Catherine Wheel Effect
of
CENTRIFUGAL / CENTRIPETAL
Forces

so-called `forces'
on objects in rotating frames

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 so-called `centrifugal force' - an outward `force' which a rotating object is said to confer on its own particles.

So that freed particles - sparks and smoke - travel outwards in apparent spirals which are actually paths determined by `gravity' and the angle at which they left the firework.

How did a `force' get created simply by rotating something?

Likewise, where did that other `force' - the centripetal force holding the remainder of the firework together in the center of the image - 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 rotating spacecraft, which has been given spin to create an artificial gravity for the astronaut.

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 a one-off outward movement must equate to a "force" pushing the astronaut onto the wall, but that `force' is really a product of the circular / rotating movement of the `wall' and the astronaut's own inertia.

[Remember your last funfair ride?]

Even so, that's where the idea of a so-called centrifugal "force" comes from.

An apparent push - of inertia - from the center outwards.

`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

Although that 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 constant centripetal `force' comes from.

A real push - against inertia - inwards toward the center.

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

(the only energy involved is that given by rotation)

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.