Resisted Motion (1 Viewer)

[djgjp]

Hey everyone,
In mechanics... resisted motion.
Would anyone know a rule for retardations?
because in the fitzpatric textbook...... in some questions with retardations mass is included in the resistance equation e.g. retatdation due to resistance in the medium varies as the square of the velocity is: resistance = mkv^2
but in another question mass is included in the resistance equation when retardation is not mentioned at all!!!
I am sooooo confused!
if anyone can help, it would be greatly apreciated!
Thankyou
Goodbye
DjGjP

 

[Rorix]

perhaps if you quoted the question in question we could explain the error of your ways

 

[Constip8edSkunk]

it depends on whether the mass is included in the constant or not...based on the question..... if not then the mass cancels of when writing the force equation , otherwise you get ma = kf(v) and value 4 mass is included in k, but it hardly matters usually as that assumes mas to be constant anyway

 

[Roobs]

Hmm..ive got pretty much the same query...but its got me stumped like a bitch:

ok in fitzpatrick 35b) q5) b) mass is not included, and so the equation of motion is

f=ma
mg-kv^2=ma
a=g-kv^2/m

and this seems logical to me

however in question 7, the equation of motion is
f=ma
mg-mkv^2=ma
a=g-kv^2

now the question ask for terminal velocity, and if set up like q5, it turns out that terminal velocity is dependant on mass, which physics will tell you isnt true
my question is why shoulf the air resistance be mkv^2, i cant understand the logic that air resistance is dependant on mass:
surface area, shae and velocity yes, but why mass?

any insight appreciated

(on a side not the conclusion our class reached was that fitz. is a wanker, and to use mkv^2, but no one could explain why)

 

[pLuvia]

Could you please also post up the question?

 

[Rax]

I know what they are talking about.
In the Fitzpatrick book in some problems in the retardation they include mass or disclude it without any real indication.
Its just written badly, dont worry, in the HSC's they all specify quite clearly the retardation they wish you to use

 

[Roobs]

Goodstuff Rax...i was at that goonelabah study day 2 btw.....but yeah thats the conclusion we came to--

For pLuvia the question was:

"a body falls towards earth with constant gravitational acceleration, g, and against a resistance which produces a retardation proportional to velocity. express v as a function of t, and hence find terminal velocity. k=1/5"

using the starting point:

f=ma
mg-kv=ma

will yeild a result where the terminal velocity is a function of m (=49m)
obviously this is incorrect (to my knowledge) and so the starting point
mg-mkv=ma

must be used (yeild books answer of =49)

*lotto i can see where you're coming from, but in this case a numerical answer was required


my query was about why the resistance should have anything to do with the mass of an
object (maybe more of a physics question, but way outside the syllabus)?

any takers

 

[Roobs]

Thanks--apparantly that concept was just foreign to me-- with all the HSC physics hoopla about Hollow balls and solid balls falling at the same rate--for the sake of simplicity i guess.

Getting off topic--but just then i did some searching and found a java applet that confirms all, for those that get their brain warped around this

http://csip.cornell.edu/Curriculum_Resources/CSIP/Bernier&Fogleman/projectile.html