Drag Force Formula

Drag Force (D) is defined as the force that resists the motion of a body with fluid. If the motion of the body exists in fluid like air it is known as aerodynamic drag. And, if the fluid is water it is known as hydrodynamic drag. The drag force always acts in the opposite direction to the flow of velocity.

Drag equation is given as

D =Cd×ρ×V2×A2Cd×ρ×V2×A2.

Where,

the Cd = drag coefficient,

ρρ = density of the medium,

V = velocity of body,

A = cross-sectional area.

Question 1:  A car travels with a speed of 90 km/h is with a drag coefficient as 0.26. If the area of cross section is 5 m2, Find the drag force.

Answer:

Given:

Velocity V = 90 km/h

drag coefficient Cd = 0.26

 Area A = 5 m2

density of fluid ρρ = 1.2 kg/m3

The drag force is given by,

D = Cd×ρ×V2×A2Cd×ρ×V2×A2

D = 0.26×1.2kg/m3×8100m/km×5m22×36000.26×1.2kg/m3×8100m/km×5m22×3600

D = 1.755 N

Therefore, the drag force of the car = 1.755 N.

Question 2: A plane moves with a speed of 800 km/h. If the area of a cross section of the plane is 100 m2, Find the drag force.

Solution:

Given:

Velocity V = 800 km/h

drag coefficient Cd = 0.26

 Area A = 100 m^2

 density of fluid ρρ = 1.2 kg/m3

The drag force is given by,

D = Cd×ρ×V2×A2Cd×ρ×V2×A2

D = 0.26×1.2kg/m3640000×100m220.26×1.2kg/m3640000×100m22

D = 9984000 N.

Therefore, the drag force of plane is 9984000 N.

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