spin drier effect?

[vic]

At a fairground ride, people can stand around the walls of a vertically
rotating cylinder. As the cylinder rotates, they are 'pressed ' against
the walls of the cylinder.
Without mentioning centrifugal force ( a no-no when teaching physics at
high school level), explain the origin of the centripetal force.
It looks to me like the circular motion is 'causing' the centripetal
force but I kind of teach my students that a centripetal force 'is
needed for circular motion in the first place.

Is there a simple answer to this?

Ideas, clarity etc appreciated.

[Randy Poe]

Post by vic
At a fairground ride, people can stand around the walls of a vertically
rotating cylinder. As the cylinder rotates, they are 'pressed ' against
the walls of the cylinder.
Without mentioning centrifugal force ( a no-no when teaching physics at
high school level), explain the origin of the centripetal force.
It looks to me like the circular motion is 'causing' the centripetal
force but I kind of teach my students that a centripetal force 'is
needed for circular motion in the first place.

The "centripetal not centrifugal" description is the one which
an observer sees who is not in the rotating frame of reference,
e.g. somebody standing on the ground.

Imagine that observer watching the ride spin. Suddenly he or
she is horrified to see the failure of the walls of the cylinder.
The people on the ride are flung out in STRAIGHT LINES
at the speed they were last moving at.

Let's now rewind the film and imagine that this catastrophe
didn't happen. What is the description? Points on the
outside of the cylinder are moving at some high speed v.
Due to restraints, people on the ride are given the same
speed. However, rather than moving in straight lines,
the walls of the cylinder continually press on them,
redirecting the linear path into a circle.

The people look squashed, obviously (says this observer)
from that inward redirecting force pushing on their backs.

If it bothers their intuition, if they think that the feeling you
get on such a ride can only come from being pushed from
the front, not the back, then ask them to imagine somebody
lying on an upward accelerating platform. A ground observer
would obviously say the person is being pushed upward by the
platform. But the person lying on the platform feels like
they are experiencing high gravity, a downward push.

Post by vic
Is there a simple answer to this?

Your statement was correct: "a centripetal force 'is
needed for circular motion in the first place." Without the
centripetal force provided by the walls, the motion of a person
given velocity v would be a straight line.

- Randy

[Sorcerer]

"vic" <***@aol.com> wrote in message news:***@b28g2000cwb.googlegroups.com...
| At a fairground ride, people can stand around the walls of a vertically
| rotating cylinder. As the cylinder rotates, they are 'pressed ' against
| the walls of the cylinder.
| Without mentioning centrifugal force ( a no-no when teaching physics at
| high school level), explain the origin of the centripetal force.

| It looks to me like the circular motion is 'causing' the centripetal
| force but I kind of teach my students that a centripetal force 'is
| needed for circular motion in the first place.
|
| Is there a simple answer to this?

Yes, there is.

Parachute onto a moving escalator or walkway, nothing happens until friction
happens.
Parachute into the rotor, nothing happens until friction happens.

In a rotating frame of reference the laws of physics change.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/gifs/coriolis.mov


Newton's LAW I.
Every body perseveres in its state of rest, or of uniform motion in a right
line, unless it is compelled to change that state by forces impressed
thereon.

Androcles' LAW I.

Every body perseveres in its state of rotation, or of uniform motion in a
circle, unless it is compelled to change that state by forces impressed
thereon.

Note that I do not disagree with Newton or Newton with me.
| Ideas, clarity etc appreciated.
|

[CWatters]

Post by vic
It looks to me like the circular motion is 'causing' the centripetal
force but I kind of teach my students that a centripetal force 'is
needed for circular motion in the first place.

Since the direction is constantly changing the object is being accelerated
towards the centre. A force is needed to accelerate an object so yes
centripetal force is needed for circular motion.

Centrifugal force felt by someone riding on the object arises from Newton's
third law.

[vic]

Post by CWatters

Post by vic
It looks to me like the circular motion is 'causing' the centripetal
force but I kind of teach my students that a centripetal force 'is
needed for circular motion in the first place.

Since the direction is constantly changing the object is being accelerated
towards the centre. A force is needed to accelerate an object so yes
centripetal force is needed for circular motion.
Centrifugal force felt by someone riding on the object arises from Newton's
third law.

Thanks for all replies so far - very helpful.