T
Tabby
Guest
On Tuesday, 9 May 2023 at 14:51:13 UTC+1, John Larkin wrote:
Ballet does terrible things to the feet. The judges don\'t care.
Ballet does terrible things to the feet. The judges don\'t care.
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W
whit3rd
Guest
On Wednesday, May 10, 2023 at 2:50:26â¯PM UTC-7, Clive Arthur wrote:
It\'s clear that there\'s no external torque, so angular momentum is
constant. Angular momentum is I * 2Ï f
thus d(angular momentum)/dt = (I * 2Ï df/dt )+ (dI/dt * 2Ïf) = 0
It\'s gotta change the spin frequency when you change angular moment \"I\", unless f = 0
so... vertical movement of air causes hurricanes, and dancers can do a really dramatic turn...
On 10/05/2023 21:25, Fred Bloggs wrote:
Energy of rotation is (angular momentum)^2/(2I) , so it obviously increases, greatly in the case of the spinning skater, with no apparent application of external torque.
That\'s all true I suspect, but IMO doesn\'t really demonstrate where the
Coriolis force comes from.
It\'s clear that there\'s no external torque, so angular momentum is
constant. Angular momentum is I * 2Ï f
thus d(angular momentum)/dt = (I * 2Ï df/dt )+ (dI/dt * 2Ïf) = 0
It\'s gotta change the spin frequency when you change angular moment \"I\", unless f = 0
so... vertical movement of air causes hurricanes, and dancers can do a really dramatic turn...
R
RichD
Guest
On May 9, Clive Arthur wrote:
No, an object in motion deviates from a straight line due to
an applied force, it doesn\'t \"make a force\".
In this case, where is the force tangential to the object\'s
instantaneous velocity, where is the kick in the rump?
Which is the question under discussion.
--
Rich
The skater does real work by moving her arms inwards in a rotating frame
of reference and it creates a torque that spins her up faster.
How does it create a torque, when it\'s RADIAL, i.e. perpendicular to the
tangential velocity?
To simplify, let\'s say she has very thin arms and is holding weights.
Let\'s also say that she takes half a rotation to pull the weights fully
in. Looking down from a stationary position above, as she spins and
draws the weights inwards, they describe a curve. A mass going round a
curve makes a force, in this case, some of that is tangential.
No, an object in motion deviates from a straight line due to
an applied force, it doesn\'t \"make a force\".
In this case, where is the force tangential to the object\'s
instantaneous velocity, where is the kick in the rump?
Which is the question under discussion.
--
Rich
R
RichD
Guest
On Tuesday, May 9, 2023 at 2:53:19â¯PM UTC-7, John Larkin wrote:
Good question. A spring lives in a 1-D space. Every point in that
space corresponds to a spring length, and a potential energy.
As the spring stretches, the \'field\' delivers/absorbs energy
from the attached object.
--
Rich
When her arms were outstretched they had significant stored potential
energy that the dancer converts into kinetic energy by her actions.
You\'re unclear on the concept. In mechanics, potential energy is
always associated with a field.
A compressed spring has potential energy. What field is that?
Good question. A spring lives in a 1-D space. Every point in that
space corresponds to a spring length, and a potential energy.
As the spring stretches, the \'field\' delivers/absorbs energy
from the attached object.
As an object moves point to point, it gains/loses energy from the field.
--
Rich
R
RichD
Guest
On Wednesday, May 10, 2023 at 5:28:42â¯AM UTC-7, Phil Hobbs wrote:
Artificial gravity:
https://www.youtube.com/watch?v=-RGGK2uyJOw
Imagine baseball in that environment -
It\'s the gyroscope effect.
Twist a miniature hand held fan -
--
Rich
Newtonian dynamics requires corrections by fictitious forces before you
can sensibly apply it in a rotating frame of reference. One of my
physics supervisors worked on rotating radars in WWII and had stories of
people knocking themselves out reaching for spanner too quickly and
forgetting that they were in a rotating frame of reference. Apparently
after a while of being rotated you largely desensitise to it.
(until you stop the rotation then the world spins in reverse)
Artificial gravity:
https://www.youtube.com/watch?v=-RGGK2uyJOw
Imagine baseball in that environment -
Those old-timey playground carousels were a good lab course too. A
couple of kids running at top speed to spin it up, then pull themselves
on board. They had tangential rails along the rim, and radials from rim
to pivot. Pulling yourself along one of the radials towards the axis
involves a lot of centrifugal force, but the really surprising thing is
the strength of the Coriolis pulling your feet sideways as you move them.
It\'s the gyroscope effect.
Twist a miniature hand held fan -
--
Rich
R
RichD
Guest
On Wednesday, May 10, 2023 at 3:29:59â¯PM UTC-7, Fred Bloggs wrote:
Evidently the cross product produces a tangential force. Which isn\'t
intuitive - 300 years of befuddled physics students will back me up -
>>> Energy of rotation is (angular momentum)^2/(2I) , so it obviously increases,
\"obvious\" only in the arithmetic sense. But physically, why should
a change in moment of inertia generate energy? Not at all obvious -
It\'s fictitious because, in the usual context of an observer in an accelerating
frame, the \'force\' doesn\'t do work. That is, the observer in a rotating frame
sees vehicles outside travel in spirals. Do they feel anything?
However, in the skating case, real work is performed, as the arms withdraw.
For dropped objects, the eastward drift results from force of
gravity; it\'s dynamical, not merely kinematic. Sans gravitational
acceleration, the earth\'s rotation won\'t produce a coriolis effect.
--
Rich
That doesn\'t answer the original question as to why the skater\'s rotation speeds up,
and why the kinetic energy of rotation increases.
The answer is Coriolis force, Fc, something that\'s been known for nearly 400 years.
Fc= -2m w* X v\'\'*
m is particle mass, or differential to be part of a sum
w* is angular velocity of rotating frame a ***vector****
v\'\'* is velocity of particle relative to the rotating frame, a ***vector***
X is vector cross-product
v\'\'* has a component due to dr\'\'*/dt, velocity radially inward, or outward, as the arms extend
Here r\'\'* is radial vector to mass m in the rotating reference frame.
Use the right hand rule w* aligned with axis of rotation, r\'\'* oriented as usual,
and you get exactly a force applied tangentially to the radius of travel of m. The force
materializes by way of acceleration of m in the rotating reference frame.
Evidently the cross product produces a tangential force. Which isn\'t
intuitive - 300 years of befuddled physics students will back me up -
>>> Energy of rotation is (angular momentum)^2/(2I) , so it obviously increases,
\"obvious\" only in the arithmetic sense. But physically, why should
a change in moment of inertia generate energy? Not at all obvious -
I should add the Coriolis force is not a real force. It\'s called a fictitious force
because the Coriolis effect is that which would arise from the fictitious force acting
on the system.
It\'s fictitious because, in the usual context of an observer in an accelerating
frame, the \'force\' doesn\'t do work. That is, the observer in a rotating frame
sees vehicles outside travel in spirals. Do they feel anything?
However, in the skating case, real work is performed, as the arms withdraw.
For dropped objects, the eastward drift results from force of
gravity; it\'s dynamical, not merely kinematic. Sans gravitational
acceleration, the earth\'s rotation won\'t produce a coriolis effect.
--
Rich
J
John Larkin
Guest
On Thu, 11 May 2023 10:21:04 -0700 (PDT), RichD
<r_delaney2001@yahoo.com> wrote:
Probably electric field.
<r_delaney2001@yahoo.com> wrote:
On Tuesday, May 9, 2023 at 2:53:19?PM UTC-7, John Larkin wrote:
When her arms were outstretched they had significant stored potential
energy that the dancer converts into kinetic energy by her actions.
You\'re unclear on the concept. In mechanics, potential energy is
always associated with a field.
A compressed spring has potential energy. What field is that?
Good question.
Probably electric field.
A spring lives in a 1-D space. Every point in that
space corresponds to a spring length, and a potential energy.
As the spring stretches, the \'field\' delivers/absorbs energy
from the attached object.
As an object moves point to point, it gains/loses energy from the field.