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Derivation of r (radius) in terms of f (frequency)? Help please. 
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#1
Apr2514, 05:10 PM

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Okay, so the job I need to do is derive an equation for the radius of an object in terms of its frequency.
These are the equations that we are allowed to use: v(Linear velocity) = rω v=2πr/T ω (angular velocity)=2πf f (frequency)= 1/T (time period) T= 2πr/v a (centripetal acceleration)=v^2/r F (centripetal force)=mv^2/r So now.. my attempt: r= v/ω = √ar/2πf = (√(a*(vT/2π))/2πf 2πfr=√(a*(vT/2π) 2πfr^2= a * (vT/2π) 2π * 2πfr^2 = a * vT ...... I'm lost I have so many variables that I need to eliminate and I'm not sure what else to do from here. Help would be very appreciated. 


#2
Apr2514, 05:17 PM

Sci Advisor
PF Gold
P: 11,923

Hi
I am not sure what you are trying to do. Radius and frequency are entirely different quantities and they are not directly related except when you state the situation that you are studying. You have written down some equations that relate them and some other quantities. What are you trying to achieve by jiggling around the original equations in the second half? I think you are trying for something that is impossible or has no meaning. Could you explain more fully, please? 


#3
Apr2514, 08:34 PM

P: 3




#4
Apr2514, 10:54 PM

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HW Helper
Thanks
PF Gold
P: 6,318

Derivation of r (radius) in terms of f (frequency)? Help please.
Well, which of the equations you listed in the OP would be useful in figuring out how a person can be pushed up against the outer wall of the ride as it turns? What is keeping the person pressed up against the wall?



#5
Apr2614, 05:17 AM

Sci Advisor
PF Gold
P: 11,923

What did you expect, for such a sophisticated problem? But 'complication' is a very relative thing. 


#6
Apr2614, 06:55 AM

P: 3

2nd attempt
Okay so I did rearranged an equation first to get... √ar = v and inserted it in to the equation: F=mv^2/r and got... F=m*(√ar^2)/r = m*a*r/r = ma 3rd attempt: v = 2πr/T √ar = 2πr/(2πr/v) √ar = v ??? (Cancelled itself out) The wrong variables keep cancelling out for some reason. I've done quite the number of attempts and "r" keeps getting cancelled out and that's one of the variables that I need. I'm not sure what I'm doing wrong? This equation has force, speed, and radius so did I pick the right equation to use or...? 


#7
Apr2614, 07:11 AM

Sci Advisor
PF Gold
P: 11,923

Why are you surprised when the variables to cancel out? You can get all sorts of results when you substitute and reduce.
You start off with the right equation ( F=mv^2/r) and then shoot yourself in the foot by 'tidying it up' too much. Put in the value you think is appropriate for the Force and then you can rearrange to get how V and r are related. Job done, bish bash bosh! 


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