# Motion of an object submerged in water.

## Main Question or Discussion Point

ignoring drag, how could you describe the motion of a cylindrical object in water, if the upwards buoyancy force is given by a certain equation? (B = [1020+0.25z]Volumexgravity)

I'm not sure if its just Fup = Fdown where Fdown is the weight and Fup is the buoyancy. However it can't be that or else the cylinder wouldn't have motion it would be stationary, wouldn't it?

## Answers and Replies

Related Mechanical Engineering News on Phys.org
tiny-tim
Science Advisor
Homework Helper
hi aero&astro!

(try using the X2 icon just above the Reply box )
I'm not sure if its just Fup = Fdown where Fdown is the weight and Fup is the buoyancy. However it can't be that or else the cylinder wouldn't have motion it would be stationary, wouldn't it?
no, Fup = Fdown would be the equilibrium equation

you need (as usual) good ol' Newton's second law …

Fup - Fdown = ma

why is it Fup - Fdown and not Fdown - Fup?

Because tiny-tim assumed a reference frame where "up" points to positive values, and "down" points to negative values.

tiny-tim
Science Advisor
Homework Helper
Yup!

thanks a lot guys, this really helped.