Physics Quiz...

[Lasse Langwadt Christensen]

mandag den 24. april 2023 kl. 23.26.19 UTC+2 skrev John Larkin:

On Mon, 24 Apr 2023 13:13:48 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

mandag den 24. april 2023 kl. 22.07.22 UTC+2 skrev John Larkin:
On Mon, 24 Apr 2023 12:49:28 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

mandag den 24. april 2023 kl. 20.12.14 UTC+2 skrev John Larkin:
On Mon, 24 Apr 2023 09:24:51 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

mandag den 24. april 2023 kl. 00.33.17 UTC+2 skrev John Larkin:
On Sun, 23 Apr 2023 15:19:40 -0700 (PDT), Fred Bloggs
bloggs.fred...@gmail.com> wrote:
On Sunday, April 23, 2023 at 1:03:04?PM UTC-4, John Robertson wrote:
On 2023/04/23 4:01 a.m., Fred Bloggs wrote:
So a vehicle is driving down the road when an entire wheel comes off and continues its direction unchanged rolling alongside the vehicle. Question is why does the wheel end up accelerating, rolling much faster than its original speed, outpacing the vehicle significantly? Answer should be obvious, but you need practical insight.
The wheel can\'t accelerate without an external force being added to it -
it has no means of self propulsion. Laws of the conservation of momentum
apply as usual.

Think of a figure skater doing one of those stationary spins. Arms outstretched is one spin rate, arms brought into the side and they turn into a blur. No external forces come into play.

The skater must use muscle power to pull her arms in. The work done in
pulling her arms in is converted to energy stored in the spinning mass
of her body, and is recoverable.

angular momentum is the product of moment of inertia and angular velocity

angular momentum can\'t just change (newtons 3rd) so when the moment of inertia get smaller (pulling arms in)
the angular velocity has to increase

no energy added, just \"stored\" differently
She did work pulling her arms in, burned a bit of breakfast and added
energy to the rotating system.

sure she spend some energy but it doesn\'t add to the rotational energy



Energy is conserved. The work she does to pull her arms in isn\'t lost,
and can be recovered. It can only be stored in the rotational energy.

what if she is standing still?

\"Think of a figure skater doing one of those stationary spins.\"
started this.

moving her arms in and out doesn\'t add to the rotational energy, the rotational energy is the same

 

[John Larkin]

On Mon, 24 Apr 2023 14:50:26 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

mandag den 24. april 2023 kl. 23.26.19 UTC+2 skrev John Larkin:
On Mon, 24 Apr 2023 13:13:48 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

mandag den 24. april 2023 kl. 22.07.22 UTC+2 skrev John Larkin:
On Mon, 24 Apr 2023 12:49:28 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

mandag den 24. april 2023 kl. 20.12.14 UTC+2 skrev John Larkin:
On Mon, 24 Apr 2023 09:24:51 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

mandag den 24. april 2023 kl. 00.33.17 UTC+2 skrev John Larkin:
On Sun, 23 Apr 2023 15:19:40 -0700 (PDT), Fred Bloggs
bloggs.fred...@gmail.com> wrote:
On Sunday, April 23, 2023 at 1:03:04?PM UTC-4, John Robertson wrote:
On 2023/04/23 4:01 a.m., Fred Bloggs wrote:
So a vehicle is driving down the road when an entire wheel comes off and continues its direction unchanged rolling alongside the vehicle. Question is why does the wheel end up accelerating, rolling much faster than its original speed, outpacing the vehicle significantly? Answer should be obvious, but you need practical insight.
The wheel can\'t accelerate without an external force being added to it -
it has no means of self propulsion. Laws of the conservation of momentum
apply as usual.

Think of a figure skater doing one of those stationary spins. Arms outstretched is one spin rate, arms brought into the side and they turn into a blur. No external forces come into play.

The skater must use muscle power to pull her arms in. The work done in
pulling her arms in is converted to energy stored in the spinning mass
of her body, and is recoverable.

angular momentum is the product of moment of inertia and angular velocity

angular momentum can\'t just change (newtons 3rd) so when the moment of inertia get smaller (pulling arms in)
the angular velocity has to increase

no energy added, just \"stored\" differently
She did work pulling her arms in, burned a bit of breakfast and added
energy to the rotating system.

sure she spend some energy but it doesn\'t add to the rotational energy



Energy is conserved. The work she does to pull her arms in isn\'t lost,
and can be recovered. It can only be stored in the rotational energy.

what if she is standing still?

\"Think of a figure skater doing one of those stationary spins.\"
started this.

moving her arms in and out doesn\'t add to the rotational energy, the rotational energy is the same

She did work to pull her extended arms in close to her body, working
against the centrifugal force. Where did that energy go?

 

[Ricky]

On Sunday, April 23, 2023 at 12:58:21 PM UTC-4, Clive Arthur wrote:

On 23/04/2023 16:39, Ricky wrote:
On Sunday, April 23, 2023 at 7:01:07 AM UTC-4, Fred Bloggs wrote:
So a vehicle is driving down the road when an entire wheel comes off and continues its direction unchanged rolling alongside the vehicle. Question is why does the wheel end up accelerating, rolling much faster than its original speed, outpacing the vehicle significantly? Answer should be obvious, but you need practical insight.

The vehicle\'s speed is controlled by the motor. It can provide both force to accelerate and drag to slow. You don\'t say anything about which is happening in this case, if either.

Someone has pointed out that the wheel\'s diameter is now larger. That would tap the rotational inertia, turning it into linear velocity, but it won\'t be a large amount, certainly single digit percent, since the size difference is on that scale. That doesn\'t sound like \"much faster\".

The effective diameter of the wheel is proportional to the radius. The
effective radius is from axle centre to road surface. Take away the
load and that\'s quite a significant change in effective diameter.

Define \"quite a significant change\" in terms of % distance. I run the specified pressures in my tires and they don\'t have a large difference from being round. Like I said, I can tell by eye, it\'s no more than 10%. Still, that\'s enough to get the wheel running 5% faster than the car.

--

Rick C.

+ Get 1,000 miles of free Supercharging
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[RichD]

On April 23, Clive Arthur wrote:

On 23/04/2023, Ricky wrote:
So a vehicle is driving down the road when an entire wheel comes
off and continues its direction unchanged rolling alongside the vehicle.
Question is why does the wheel end up accelerating, rolling much faster
than its original speed, outpacing the vehicle significantly?

Someone has pointed out that the wheel\'s diameter is now larger.
That would tap the rotational inertia, turning it into linear velocity,

huh?

The effective diameter of the wheel is proportional to the radius. The
effective radius is from axle centre to road surface. Take away the
load and that\'s quite a significant change in effective diameter.

um, if the wheel\'s diameter (the tire, actually) increases,
its moment of inertia increases. Angular momentum
is presumably conserved, hence the tire SLOWS.

Good to see that everyone here aced Physics 101 -

--
Rich

 

[Ricky]

On Sunday, April 23, 2023 at 1:03:04 PM UTC-4, John Robertson wrote:

On 2023/04/23 4:01 a.m., Fred Bloggs wrote:
So a vehicle is driving down the road when an entire wheel comes off and continues its direction unchanged rolling alongside the vehicle. Question is why does the wheel end up accelerating, rolling much faster than its original speed, outpacing the vehicle significantly? Answer should be obvious, but you need practical insight.
The wheel can\'t accelerate without an external force being added to it -
it has no means of self propulsion. Laws of the conservation of momentum
apply as usual.

Exactly! So with the increased radius of an unloaded wheel, it is spinning faster than its forward velocity would match. So the tire both slows its spinning by some factor and speeds its forward velocity.

Rather the vehicle the wheel left has started to decelerate due to
increased drag on the other three axles and as the vehicle side missing
a wheel drops to the pavement.

LOL Not all cars will drop the corner without the wheel. Or I should say, not all wheels are on corners that will drop to the ground when the wheel is removed.

--

Rick C.

-- Get 1,000 miles of free Supercharging
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[John Larkin]

On Mon, 24 Apr 2023 14:45:36 -0700 (PDT), Tabby <tabbypurr@gmail.com>
wrote:

On Monday, 24 April 2023 at 19:12:14 UTC+1, John Larkin wrote:
On Mon, 24 Apr 2023 09:24:51 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

mandag den 24. april 2023 kl. 00.33.17 UTC+2 skrev John Larkin:
On Sun, 23 Apr 2023 15:19:40 -0700 (PDT), Fred Bloggs
bloggs.fred...@gmail.com> wrote:
On Sunday, April 23, 2023 at 1:03:04?PM UTC-4, John Robertson wrote:
On 2023/04/23 4:01 a.m., Fred Bloggs wrote:
So a vehicle is driving down the road when an entire wheel comes off and continues its direction unchanged rolling alongside the vehicle. Question is why does the wheel end up accelerating, rolling much faster than its original speed, outpacing the vehicle significantly? Answer should be obvious, but you need practical insight.
The wheel can\'t accelerate without an external force being added to it -
it has no means of self propulsion. Laws of the conservation of momentum
apply as usual.

Think of a figure skater doing one of those stationary spins. Arms outstretched is one spin rate, arms brought into the side and they turn into a blur. No external forces come into play.

The skater must use muscle power to pull her arms in. The work done in
pulling her arms in is converted to energy stored in the spinning mass
of her body, and is recoverable.

angular momentum is the product of moment of inertia and angular velocity

angular momentum can\'t just change (newtons 3rd) so when the moment of inertia get smaller (pulling arms in)
the angular velocity has to increase

no energy added, just \"stored\" differently
She did work pulling her arms in, burned a bit of breakfast and added
energy to the rotating system. When she extends them back out, work is
done on her arms. Of course, muscles don\'t recover energy when
back-driven but some other gadget, a prosthetic arm maybe, could.

One could build a robot lady to do the skater spin thing. If it could
recover the energy supplied when it extends its arms, it could spin up
and down forever without needing additional power.

In a vacuum. On a gas bearing. Which requires an infinitely powerful pump

Energy is conserved.

A robot skater lady could use a DC motor/generator to extend and later
pull the arms in. Extension can generate power to be saved in a
battery or capacitor. That can be used later to pull the arms in.

 

[RichD]

On April 23, Ralph Mowery wrote:

I was in a car that a rear wheel came off while the car was doing 40
mph. The wheel passed us and a few seconds later the rear of the car
started to fall.

So there was a 3 second delay, before gravity kicked in?
Because the car\'s velocity carried it forward before it started to fall?

https://tenor.com/view/coyote-loony-tunes-bye-farewell-waving-gif-16026851

--
Rich

 

[Clive Arthur]

On 24/04/2023 23:12, RichD wrote:

On April 23, Clive Arthur wrote:
On 23/04/2023, Ricky wrote:
So a vehicle is driving down the road when an entire wheel comes
off and continues its direction unchanged rolling alongside the vehicle.
Question is why does the wheel end up accelerating, rolling much faster
than its original speed, outpacing the vehicle significantly?

Someone has pointed out that the wheel\'s diameter is now larger.
That would tap the rotational inertia, turning it into linear velocity,

huh?

The effective diameter of the wheel is proportional to the radius. The
effective radius is from axle centre to road surface. Take away the
load and that\'s quite a significant change in effective diameter.

um, if the wheel\'s diameter (the tire, actually) increases,
its moment of inertia increases. Angular momentum
is presumably conserved, hence the tire SLOWS.

Good to see that everyone here aced Physics 101 -

--
Rich

Look at it this way. When the car is running normally, there is a
distance between the centre of rotation of the axle and the road. I\'m
calling that the effective radius. The tyre isn\'t a circle, it\'s
squished due to the weight of the car.

Every pico degree rotation of the axle corresponds to a distance moved,
proportional only to this effective radius. The parts of the tyre which
are not in contact with the ground don\'t make no never-mind.[1]

Take the weight off the wheel by detaching it (almost instantly) and the
effective radius increases. It\'s still spinning at X RPM but it\'s now
\'bigger\'. So it goes faster.

Don\'t overthink it.

[1] Crap design, the wheel. At any one time, most of it is doing nothing.

--
Cheers
Clive

 

[RichD]

On April 23, John Larkin wrote:

So a vehicle is driving down the road when an entire wheel comes off
and continues its direction unchanged rolling alongside the vehicle.
Question is why does the wheel end up accelerating, rolling much faster
than its original speed, outpacing the vehicle significantly?

A wheel can store energy in the rubber. Before it was turned loose,
there was torque, which flexed the rubber.

The continuous torque is what rotates the wheel, against
losses. It the torque is removed, it slows. (!!)

> And compression.

The compression is the flattened portion, against the road.
Weight and gravity. When it decompresses, it bounces straight
up. No torque.

--
Rich

 

[RichD]

On April 23, Fred Bloggs wrote:

So a vehicle is driving down the road when an entire wheel comes off
and continues its direction unchanged rolling alongside the vehicle.
Question is why does the wheel end up accelerating, rolling much
faster than its original speed, outpacing the vehicle significantly?

Mythbusters, paging Dr. Mythbusters, report to sci.elect.design, urgent -

--
Rich

 

[Lasse Langwadt Christensen]

tirsdag den 25. april 2023 kl. 00.12.48 UTC+2 skrev RichD:

On April 23, Clive Arthur wrote:
On 23/04/2023, Ricky wrote:
So a vehicle is driving down the road when an entire wheel comes
off and continues its direction unchanged rolling alongside the vehicle.
Question is why does the wheel end up accelerating, rolling much faster
than its original speed, outpacing the vehicle significantly?

Someone has pointed out that the wheel\'s diameter is now larger.
That would tap the rotational inertia, turning it into linear velocity,
huh?
The effective diameter of the wheel is proportional to the radius. The
effective radius is from axle centre to road surface. Take away the
load and that\'s quite a significant change in effective diameter.
um, if the wheel\'s diameter (the tire, actually) increases,
its moment of inertia increases. Angular momentum
is presumably conserved, hence the tire SLOWS.

depending on the weight distribution the moment of inertia might
not increase as much as the diameter increased

 

[Ricky]

On Sunday, April 23, 2023 at 6:17:50 PM UTC-4, Fred Bloggs wrote:

On Sunday, April 23, 2023 at 9:39:16 AM UTC-4, Martin Brown wrote:
On 23/04/2023 13:44, Fred Bloggs wrote:
On Sunday, April 23, 2023 at 7:49:24 AM UTC-4, Anthony William Sloman
wrote:
On Sunday, April 23, 2023 at 9:01:07 PM UTC+10, Fred Bloggs wrote:
So a vehicle is driving down the road when an entire wheel comes
off and continues its direction unchanged rolling alongside the
vehicle. Question is why does the wheel end up accelerating,
rolling much faster than its original speed, outpacing the
vehicle significantly? Answer should be obvious, but you need
practical insight.
The only thing that could accelerate it would be the airflow around
the vehicle body, which would be associated with trailing
vortices.

It seems unlikely than any of them could make the wheel rotate much
faster than it\'s original speed - it would outrun the car and the
trailing vortices - unless it was moving sideways with respect to
the rest of the car, and out into the wake.

Alright, you\'re getting close. From what I\'ve personally observed, a
wheel dropping off the vehicle leaves the vehicle behind in the dust,
it really takes off.
Its diameter changes quite radically when it is no longer being squashed
by the weight of the car and so although it was previously rolling with
constant velocity at radius r when loaded the sudden loss of load means
the tyre expands to about 10-15% larger radius R ~ 1.1r where it is in
contact with the road. The car may also slow down with one wheel missing
if the driver has any road sense at all.

Angular momentum is conserved in the short term so the wheel now has a
linear velocity increase by about 10-15% compared to the car.
Doesn\'t the increased radius increase the angular moment of inertia about the centroid, thereby slowing the rotational speed to conserve angular momentum?

It\'s not the entire wheel that enlarges, just the point of contact with the road. So the delta rotational speed you are talking about is very small.


> But you hit on one of two causes for speed increase. Having the weight of the vehicle suddenly removed means the effective friction between the tire and the road, caused by having to overcome that weighted flat spot where road and tread meets, is completely eliminated.

Which is only relevant while driven. Once off the car, and at it\'s full roundness, it has momentum and will tend to preserve that.


> The flat spot effect has been traditionally modelled as continuously have to apply translational force to push the tire out of a rut the depth of the flat spot. These days they probably use finite element analysis to make it more accurate, but that\'s essentially it. But it doesn\'t explain the increase in rotational speed.

It would not be the depth of the flat spot, because some of the force is elastic, and pushes the tire faster on the trailing edge.


> There\'s a second element hit on by JL, and that is the ability of tire to store energy in its elasticity. These runaway tires almost always break into a bounce, they can bounce really high, and then they really start flying..

LOL Bouncing will not make it run faster at all.

BTW Wheels shouldn\'t drop off properly maintained vehicles.

I only know of one hapless individual with no engineering or mechanical
sense at all who was actually passed by one of their own wheels.
Worst thing is when they bounce into oncoming traffic, which almost always causes a serious wreck. Or they could hit a pedestrian on a sidewalk, which almost always kills them.

You mean like the \"Totally irrelevant\" video of the car that was sent flying some six feet into the air? Then, ironically, the car gets rear ended by the wheel that flew off the pickup! .

This is also what happens when Indy cars run over another\'s wheel in passing.

--

Rick C.

-+ Get 1,000 miles of free Supercharging
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[Ricky]

On Sunday, April 23, 2023 at 6:19:44 PM UTC-4, Fred Bloggs wrote:

On Sunday, April 23, 2023 at 1:03:04 PM UTC-4, John Robertson wrote:
On 2023/04/23 4:01 a.m., Fred Bloggs wrote:
So a vehicle is driving down the road when an entire wheel comes off and continues its direction unchanged rolling alongside the vehicle. Question is why does the wheel end up accelerating, rolling much faster than its original speed, outpacing the vehicle significantly? Answer should be obvious, but you need practical insight.
The wheel can\'t accelerate without an external force being added to it -
it has no means of self propulsion. Laws of the conservation of momentum
apply as usual.
Think of a figure skater doing one of those stationary spins. Arms outstretched is one spin rate, arms brought into the side and they turn into a blur. No external forces come into play.

Now, think of that figure skater pulling in one finger. That\'s about the extent of the flat spot on the tire resuming it\'s full circular shape. It\'s a tiny effect and is greatly outweighed by the impact of the larger radius and the very slightly slower rotation.

--

Rick C.

+- Get 1,000 miles of free Supercharging
+- Tesla referral code - https://ts.la/richard11209

 

[Ricky]

On Sunday, April 23, 2023 at 7:36:36 PM UTC-4, Tabby wrote:

On Sunday, 23 April 2023 at 23:48:05 UTC+1, John Larkin wrote:
On Sun, 23 Apr 2023 15:30:24 -0400, bitrex <us...@example.net> wrote:

On 4/23/2023 8:44 AM, Fred Bloggs wrote:
On Sunday, April 23, 2023 at 7:49:24?AM UTC-4, Anthony William Sloman wrote:
On Sunday, April 23, 2023 at 9:01:07?PM UTC+10, Fred Bloggs wrote:
So a vehicle is driving down the road when an entire wheel comes off and continues its direction unchanged rolling alongside the vehicle. Question is why does the wheel end up accelerating, rolling much faster than its original speed, outpacing the vehicle significantly? Answer should be obvious, but you need practical insight.
The only thing that could accelerate it would be the airflow around the vehicle body, which would be associated with trailing vortices.

It seems unlikely than any of them could make the wheel rotate much faster than it\'s original speed - it would outrun the car and the trailing vortices - unless it was moving sideways with respect to the rest of the car, and out into the wake.

Alright, you\'re getting close. From what I\'ve personally observed, a wheel dropping off the vehicle leaves the vehicle behind in the dust, it really takes off.

It seems like a potentially complex process, particularly if it\'s one of
the drive wheels in a modern car with traction control, and/or the wheel
isn\'t in full contact with the pavement when it separates.
If it bounces up before it breaks off, the differential (assume it has
one) could spin it way up.
And no mention yet of the fact that the last bolt to let go is going to put force on the tyre in some direction. What direction I couldn\'t say.
It\'s too complex to analyze.
+1

We have video. Mostly, the wheel shot sideways like it was on a rocket. But that was because the car hit it before it could make significant contact with the road.

I really don\'t see the bolts having much to do with it. Maybe you are not aware that there is a hole in the wheel that fits over the hub. Even when all the bolts break or the nuts are unscrewed, the wheel still has to come off the hub.

--

Rick C.

++ Get 1,000 miles of free Supercharging
++ Tesla referral code - https://ts.la/richard11209

 

[Ricky]

On Monday, April 24, 2023 at 12:24:56 PM UTC-4, Lasse Langwadt Christensen wrote:

mandag den 24. april 2023 kl. 00.33.17 UTC+2 skrev John Larkin:
On Sun, 23 Apr 2023 15:19:40 -0700 (PDT), Fred Bloggs
bloggs.fred...@gmail.com> wrote:
On Sunday, April 23, 2023 at 1:03:04?PM UTC-4, John Robertson wrote:
On 2023/04/23 4:01 a.m., Fred Bloggs wrote:
So a vehicle is driving down the road when an entire wheel comes off and continues its direction unchanged rolling alongside the vehicle. Question is why does the wheel end up accelerating, rolling much faster than its original speed, outpacing the vehicle significantly? Answer should be obvious, but you need practical insight.
The wheel can\'t accelerate without an external force being added to it -
it has no means of self propulsion. Laws of the conservation of momentum
apply as usual.

Think of a figure skater doing one of those stationary spins. Arms outstretched is one spin rate, arms brought into the side and they turn into a blur. No external forces come into play.

The skater must use muscle power to pull her arms in. The work done in
pulling her arms in is converted to energy stored in the spinning mass
of her body, and is recoverable.
angular momentum is the product of moment of inertia and angular velocity

angular momentum can\'t just change (newtons 3rd) so when the moment of inertia get smaller (pulling arms in)
the angular velocity has to increase

no energy added, just \"stored\" differently

When the skater pulls their arms in against a force, work is being done. This work is not against friction, so it has to manifest as an increase in speed, even if that\'s not the only thing going on.

--

Rick C.

--- Get 1,000 miles of free Supercharging
--- Tesla referral code - https://ts.la/richard11209

 

[Ricky]

On Monday, April 24, 2023 at 4:13:53 PM UTC-4, Lasse Langwadt Christensen wrote:

mandag den 24. april 2023 kl. 22.07.22 UTC+2 skrev John Larkin:
On Mon, 24 Apr 2023 12:49:28 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

mandag den 24. april 2023 kl. 20.12.14 UTC+2 skrev John Larkin:
On Mon, 24 Apr 2023 09:24:51 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

mandag den 24. april 2023 kl. 00.33.17 UTC+2 skrev John Larkin:
On Sun, 23 Apr 2023 15:19:40 -0700 (PDT), Fred Bloggs
bloggs.fred...@gmail.com> wrote:
On Sunday, April 23, 2023 at 1:03:04?PM UTC-4, John Robertson wrote:
On 2023/04/23 4:01 a.m., Fred Bloggs wrote:
So a vehicle is driving down the road when an entire wheel comes off and continues its direction unchanged rolling alongside the vehicle. Question is why does the wheel end up accelerating, rolling much faster than its original speed, outpacing the vehicle significantly? Answer should be obvious, but you need practical insight.
The wheel can\'t accelerate without an external force being added to it -
it has no means of self propulsion. Laws of the conservation of momentum
apply as usual.

Think of a figure skater doing one of those stationary spins. Arms outstretched is one spin rate, arms brought into the side and they turn into a blur. No external forces come into play.

The skater must use muscle power to pull her arms in. The work done in
pulling her arms in is converted to energy stored in the spinning mass
of her body, and is recoverable.

angular momentum is the product of moment of inertia and angular velocity

angular momentum can\'t just change (newtons 3rd) so when the moment of inertia get smaller (pulling arms in)
the angular velocity has to increase

no energy added, just \"stored\" differently
She did work pulling her arms in, burned a bit of breakfast and added
energy to the rotating system.

sure she spend some energy but it doesn\'t add to the rotational energy



Energy is conserved. The work she does to pull her arms in isn\'t lost,
and can be recovered. It can only be stored in the rotational energy.
what if she is standing still?

Then it takes no work to move the arms inward.

--

Rick C.

--+ Get 1,000 miles of free Supercharging
--+ Tesla referral code - https://ts.la/richard11209

 

[Ricky]

On Monday, April 24, 2023 at 6:12:48 PM UTC-4, RichD wrote:

On April 23, Clive Arthur wrote:
On 23/04/2023, Ricky wrote:
So a vehicle is driving down the road when an entire wheel comes
off and continues its direction unchanged rolling alongside the vehicle.
Question is why does the wheel end up accelerating, rolling much faster
than its original speed, outpacing the vehicle significantly?

Someone has pointed out that the wheel\'s diameter is now larger.
That would tap the rotational inertia, turning it into linear velocity,
huh?
The effective diameter of the wheel is proportional to the radius. The
effective radius is from axle centre to road surface. Take away the
load and that\'s quite a significant change in effective diameter.
um, if the wheel\'s diameter (the tire, actually) increases,
its moment of inertia increases. Angular momentum
is presumably conserved, hence the tire SLOWS.

Good to see that everyone here aced Physics 101 -

Huh? Yourself. The radius at the point of contact is larger after the load of the car is removed from the wheel. It has the impact of changing the ratio between the rotational velocity and the linear velocity, but has minimal impact on the actual moment of inertia, since it only impacts a small portion of the wheel mass.

--

Rick C.

-+- Get 1,000 miles of free Supercharging
-+- Tesla referral code - https://ts.la/richard11209

 

[Ricky]

On Monday, April 24, 2023 at 6:45:09 PM UTC-4, RichD wrote:

On April 23, John Larkin wrote:
So a vehicle is driving down the road when an entire wheel comes off
and continues its direction unchanged rolling alongside the vehicle.
Question is why does the wheel end up accelerating, rolling much faster
than its original speed, outpacing the vehicle significantly?

A wheel can store energy in the rubber. Before it was turned loose,
there was torque, which flexed the rubber.

The continuous torque is what rotates the wheel, against
losses. It the torque is removed, it slows. (!!)

Only if there are no other effects. Look at any of the videos. The wheel continues moving at a high rate of speed, slowing very, very little.

The other effect, that increases the wheel velocity, only exists for a moment. Then, your friction effects are the only effects in play and the wheel very gradually slows.

And compression.

The compression is the flattened portion, against the road.
Weight and gravity. When it decompresses, it bounces straight
up. No torque.

If it never came into contact with the road, that would be the end of it. The fact remains that the rotation is faster than the corresponding linear velocity once it does impact the road again, because the point of contact is at a greater radius than when on the car. So, on contact with the road, it will gain speed.

--

Rick C.

-++ Get 1,000 miles of free Supercharging
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[whit3rd]

On Monday, April 24, 2023 at 2:50:30 PM UTC-7, Lasse Langwadt Christensen wrote:

mandag den 24. april 2023 kl. 23.26.19 UTC+2 skrev John Larkin:

\"Think of a figure skater doing one of those stationary spins.\"
started this.

moving her arms in and out doesn\'t add to the rotational energy, the rotational energy is the same

False. Moving arms in and out changes the angular inertial moment, and the conserved quantity
of rotation in that instant is the angular momentum, in the absence of torque... not the energy.

Rotational energy is proportional to squared momentum over moment, so when the angular
moment decreases, energy rises.

 

[Martin Brown]

On 24/04/2023 19:11, John Larkin wrote:

On Mon, 24 Apr 2023 09:24:51 -0700 (PDT), Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

mandag den 24. april 2023 kl. 00.33.17 UTC+2 skrev John Larkin:
On Sun, 23 Apr 2023 15:19:40 -0700 (PDT), Fred Bloggs
bloggs.fred...@gmail.com> wrote:
On Sunday, April 23, 2023 at 1:03:04?PM UTC-4, John Robertson
wrote:
On 2023/04/23 4:01 a.m., Fred Bloggs wrote:
So a vehicle is driving down the road when an entire wheel
comes off and continues its direction unchanged rolling
alongside the vehicle. Question is why does the wheel end
up accelerating, rolling much faster than its original
speed, outpacing the vehicle significantly? Answer should
be obvious, but you need practical insight.
The wheel can\'t accelerate without an external force being
added to it - it has no means of self propulsion. Laws of the
conservation of momentum apply as usual.

Think of a figure skater doing one of those stationary spins.
Arms outstretched is one spin rate, arms brought into the side
and they turn into a blur. No external forces come into play.

The skater must use muscle power to pull her arms in. The work
done in pulling her arms in is converted to energy stored in the
spinning mass of her body, and is recoverable.

Moving the arms in provides the force that spins her up but the
fundamental rule is conservation of angular momentum.

angular momentum is the product of moment of inertia and angular
velocity

angular momentum can\'t just change (newtons 3rd) so when the moment
of inertia get smaller (pulling arms in) the angular velocity has
to increase

no energy added, just \"stored\" differently

She did work pulling her arms in, burned a bit of breakfast and
added energy to the rotating system. When she extends them back out,
work is done on her arms. Of course, muscles don\'t recover energy
when back-driven but some other gadget, a prosthetic arm maybe,
could.

It *is* adding energy to the system by moving her arms against a
resisting force. Though more importantly it is radically changing her
mass distribution so that it is all much nearer the spin axis.

Conservation of angular momentum is more fundamental here which means
that she spins much faster with arms in (and with more stored energy).
Choosing the correct conservation law is essential!

Moving her arms provides the force and torque needed to spin her up but
doesn\'t alter her total angular momentum at all. She would spin as fast
if she raised here arms above her head as by bringing them to her side.
That state with arms high (and hands clasped together is the fastest
possible spin rate but harder to stay balanced on point) also has more
stored potential energy since her centre of mass has been raised in
addition. This is quite a nice (no maths) review of the physics of ballet.

https://www.discovermagazine.com/the-sciences/the-physicist-who-figured-out-ballet

One could build a robot lady to do the skater spin thing. If it
could recover the energy supplied when it extends its arms, it could
spin up and down forever without needing additional power.

I expect USPTO is just waiting for your application - although ISTR they
are officially no longer accepting patents on perpetual motion machines. >
> Energy is conserved.

No it isn\'t.

She has to do work against the centrepetal (old school centrifugal)
forces that act on her outstretched arms to move them. By bringing them
in to her side she *is* doing work against that force which results in a
net torque on her body which acts to spin her up so that her angular
velocity increases markedly as her moment of inertia decreases.

Conservation of angular momentum in a frictionless system with no
external forces acting is a much stronger conservation law in this
situation.

L = I.w = i.W (1)

Where

I = moment of inertia with arms out, w = initial spin rate
i = moment of inertia arms folded, W = final spin rate

It is fairly easy to see that the energy of the two systems are not the
same and energy stored increases as her moment of inertia decreases.
The initial energy with arms outstretched is:

E0 = I.w^2/2

Whilst with her arms in by her side it is

E1 = i.W^2/2

Where from (1) W = I.w/i

E1 = i.(I.w/i)^2 = (I/i).E0

Stars becoming pulsars do an even more extreme version of this trick
with a star that once spun round every 20 days or so reduced to
fractions of second spin rates at near nuclear matter density.

--
Martin Brown