I survived, but now some questions remain

[Johan Bornman]

yep' date=' Been in marketing too long. Failed to see the vehicle as the energy sink. [slaps forehead, and heads off to self flaggulation']

 

Lefty, I don't think the energy sink concept works. Whence still attached, the wheel and truck are one, none sapping or passing energy onto each other.

 

The total kinetic energy in the truck is the sum of the kinetic energy in all its components. This is smaller than the input energy into the system by the total energy sapped by wind and rolling resistance.

 

Upon release, the wheel still has the same kinetic energy as before, but far less rolling resistance.

 

We know this rolling resistance is large because even when the wheel hits the full airstream when it is no longer protected from it by the truck, it stil accellerates.

 

 

Continue the flaggelation or slap forehead a  bit more.

 

 

 

 

 

 

[Johan Bornman]

From my boet' date=' the physics prof:

 

I reckon the key is to understand the effect of the flat contact spot where the tyre touches the road.  The flatness effectively reduces the wheel's radius, but only at that one point in the tyre.  So, when the wheel detaches, its radius increases (effectively, no weight on it, so no flat spot, so the effective radius is larger).  But the wheel's mass distribution stays the same, so the angular momentum stays the same, so the spinning rate stays the same.  Same spinning rate x wider radius = goes faster.  If the increase in radius was accompanied by a corresponding outward movement of mass, the wheel would slow down (like a pirouetting ice skater extending her arms).  The only outward movement is at the flat spot, which is a negligible change in mass distribution, but because all the road contact is through the flat spot, the effective radius of the whole tyre changes. 

Will provide mathmatical proof for beer.

 [/quote']

 

Hmmmmm....how about a virtual beer?

 

I buy the story that the mass moves outwards and therefore the wheel has to slow down a bit. However, this is a small, perhaps negligible change in mass positioning. We know the wheel accellerates - observation tells us that..You can calculate the change in mass positioning and if we assume the flat spot on a loaded tyre is 3 or four degrees - perhaps 6, lets be conservative (easy to measure but I don't have a truck here right now), and the depth of the indent is 25mm and we know the mass of rubber mixed with steel and, we know the width of the tyre, we can calculate it.

 

However, observaton shows that it is is very small indeed.

 

Therefore, the effect is negligible an this hypothesis misses the biggest additive energy, which is the removed resistance.

[mudda]

The wheels on the truck could have resistance from alignment problems i.e a fraction of a degree off.

 

when the wheel is released from the truck

 

1) It has less rolling resistance from aligment issue

2) its rotational force is only applied to itself, and not absorbed and used by the object it was attached to.

 

I think of it this way. If I spin a wheel up (the wheel on its own) to 120km and drop it to the ground , it will leave a nice black line and shoot off for distance x.

 

If I spin up the same single wheel, but this time it is attached to car and  drop it to the ground, it might only move the whole car a little bit, when the wheel spinning at 120km attached to the car touches the ground.

 

 

 

[TheLegend]

There's hamsters in the wheel. They struggle with lower roatations and the according harder resistance. But once they are set free, the pressure from the axle gone, they can go ahead and do they spinning of high rotations thing. simple...

[Johan Bornman]

The wheels on the truck could have resistance from alignment problems i.e a fraction of a degree off.

when the wheel is released from the truck

1) It has less rolling resistance from aligment issue
2) its rotational force is only applied to itself' date=' and not absorbed and used by the object it was attached to.

I think of it this way. If I spin a wheel up (the wheel on its own) to 120km and drop it to the ground , it will leave a nice black line and shoot off for distance x.

If I spin up the same single wheel, but this time it is attached to car and  drop it to the ground, it might only move the whole car a little bit, when the wheel spinning at 120km attached to the car touches the ground.


[/quote']

 

Yes it could have alignment problems and as you say, that too will sap energy. So will chewing gum or a maltest dog stuck in its thread. But those are all red herrings. The wheel accellerates. Finish and klaar, in spite of all the theoretical obstacles thrown at it. Look at Nick's explanation. The example you cite proves nothing. It only confirms your understanding of preservation of energy.

 

 

 

 

 

 

[GoLefty!!]

yep' date=' Been in marketing too long. Failed to see the vehicle as the energy sink. [slaps forehead, and heads off to self flaggulation']

 

Lefty, I don't think the energy sink concept works. Whence still attached, the wheel and truck are one, none sapping or passing energy onto each other.

 

The total kinetic energy in the truck is the sum of the kinetic energy in all its components. This is smaller than the input energy into the system by the total energy sapped by wind and rolling resistance.

 

Upon release, the wheel still has the same kinetic energy as before, but far less rolling resistance.

 

We know this rolling resistance is large because even when the wheel hits the full airstream when it is no longer protected from it by the truck, it stil accellerates.

 

 

Continue the flaggelation or slap forehead a  bit more.

 

 

 

 

 

 

 

 

I beleive I was using sarcasm...

Somehow the smiley did not post

[Johan Bornman]

 

 

I beleive I was using sarcasm...

Somehow the smiley did not post

 

Don't worry, it's two-way good humour. This is fun, keep it up. Smileys are for people who can't read nuances.

 

[Johan Bornman]

There's hamsters in the wheel. They struggle with lower roatations and the according harder resistance. But once they are set free' date=' the pressure from the axle gone, they can go ahead and do they spinning of high rotations thing. simple... [/quote']

 

 

No, not a good analogy.  At the moment the wheel is severed from the truck, the hamsters stop running. The engine is in the truck remember, the wheels have no propulsion of their own.

 

 

 

 

 

[nellie]

There's hamsters in the wheel. They struggle with lower roatations and the according harder resistance. But once they are set free' date=' the pressure from the axle gone, they can go ahead and do they spinning of high rotations thing. simple... [/quote']

 

 

No, not a good analogy.  At the moment the wheel is severed from the truck, the hamsters stop running. The engine is in the truck remember, the wheels have no propulsion of their own.

 

 

JB...don't criticise him...that's pure GeNiUs coming from a German who wears pink socks!!

[Johan Bornman]

 

JB...don't criticise him...that's pure GeNiUs coming from a German who wears pink socks!!

 

Really? Naw..cant be. Come closer, I want to ask you a question but we have to wisper. Does he wear those pink socks with sandals? Do you have any photos of him in lederhosen?

[GoLefty!!]

I still cannot see how the wheel will accelerate. The resistance forces change magnitude but not direction. Therefore the acceleration is still negative (deceleration) but the magntitude has changed.

 

that change is not acceleration (positive).

 

If you apply brakes to a vehicle in motion the acceleration is negative. If you reduce the pressure on the brakes, the acceleration is still negative but the magnitude has changed.

 

The wheel being released from the truck is similar. Unless the resistive forces change direction the wheel cannot accelerate positively.

[bruce]

The problem is actually not well enough defined for any answer to be deemed correct.

 

Does the vehicle continue at the same speed, or does it slow?  This changes the situation significantly, and from JB's perspective he would not have been able to tell whether the wheel accelerated or the vehicle decelerated, as both would appear the same from his point of view.

 

If you make the following assumptions:

- Vehicle continues at same speed.

- Wheel accelerates i.e. gains speed when it detaches.

 

Then it is clear that the balance of forces changes resulting in a nett forward force causing acceleration.  Assuming that rolling resistance decreases, this does not account for a nett positive force, it only accounts to a decrease in negative force (positive force being in the direction of motion, negative opposing motion).  Something must still be pushing the wheel forward.  Since the wheel is detached, the only possible source of this forward force is inertia, which we know the wheel has due to it's forward motion, and it's rotation.

 

As for the rotational energy of the wheel, the truck puts energy into the wheel when the wheel get's accelerated, and the wheel puts energy back into the truck when it decelerates.

 

[nellie]

 

JB...don't criticise him...that's pure GeNiUs coming from a German who wears pink socks!!

 

Really? Naw..cant be. Come closer' date=' I want to ask you a question but we have to wisper. Does he wear those pink socks with sandals? Do you have any photos of him in lederhosen?

[/quote']

 

went through some old fodies...

 

his favourites....

 

http://www.photoshopdiva.com/images/personal/birkies.jpg

 

when he has to look half-decent...

http://scienceblogs.com/transcript/upload/2006/08/SS2.jpg

 

 

 

 

and hey...i found one of Mud Dee too.... (here you can see his nipples too..for those of you who were wondering...)

http://lh5.ggpht.com/_QnJT1nZA16M/RoR1bUaXdwI/AAAAAAAAB8w/hJbl4qXh29o/DSC_0676.jpg

 

 

 

 

Sorry Legend...JB made me do it!!!

[TheLegend]

 

 

JB...don't criticise him...that's pure GeNiUs coming from a German who wears pink socks!!

 

Really? Naw..cant be. Come closer' date=' I want to ask you a question but we have to wisper. Does he wear those pink socks with sandals? Do you have any photos of him in lederhosen?

[/quote']

 

went through some old fodies...

 

his favourites....

 

http://www.photoshopdiva.com/images/personal/birkies.jpg

 

when he has to look half-decent...

http://scienceblogs.com/transcript/upload/2006/08/SS2.jpg

 

 

[...]

 

 

Sorry Legend...JB made me do it!!!

 

backstabber!

 

[GoLefty!!]

agreed, not enough info to effectively provide an answer.

 

I assumed:

 

1) Constant speed of the truck.

2) torque  on the wheel was constant. However the torque at instant of release would provide the final speed of the wheel at the instant of departure so the torwue effect is Zero for the purpose of this discussion.

 

3) Friction and wind resistance are opposing forces.

4) the wheel did not gain mass by picking up crap on the road.

 

 

Under deceleration, the wheel  will also decelerate and it wil feed energy into the truck as well as the air and brake fluid and material.

At the instant of separation the deceleration my reduce but the nett effecgt is still deceleration, only at a different magnitude.

 

There is no positive acceleration of the wheel. It is an optical illusion.

[TheLegend]

 

There's hamsters in the wheel. They struggle with lower roatations and the according harder resistance. But once they are set free' date=' the pressure from the axle gone, they can go ahead and do they spinning of high rotations thing. simple... [/quote']

 

 

No, not a good analogy.  At the moment the wheel is severed from the truck, the hamsters stop running. The engine is in the truck remember, the wheels have no propulsion of their own.

 

 

 

 

 

 

not an analogy but a fact!