The Lost Wheel Thread

[oakdale]

I think this little conundrum is caused by confusing speed with energy.

When a car with a full sized spare wheel is travelling at 60mph, all 5 wheels have the same linear kinetic energy but the the road wheels have rotational (angular) kinetic energy as well, the dissipation of the angular energy of a released wheel causes it to go faster than its linear momentum would on its own.

[Kawasicki]

This topic is daft. As soon as it leaves the vehicle the wheel loses energy. Why does it travel so far and do so much damage on impact? Because it is a heavy flywheel it has a lot of energy. There is nothing interesting happening at all.

[98elise]

TartanPaint said:

How I see it (and I am NOT good at this stuff..)

While attached to the trailer, the wheel is a flywheel. More energy is being put into the wheel than is being translated into useful work (rotation). The wheel doesn't spin faster than the forward motion of the trailer, because the excess energy input into the wheel is balanced by friction against the road slowing the wheel down (caused by the weight of the trailer and the drag from air resistance and friction). This balance of forces means the wheel, not surprisingly, turns at the speed the trailer is moving at relative to the road. That's all fairly intuitive except that the wheel is a flywheel and flywheels store excess energy.

Freed from the trailer's drag and weight, the wheel should behave like any other flywheel, and release its energy until it's all used up and the balance is restored in the system. I think it makes sense to me that this would be released initially as acceleration relative to the road, and probably some bounce like a tiddlywink with the force of the weight of the trailer removed. It will continue to accelerate until all excess rotational energy is all used up as forward motion and height gain. Then it's just like any other system... no energy input (ignoring hills) so friction and gravity win and it slows down and stops.

I think.

No additional energy is being stored that can be converted into acceleration. A fly wheel can store energy but as soon as you stop adding energy it starts to slow down.

a=f/m

[98elise]

....also if detaching a wheel from a car axle caused it to accelerate, you could build a device that spun a wheel to a fixed speed then detached it from the hub via a clutch, let it accelerate, then declutch to reattach, it will now be adding more energy back into the hub that it left with.

Keep doing that any you've invented a free energy source.

[oakdale]

98elise said:

....also if detaching a wheel from a car axle caused it to accelerate, you could build a device that spun a wheel to a fixed speed then detached it from the hub via a clutch, let it accelerate, then declutch to reattach, it will now be adding more energy back into the hub that it left with.

Keep doing that any you've invented a free energy source.

The wheel could not accelerate because it's not free to do so.

Further to my earlier post, I'm going to try and explain this in a different way.

It's easy to visualise that a wheel released from a vehicle at 60mph would, due to angular energy (or flywheel effect if you like) initially travel at
60mph then progressively slow down due to friction and air resistance but the wheel also has linear kinetic energy acting on its centre of mass
(axis) in the direction of travel, in addition to its rotational energy.

This linear kinetic energy is not a speed, it's energy and energy is force that causes displacement.

The wheel has to accelerate, it's basic Newtonian physics.

[Kawasicki]

oakdale said:

98elise said:

....also if detaching a wheel from a car axle caused it to accelerate, you could build a device that spun a wheel to a fixed speed then detached it from the hub via a clutch, let it accelerate, then declutch to reattach, it will now be adding more energy back into the hub that it left with.

Keep doing that any you've invented a free energy source.

The wheel could not accelerate because it's not free to do so.

Further to my earlier post, I'm going to try and explain this in a different way.

It's easy to visualise that a wheel released from a vehicle at 60mph would, due to angular energy (or flywheel effect if you like) initially travel at
60mph then progressively slow down due to friction and air resistance but the wheel also has linear kinetic energy acting on its centre of mass
(axis) in the direction of travel, in addition to its rotational energy.

This linear kinetic energy is not a speed, it's energy and energy is force that causes displacement.

The wheel has to accelerate, it's basic Newtonian physics.

Energy doesn’t act on anything. Force does. Energy is not a force. The only forces acting on a spinning wheel that has separated from a vehicle are due to gravity, air and rolling resistance. Air and rolling resistance slow the wheel.

Either you have a strange understanding of Newtons laws or I am debating with a troll/need a whoosh parrot picture.

[98elise]

Nanook said:

98elise said:

....also if detaching a wheel from a car axle caused it to accelerate, you could build a device that spun a wheel to a fixed speed then detached it from the hub via a clutch, let it accelerate, then declutch to reattach, it will now be adding more energy back into the hub that it left with.

Keep doing that any you've invented a free energy source.

What now?

When you 'declutch' (you mean clutch) to reattach, it now has to spin the hub back up.

Have you ever changed gear in a manual car?!

Clutch yes...too much beer last night!

[98elise]

Kawasicki said:

oakdale said:

98elise said:

....also if detaching a wheel from a car axle caused it to accelerate, you could build a device that spun a wheel to a fixed speed then detached it from the hub via a clutch, let it accelerate, then declutch to reattach, it will now be adding more energy back into the hub that it left with.

Keep doing that any you've invented a free energy source.

The wheel could not accelerate because it's not free to do so.

Further to my earlier post, I'm going to try and explain this in a different way.

It's easy to visualise that a wheel released from a vehicle at 60mph would, due to angular energy (or flywheel effect if you like) initially travel at
60mph then progressively slow down due to friction and air resistance but the wheel also has linear kinetic energy acting on its centre of mass
(axis) in the direction of travel, in addition to its rotational energy.

This linear kinetic energy is not a speed, it's energy and energy is force that causes displacement.

The wheel has to accelerate, it's basic Newtonian physics.

Energy doesn’t act on anything. Force does. Energy is not a force. The only forces acting on a spinning wheel that has separated from a vehicle are due to gravity, air and rolling resistance. Air and rolling resistance slow the wheel.

Either you have a strange understanding of Newtons laws or I am debating with a troll/need a whoosh parrot picture.

Agreed. Remove force from the equation and you have no acceleration. You still have losses so the wheel starts to slow down.

The reason you may see a lost wheel overtake the car it was attached to, is the car is slowing at a greater rate than the wheel.

[TartanPaint]

98elise said:

Agreed. Remove force from the equation and you have no acceleration. You still have losses so the wheel starts to slow down.

The reason you may see a lost wheel overtake the car it was attached to, is the car is slowing at a greater rate than the wheel.

In that case, if an external force is required for acceleration, can you explain the balance of forces in the usual examples of conservation of angular momentum (e.g. figure skater, roundabout etc)?

[4x4Tyke]

I think the majority of cases the biggest factor is psychological not physical, the vehicle stops quickly and the wheel is perceived to speed off.

In some cases there is probably a slingshot effect from the failure, think about how a sling, spear thrower or trebuchet works, a lever is being used to impart additional mechanical force.

https://www.youtube.com/watch?v=9-Hwxw4fgqk

All the studs are not going to fail at the same instant in time. A driven hub is going to continue to spin, at some point during the failure only one stud will remain and the hub will continue to spin until at some point it fails. During this period the wheel while be offset to the hubs centre. This effect with be larger on a driven hub, that will want to increase its angular momentum, the rotation of the wheel will be increased.

[Kawasicki]

TartanPaint said:

98elise said:

Agreed. Remove force from the equation and you have no acceleration. You still have losses so the wheel starts to slow down.

The reason you may see a lost wheel overtake the car it was attached to, is the car is slowing at a greater rate than the wheel.

In that case, if an external force is required for acceleration, can you explain the balance of forces in the usual examples of conservation of angular momentum (e.g. figure skater, roundabout etc)?

The angular momentum is conserved. No external force is applied. The balance of forces is zero, at least in a vacuum.

[Kawasicki]

Nanook said:

The angular momentum is conserved, and yet, the velocity can increase, by decreasing the intertia.

Yep, but no external force is required and no energy is created

[98elise]

Nanook said:

98elise said:

Nanook said:

98elise said:

....also if detaching a wheel from a car axle caused it to accelerate, you could build a device that spun a wheel to a fixed speed then detached it from the hub via a clutch, let it accelerate, then declutch to reattach, it will now be adding more energy back into the hub that it left with.

Keep doing that any you've invented a free energy source.

What now?

When you 'declutch' (you mean clutch) to reattach, it now has to spin the hub back up.

Have you ever changed gear in a manual car?!

Clutch yes...too much beer last night!

My point though, was that your statement is wrong. I wasn't fussed about the terminology, but your idea that you could re-attach and add free energy?

You could build the device you speak of. There's an example of a toy on this thread. But when you re-attach, and assuming everything is frictionless, you still have to accelerate the rest of the rotating assembly (hub) that you've just re-attached to, back up to speed.

There's no free energy


Consider a rotating body. It has a certain mass, a certain inertia, a certain angular velocity, and a certain energy as a result of all of those things.

Now, you decrease the mass of the rotating assembly.

What happens to the angular velocity?

The wheel has not decreases mass. If you consider it to have decreased then so has the hub (when it lost the wheel mass). Do both accelerate as a result of the detachment?

I don't pretend to be an expert, but I cannot see why it should accelerate by becoming detached.

[TartanPaint]

Nanook said:

Kawasicki said:

Nanook said:

The angular momentum is conserved, and yet, the velocity can increase, by decreasing the intertia.

Yep, but no external force is required and no energy is created

OK.

But the point was, that if the inertia decreased, the angular velocity can increase.

So, the wheel would rotate faster. Which I think you said couldn't happen?

That's exactly where I was going.

[Kawasicki]

Nanook said:

Kawasicki said:

Nanook said:

The angular momentum is conserved, and yet, the velocity can increase, by decreasing the intertia.

Yep, but no external force is required and no energy is created

OK.

But the point was, that if the inertia decreased, the angular velocity can increase.

So, the wheel would rotate faster. Which I think you said couldn't happen?

If the wheel increases in rotational speed after it leaves the vehicle then there must a corresponding drop in translational speed. No energy is being added to the wheel, the wheel will slow down.

[SpeckledJim]

Aren't we discussing why something happens, which doesn't actually happen at all?

A rolling wheel slows down, unless something is actively speeding it up.

[TartanPaint]

I feel like we're, pardon the pun, going round in circles.

I'm trying to imagine an experiment.


Imagine 3 identical wheels, suspended from an imaginary, static wheel-releasing dropper mechanism of some sort, hanging from the ceiling. Some sort of Barnes-Wallace Lancaster bomb releaser.

Spin wheel 1 forwards.
Don't spin wheel 2 at all.
Spin wheel 3 backwards.

Drop them all. What do they do?

Wheel 1 will roll/bounce forwards and come to rest away from the dropper.
Wheel 2 bounces a bit, wobbles and falls over somewhere directly underneath the dropper.
Wheel 3 rolls/bounces backwards and comes to rest away from the dropper, in the other direction from wheel 1.

I'm stating things here as I imagine them to happen, but feel free to argue if I'm wrong.

Wheels 1 and 3 have more energy because they have a rotational energy component, which wheel 2 does not.

Now do the same thing again, but with the imaginary wheel-dropping Lancaster bomber device flying along above a road at 70mph.

Drop them all. What do they do?

Wheel 2 first. It has some kinetic energy, but no rotational inertia, because it's not spinning. It bounces along at 70mph and slows down as air resistance and friction get the better of it and it'll fall behind the dropper.

Wheel 3 doesn't keep pace with the dropper at all. Although it has more energy than wheel 2, it's angular momentum is directional (momentum is a vector). It'll go shooting off backwards relative to the dropper. Every time it touches the road, its "backspin" will slow it down sharply. That's sort of intuitive, right?

Wheel 1 has a load of additional rotational kinetic energy, the same as wheel 3. But its angular momentum is acting in the opposite direction this time. In other words, in the direction of travel of the dropper.

I think that because wheel 1 and wheel 3 have the same amount of energy (total, kinetic plus rotational), wheel 1 will shoot forwards just as hard as wheel 3 shoots backwards.

Edited by TartanPaint on Friday 5th October 16:18

[SpeckledJim]

TartanPaint said:

I feel like we're, pardon the pun, going round in circles.

I'm trying to imagine an experiment.


Imagine 3 identical wheels, suspended from an imaginary, static wheel-releasing dropper mechanism of some sort, hanging from the ceiling. Some sort of Barnes-Wallace Lancaster bomb releaser.

Spin wheel 1 forwards.
Don't spin wheel 2 at all.
Spin wheel 3 backwards.

Drop them all. What do they do?

Wheel 1 will roll/bounce forwards and come to rest away from the dropper.
Wheel 2 bounces a bit, wobbles and falls over somewhere directly underneath the dropper.
Wheel 3 rolls/bounces backwards and comes to rest away from the dropper, in the other direction from wheel 1.

I'm stating things here as I imagine them to happen, but feel free to argue if I'm wrong.

Wheels 1 and 3 have more energy because they have a rotational energy component, which wheel 2 does not.

Now do the same thing again, but with the imaginary wheel-dropping Lancaster bomber device flying along above a road at 70mph.

Drop them all. What do they do?

Wheel 2 first. It has some kinetic energy, but no rotational inertia, because it's not spinning. It bounces along at 70mph and slows down as air resistance and friction get the better of it and it'll fall behind the dropper.

Wheel 3 doesn't keep pace with the dropper at all. Although it has more energy than wheel 2, it's angular momentum is directional (momentum is a vector). It'll go shooting off backwards relative to the dropper. Every time it touches the road, its "backspin" will slow it down sharply. That's sort of intuitive, right?

Wheel 1 has a load of additional rotational kinetic energy, the same as wheel 3. But its angular momentum is acting in the opposite direction this time. In other words, in the direction of travel of the dropper.

I think that because wheel 1 and wheel 3 have the same amount of energy (total, kinetic plus rotational), wheel 1 will shoot forwards just as hard as wheel 3 shoots backwards.

Edited by TartanPaint on Friday 5th October 16:18

But the plane is actually a car and it's doing 70mph. Its wheels are all doing 70mph as well.

Once the engine stops driving the wheel, the wheel will instantly start slowing down as drag and rolling resistance take their bites out of its momentum.

The wheel isn't doing 70mph in addition to the 70mph of the car. It's doing the same 70mph as the car.

[TartanPaint]

I disagree. The wheel is doing 70mph linearly (because it's tied to the axle) and rotationally (because it's in contact with the road). It's not the same 70mph. It's additional rotational energy.

Imagining it being spun up and dropped from a Lancaster is just a thought exercise to separate the two things more clearly.

[SpeckledJim]

TartanPaint said:

I disagree. The wheel is doing 70mph linearly (because it's tied to the axle) and rotationally (because it's in contact with the road). It's not the same 70mph. It's additional rotational energy.

Imagining it being spun up and dropped from a Lancaster is just a thought exercise to separate the two things more clearly.

If the plane/car is doing 70mph, and the wheel is spinning at a rate such that a point on the perimeter is doing 70mph around the centre of the wheel, then dropping in onto the ground (ignoring bouncing) will see it drop gradually backwards relative to the plane/car, as drag and friction slow the rate at which it is spinning.

When the centre of the wheel (attached to the car) is doing 70mph, along with the car, then the top of the wheel is doing 140mph relative to the ground, and the bottom of the wheel is doing 0mph relative to the ground. Any point on the perimeter is doing an average of 70mph relative to the ground. Otherwise it wouldn't be keeping pace with the car.

Once you detach the wheel from the car which is providing the force to cause that rotation, it'll slow down. What force could cause it to do anything different?