Re: 757 engine (fan?) noise during freewheel

From:         M.J.Jennings@amtp.cam.ac.uk (Michael Jennings)
Organization: University of Cambridge DAMTP
Date:         03 Nov 95 04:23:14 
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In article <airliners.1995.1548@ohare.Chicago.COM>,
Jon Ward  <Jon_Ward@blibble.demon.co.uk> wrote:
>           barr@netcom.com "Keith Barr" writes:
>> Centrifugal force holds the blades in the proper position when the engine is
>> running.
>
>Hmmm, correct me if I am wrong, but I am _sure_ that there is no thing as
>centrifugal force. There is a combination of centripetal acceleration and
>tangental velocity in circular motion, which is what I assume holds the fan
>blades in place.
>
	It does depend slightly on how you look at it. In an accelerating
frame of reference (of which a rotating frame is an example), a force is
appied on objects in the frame, due to the accelerating frame of reference.
This force is applied outwards from the centre of rotation, and is called
'centrifugal force'. It is an example of a class of forces called
'fictitious forces' or 'inertial forces'. If you are sitting in a frame
of reference that is rotating, then the force is perfectly real, which
is why I don't like the expression 'fictitious force'. If you look at
the same physical situation from a frame of reference that is not accelerating,
then it is true that the centrifugal force is not present. However, this
is no more or less valid than looking at the problem from an accelerating
frame of reference. Whether it is more or less useful depends on the
situation.
--
Michael Jennings
Department of Applied Mathematics and Theoretical Physics
The University of Cambridge. 		mjj12@damtp.cambridge.ac.uk

Disclaimer: the opinions presented here are mine alone,
but they should be yours too because they're right.