Expert: Mark Janus - 2/12/2011

Question
Dear Mark, if you have a falling particle with a irregular shape, you can calculate what the diameter of a sphere (with the same density) would have to be to reach the same terminal velocity.  Would this equivalent diameter be the same for air as well as liquid?
Best regards,
Cas

Answer
No...  the terminal velocity of an object is a function of (depends on) the drag of the object...  while the drag coefficient (non-dimensional drag) of a sphere is independent of size and fluid type, the physical drag is dependent on these...  thus since the density of a liquid is inevitably greater than that of air, the diameter of the equivalent sphere would necessarily be different.

Hope this helps,
mj