Calculating Water Friction for Dropped Objects

[waterfriction]

In my current scenario, I have something that was dropped from above water. I have found the theoretical acceleration of the object when it is under water (it had entered the water with some initial downward velocity) already from boyancy - weight = ma. However, in reality that acceleration is going to be smaller because of water friction. How do I take into account water friction? I know it can't be u*N which is what friction is on land.

Thanks

 

[Pyrrhus]

Try http://www.hypertextbook.com/physics/matter/drag/" (it has a reasonable explanation). Just remember the problem with drag in a fluid is not as easy as you might think.

 

[waterfriction]

Thanks for the link. How can I use the value of drag though? What equation can I use to then find out the increase in time drag creates over an ideal fluid or the lessening in velocity?

 

[Pyrrhus]

Well for all purposes you can use $F_{drag} = C_{drag} \frac{\rho v^{2}}{2} A$ as long as you keep small speeds, therefore there will be a small Reynolds number (laminar flow). A is surface area, and $C_{drag} = \frac{24}{N_{reynolds}}$ in this case.

 

[waterfriction]

Thanks again. I'm assume that the best way to find the F drag with a changing velocity is to take the average and use that as v, right?

 

[Pyrrhus]

Well it depends on your approach. Do you want a variable or constant acceleration?, if its the former then you'll need to solve the differential equation, for the later you could use the average velocity.