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PLEASE HELP! in Deriving Terminal Velocity Equation

  • Thread starter hibachii
  • Start date
17
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PLEASE HELP! in Deriving Terminal Velocity Equation :)

1. Homework Statement

The terminal velocity of a mass m, moving at 'high speeds' through a fluid of density
ρ(kg m^-3), is given by v = sqrt(2mg/DρA) where A is the cross sectional area of the object (m^2) and D a dimensionless "drag coefficient".

i) Show that equation is dimensionally correct
ii) Estimate the terminal velocity of an Australian $1 coin. Take D to be ~0.3.


2. Homework Equations

v = sqrt(2mg/DρA)


3. The Attempt at a Solution

I'm completely stuck. I have no clue as in how to even start. I'm so sorry :(
 

Answers and Replies

tiny-tim
Science Advisor
Homework Helper
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welcome to pf!

hi hibachii! welcome to pf! :smile:

(have a square-root: √ and try using the X2 tag just above the Reply box :wink:)

what are the dimensions of each symbol in √(2mg/DρA)?

check that when you combine them they come to the same as v :wink:
 
1
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Re: PLEASE HELP! in Deriving Terminal Velocity Equation :)

I'm going to assume your in the same Physics Class as I am.

If you go to Page 7 of the the book "Physics for Scientists and Engineers" by Serway it explains how to do Dimensional Analysis and that will explain how to do (i).

For (ii) you will need to look up a value for the density of air and then estimate the mass and area of the coin remembering that the coin would be rotating as it fell through the air
 
17
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Re: PLEASE HELP! in Deriving Terminal Velocity Equation :)

Cheers guys. Helped alot.

P.S Jazrad what class are you in for physics 101?
 
17
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Re: PLEASE HELP! in Deriving Terminal Velocity Equation :)

Hey jazrad thanks but im still stuck. Ive looked at page 7 and 8 but it only shows dimensions for velocity and acceleration. Ive still got alot of unknowns like D, A, m and ρ.
I tried subbing in L/T for v and L/T^2 for g but im still left stuck. Please give some more help although you have provided me with a very good start.
Cheers
 
tiny-tim
Science Advisor
Homework Helper
25,789
249
hey hibachii! :smile:

(try using the X2 icon just above the Reply box :wink:)
Ive still got alot of unknowns like D, A, m and ρ.
(D is given as dimensionless :wink:)

to find the dimensions of an unfamiliar variable, decde what units it's measured in (m/s2 for example), then convert each unit to dimensions (m/s2 = L/T2)

start with ρ, the density …

what units is it measured in? :smile:
 

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