# Terminal velocity with Reynods# and Drag Coeff.

Hi guys i've been reading on the boards for a few days now and everyone here seems really helpful, so i've decided to come and ask for some guidence on a problem i've been having

## Homework Statement

I have to find the terminal velocity of a smooth wooden ball.
rho(ball)=420 kg/m^3
rho(air)=1.1614kg/m^3
mu=1.846x10^-5 N-s/m^2
dia(ball)=.2m

## Homework Equations

I am given a table of about 25 reynolds numbers and drag coeff values, i can post them if it is necessary

## The Attempt at a Solution

My approach was to use the terminal velocity equation

mg=(.5)(rho of ball)(v)^2(A)(drag coeff)

to guess a value for the velocity, then calculate the drag coeff, then use my guessed velocity to calculate reynolds number using

Re=[(rho ball)(v)(dia ball)]/mu

Then my plan was to take my calculated reynolds number(which i hoped was the same as one of the reynolds numbers in my given table) and then take the corresponding drag coeff and see if it matched my calulated drag coeff, and if it i would know that the guess i made for the terminal velocity was correct. But i cannot get this approach to work, my calulated Re values are much larger than my table values. Any suggestions?

PhanthomJay
Homework Helper
Gold Member
Hi guys i've been reading on the boards for a few days now and everyone here seems really helpful, so i've decided to come and ask for some guidence on a problem i've been having

## Homework Statement

I have to find the terminal velocity of a smooth wooden ball.
rho(ball)=420 kg/m^3
rho(air)=1.1614kg/m^3
mu=1.846x10^-5 N-s/m^2
dia(ball)=.2m

## Homework Equations

I am given a table of about 25 reynolds numbers and drag coeff values, i can post them if it is necessary

## The Attempt at a Solution

My approach was to use the terminal velocity equation

mg=(.5)(rho of ball)(v)^2(A)(drag coeff)

to guess a value for the velocity, then calculate the drag coeff, then use my guessed velocity to calculate reynolds number using

Re=[(rho ball)(v)(dia ball)]/mu

Then my plan was to take my calculated reynolds number(which i hoped was the same as one of the reynolds numbers in my given table) and then take the corresponding drag coeff and see if it matched my calulated drag coeff, and if it i would know that the guess i made for the terminal velocity was correct. But i cannot get this approach to work, my calulated Re values are much larger than my table values. Any suggestions?
Your terminal velocity equation is incorrect. What densiy (and what area) should you be using?