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I am in high school and am confused regarding drag, viscosity and friction.

As far as I know, viscosity of a fluid is non-uniformity in velocity of the fluid particles due to sliding of elementary layers of the fluid on top of each other.

Between $2$ solid surfaces, friction occurs to welding (formation of bonds) between the molecules of the $2$ surfaces due to irregularities already present in them. So, I guess friction on a solid object in a fluid is due to the attractive interaction between the particles of the fluid and the surface of the object.

In this light, aren’t viscous force and friction force due to a fluid on a solid object in the fluid the same thing? And is drag a redundant term here or is it some other type of retarding force? Any help would be highly appreciated.

Qmechanic
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Rϵλατινιτy
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1 Answers1

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Friction is one component of the viscous drag and as you imply, it is from the rate of change of the flow speed with distance from the surface.

However, it isn't the only viscous effect. The growth of the boundary layer (the region close to the object where the speed is changing) along the length of the object effectively amounts to a change in shape of the object, as far as the flow is concerned, making it more asymmetrical fore and aft. This causes a corresponding asymmetry in the pressure distribution and an additional contribution to the drag.

At speeds close to or above the speed of sound (transonic or supersonic flows), the formation of shock waves causes additional drag.

In 3-d cases, there is also a drag that would occur even in the absence of viscous effects (induced drag, or vortex drag), but your question seems to only be asking about viscous drag, so I won't elaborate on that.

D. Halsey
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