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Copyright © 2009 Ray Dickenson
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Catherine Wheel Effect
of
LAZY CENTRIFUGAL / CENTRIPETAL
Forces

so-called `forces'
on objects in rotating frames

catherine wheel

A Catherine Wheel

(from St. Catherine)


A rotating firework which gets its dramatic effect by throwing out spirals of colored sparks, flame and smoke.

And the `throwing' is done by what?
A `centrifugal force' - an outward `force' which a rotating object confers on its own particles.

So that freed particles of sparks and smoke travel outwards in spirals.


How was an outward `force' created by rotating something?


Likewise, where did that other `force' - the inward force holding the remainder of the firework together - come from?




In a rotating spacecraft



Let's transfer to an environment where we can see, or rather sense, these `forces'

Here we see a spacecraft, which is being `spun' / rotated to provide artificial gravity for the astronaut.




cross-section of spacecraft


Centrifugal
As you see, the astronaut at "A" would fly off towards "D" if not for the enclosing wall of the spacecraft.
(Because things tend to carry on doing what they were doing. See inertia below)

We instinctively feel such an outward movement must equate to a "force" pushing the astronaut onto the wall, but that `force' is really the astronaut's own inertia acting against the circular / rotating movement of the wall.

[Remember your last funfair ride?]

Yup, that's where the centrifugal "force" comes from.

An outwards push of inertia.



`Inertia' note:

remember from `inertmass' that objects don't easily move, or stop if they're already moving?

It's always known as inertia if not moving, & sometimes called momentum if moving, but it's the same phenomenon

objects tend to carry on doing what they were doing


cross-section of spacecraft



Centripetal
Although the spinning spacecraft's wall is moving sideways, it's also always moving `inward' / `upward' - from the viewpoint of the astronaut's inertial mass.

So the astronaut, now shown at "B", is always being pushed `inward' / `upward' by the wall's circular rotation - towards "C", the center of the spacecraft.

That acceleration - (change in speed or direction, remember) - gives the astronaut a "g force" or sense of "gravity", like you'd feel in a rising elevator.

That's where the centripetal "force" comes from.

A push inwards (towards the center) against inertia.



`centrifugal' & `centripetal' aren't
"MAGIC FORCES"

Rotation powers both effects




Inertial Mass in Rotation


Reaction of mass held in rotation, or released:
"Centripetal or Centrifugal Force"

Reaction of mass to varying radius of rotation:
"Conservation of Angular Momentum"

Reaction of mass to varying angles of rotation:
"Precession"

Reactions of mass to other movements:
"Coriolis Force"




"Centrifugal force" and "Centripetal force"
is the same phenomenon:

reaction of a body's inertia when

released from rotation (centrifugal),
or
retained in rotation (centripetal).


The astronaut's `acceleration' - towards the center of rotation - can be calculated by:
m u2/r
Where m is astronaut's mass, u is the constant speed of rotation, and r is distance to center of spacecraft.



`MYSTERIOUS FORCES'

"INERTIA"

"INERTIA or MOMENTUM"

"Spinning-Top Effect of PRECESSION Forces"

"Hurricane, Whirlwind & Bathtub Effects of CORIOLIS Forces"

"Catherine Wheel Effect of CENTRIFUGAL / CENTRIPETAL Forces"


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