# Buoyancy force and drag

Why does an object in a fluid experiance both a buoyancy force and a drag force? How are they different and what causes these two different forces?
Thanks. :-)

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The bouyant force is caused by the pressure differential across the top and bottom of the object. It exists regardless of motion.

Drag force is purely a motional force.

Both are caused by the impacts of nearby fluid molecules.

Thanks, but what do you mean by "pressure differential across the top and bottom of the object"? Could you please explain what this means, what causes this, etc?
Thanks. :-)

suffian
Pressure is a measure of the force applied per unit of area. In the case of an body in the air, the pressure results from air molecules impinging the surface of the body at high speeds. Because the concentration of air molecules decreases as one rises from the earth (notice that there is a high concentration near the surface but clearly no air in space, the fall off is continous), the pressure exerted on the lower portion of the body is greater than that exerted on the upper portion of the body (less air molecules impinging the upper surface at a higher altitude). This difference in pressure, or pressure differential, is what causes the bouyant force.

Air drag is a little different in that it results from the body colliding into a large number of air molecules as it travels through the air. The faster it travels, the more air molecules the body has to push out of its way in a unit of time, so there is greater resistance at higher speeds.

With drag, is it that the object is colliding with eg - air molecules more often or hitting them harder, hence experiancing a greater force?