# Drag force and mass

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

## Answers and Replies

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?

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.

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.

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.

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

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