through a horizontal pipeline of varying cross-sections. If the pressure of water equals 6cm of mercury at a point where the velocity of flow is 30 cm/s, Calculate the pressure at the other point where the velocity of flow is 50 m/s. (5 Marks)
Ans. At first point, v1 = 30 cm/s = 0.3 m/s
P1 = ρgh = 6 x 10-2 x 13600 x 9.8
P1 = 7997 N/m2
At the other point, V2 = 50cm/s = 0.5m/s
From Bernoulli’s equation, P + (1/2)ρv2+ρgh = constant
P1 + 1/2ρv12 = P2 + 1/2ρv22
7997+(1/2) x 1000 x (0.3)2 = P2 + (1/2) x 1000 x (0.5)2
P2 = 7917N/m2
= ρgh2 = H2 x 13600 x 9.8
h2 = 5.9 cm Hg
Ques. What do you understand about the term “Magnus Force”? (2 Marks)
Ans. The Magnus force is considered the result of Newton’s third law of motion. According to the figure, it is the equal and opposite force that air applies on the ball as a reaction to the force the ball imposes on the air. The body pushes the air in one direction, and the air pushes the body in the other direction. The force is described by an arrow perpendicular to the axis of rotation, in the direction of the pressure differential.
Ques. What are lift and drag? Write the formula for lift. (3 Marks)
Ans. A lift can be considered as a part of the aerodynamic force which lies perpendicular to the direction of flow. And drag is considered to be a part that stays parallel to the flow of motion.
The formula for lift is -
Lift(L) = 4/3(4π2b3sρv)
Where b is the radius, v is velocity, ρ is density and s is rotation of the object.
Ques. What are the four forces of flight? (2 Marks)
Ans. As airplanes fly due to the pressure difference between the streamlines of the upper end and lower surface of the airfoil. This pressure difference gives rise to the dynamic lift. Hence, there are three other forces besides the lift that help airplanes fly. These forces are -
- Thrust
- Drag
- Weight
These four forces of flight are also called aerodynamic forces because they show their occurrence during the motion(flight) of an airplane.
Ques. Define the term Aerodynamic force. (2 Marks)
Ans. Aerodynamic force is a type of force that is applied on a body when there exists a relative velocity between the air and the object. There are two basic sources of aerodynamic force: pressure distribution and frictional shear stress distribution applied by the airflow on the object’s surface.
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