# Drag force and mass

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## Main Question or Discussion Point

Why drag force doesn't Depend on the mass of the Object?

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Drakkith
Staff Emeritus
Well, what does drag force depend on?

if the drag force is the reaction force on the elastic collision between air molecules and the object , so it have to depend on the momentum of the air molecules and object momentum, is that right?

FactChecker
Gold Member
Drag force (and all aerodynamic forces) are the forces caused by the air flowing around an object. Only the shape of the object matters. It doesn't matter if the object is hollow or filled with lead.

okay ,but if you see it as a collision and the air molecule mass is constant and its initial velocity is also constant, and the object velocity is also constant but its mass is changing so the reaction force will change , so drag force must depend mass.

A Lazy Shisno
The drag equation suggests it's the object's density, cross-sectional area, and shape that affects the force acting on it by the air.

FactChecker
Gold Member
The drag equation suggests it's the object's density, cross-sectional area, and shape that affects the force acting on it by the air.
That's the air density, not the object's density.

A Lazy Shisno
That's the air density, not the object's density.
Sorry, yes, you're right. I'm not sure how I forgot that.

The "drag equation" also is not based upon first principles and does not capture all of the relevant physics contributing to drag. It's just an empirical relationship between the drag force, area, and the dynamic pressure ($\rho V^2/2$). The drag coefficient relates those quantities across a wide range of conditions, but tells you nothing about what actually contributes to the drag. Viscosity, ratio of specific heats, and Mach number can also all play a role, for example. So can the laminar-turbulent state of the boundary layer, the surface quality, and in extremely high-speed flows, chemical reactions.