Upthrust and viscous drag

I know that upthrust is equal to the volume of fluid displaced. But if weight and viscous drag act then where does upthrust come into it?

I suppose in overly simplistic terms, upthrust is a reaction force that happens due to the fact that the fluid can't move out of the way fast enough, and viscous drag is a bit like friction due to the body sliding past the fluid, (I think). Both of them act to oppose the motion of the falling body.

upthrust is just equivalent to the pressure force which the displaced fluid exerts (equal to the mass of the fluid displaced). This is a constant force when a body is fully submerged in the fluid.

I think viscosity is proportional to velocity though, so as you get faster, viscous drag forces increase until the sum of the upthrust and viscous drag is equal to the weight of the object and the object undergoes no further acceleration and continues at a constant velocity.

Well it being equal to the mass of the fluid displaced is true in most cases, and it is usually constant. However that is not always true

--------------



Viscosity is constant (well under standard conditions), viscous drag forces do change with velocity, but it isn't a linear relationship but a very complicated one.

--------------

But anyways, the rest of what you've said is true