I survived, but now some questions remain

[GoLefty!!]

OK Guys... Bite size pieces.....

 

1. Can the wheel accelarate if no additional energy is added? (Y/N)

 

Energy is the confusion here.

If no additional Force is added to the wheel, then it cannot accelerate.

 

Energy is a another matter. You can add energy ie heat, that will not result in acceleration. So you need to define what energy and where it is coming from.

 

the quantity that has the most direct bearing on this discussion is Force, which is not necessarily energy but it could be the result of energy, ie the the tyre heats up and the tyre ruptures at a point where the stored energy release is converted to a force which propels the wheel.

GoLefty!!2008-11-17 08:21:45

[greatwhite]

OK Guys... Bite size pieces.....

 

1. Can the wheel accelarate if no additional energy is added? (Y/N)

 

Energy is the confusion here.

If no additional Force is added to the wheel' date=' then it cannot accelerate.

 

Energy is a another matter. You can add energy ie heat, that will not result in acceleration. So you need to define what energy and where it is coming from.

 

the quantity that has the most direct bearing on this discussion is Force, which is not necessarily energy but it could be the result of energy, ie the the tyre heats up and the tyre ruptures at a point where the stored energy release is converted to a force which propels the wheel.

[/quote']

 

Now you are getting picky - correct in so far as the force being the main method for adding energy, but the question still remains:

 

Can the wheel accelarate if no additional energy is added? (Y/N)

[bruce]

OK Guys... Bite size pieces.....

 

1. Can the wheel accelarate if no additional energy is added? (Y/N)

 

It can accelerate if stored energy is released.  No additional energy is added.

 

[greatwhite]

OK Guys... Bite size pieces.....

1. Can the wheel accelarate if no additional energy is added? (Y/N)

It can accelerate if stored energy is released.  No additional energy is added.

Wrong!

The stored enegry will gradually be 'eroded' by losses (friction with air and ground) and by change in potential energy (going up or down hill) but it can only accelarate if energy is added.

greatwhite2008-11-17 09:11:28

[bruce]

Wrong

 

A stationary wheel is released on a decline - potential energy is converted to kinetic energy and the wheel accelerates.  No additional energy has been added.

 

A spinning flywheel is placed on the ground.  Not additional energy has been added (energy is held in the wheel due to rotational kinetic energy), the wheel accelerates when it touches the ground due to the release of stored rotational energy.

[greatwhite]

 

A stationary wheel is released on a decline - potential energy is converted to kinetic energy and the wheel accelerates.  No additional energy has been added.

 

I don't recall there being a nice steep downhill around in the scenario JB painted for us?

 

 

A spinning flywheel is placed on the ground.  Not additional energy has been added (energy is held in the wheel due to rotational kinetic energy)' date=' the wheel accelerates when it touches the ground due to the release of stored rotational energy.

[/quote']

 

With this scenario all you have done is converted rotational to linear energy - also, you fill find a large DROP in rpm of the wheel as it picks up linear velocity from a stand still

 

If I drop a rotating wheel off a truck and the surface velocity on the wheel was the same as the ground speed of the truck, there is no differential in velocity between the wheel surface and the ground and therefore no potential to accelarate

[bruce]

Greatwhite, you asked a deliberately simplified question. I gave you an answer that you declared categorically wrong without any conditions. I have given you two conditions which prove my point. Now you chose to reintroduce the truck/wheel scenario.

[GoLefty!!]

This thread should be renamed the twist and twirly thing thread

[AttieSlabbert]

the wheel will go faster because of the loss of mass it had to carry and the law of conservation of momentum / energy. 

The moment the wheel is detached from the axle- 2 things happen at the same time -

1. it looses the mass

2. It still has the same kinetic energy as when it was attached to the axle.

Less mass, same amount of energy= acceleration

(that's why small guys climb faster than big guys using the same amount of energy)

 

after that it will start to loose energy and go slower because of external forces

Many other factors, but I think  this sums it up.

[4eyes]

WOW - Check out the brains on Attie!!!!! You must have been a Science Teacher!

[AttieSlabbert]

You're up early Mr 4 Eyes!!  Any eye comments from that side? Maybe the wheel will have more momentum if it wears Oakleys!!

[greatwhite]

Greatwhite' date=' you asked a deliberately simplified question. I gave you an answer that you declared categorically wrong without any conditions. I have given you two conditions which prove my point. Now you chose to reintroduce the truck/wheel scenario.[/quote']

 

I did and perhaps I was overzelous. That said, neither of the conditions you mentioned existed, so the question still remains, where did the extra energy come from?

 

The only way I can see energy being added is during the failure of hub or stub shaft and I'll be the 1st to admit that I'm reaching a bit here - 2 examples come to mind:

1. During the shearing process of the stub shaft the CoG (Centre of Gravity) of the wheel is no longer in line with the axis of rotation for a rev or 2, the extra energy is imparted in the wheel as a result - when the last of the shaft fails the already eccentricly rotating wheel if flung forward

2. All but 1 wheel nut have failed, again allowing the wheel to run eccentricly for a short while before being flung free (many hubs also locate the wheel so is less probable)

 

At the end of it all, I'm still with GoLefty on his early analysis - aptly described by JB in his 1st post: ..."***, hier kom 'n wiel."  Wide eyes, wheel coming toward you....damn! it look like its accelarating at me.

[greatwhite]

the wheel will go faster because of the loss of mass it had to carry and the law of conservation of momentum / energy. 

The moment the wheel is detached from the axle- 2 things happen at the same time -

1. it looses the mass

2. It still has the same kinetic energy as when it was attached to the axle.

Less mass' date=' same amount of energy= acceleration

(that's why small guys climb faster than big guys using the same amount of energy)

 

after that it will start to loose energy and go slower because of external forces

Many other factors, but I think  this sums it up.

[/quote']

 

1. the mass remains unchanged - what was the truck stays the truck, what was the wheel stay the wheel

2. Same mass, same amount of energy= no change 

[AttieSlabbert]

 

1. the mass remains unchanged - what was the truck stays the truck' date=' what was the wheel stay the wheel

2. Same mass, same amount of energy= no change 

[/quote']

 

Let me see-

wheel only - maybe 40kg

wheel and truck - maybe 2500kg

Take away the Wheel from the truck - the wheel loses about 2460kg

OK - mass of wheel only, stays the same, but part of the weight of the truck is added to the wheel when it is attached to the axle which is attached to the truck. Which the wheel loses when it is not attached to the axle anymore. Weight and mass not being the same.

[greatwhite]

 

1. the mass remains unchanged - what was the truck stays the truck' date=' what was the wheel stay the wheel

2. Same mass, same amount of energy= no change 

[/quote']

 

Let me see-

wheel only - maybe 40kg

wheel and truck - maybe 2500kg

Take away the Wheel from the truck - the wheel loses about 2460kg

OK - mass of wheel only, stays the same, but part of the weight of the truck is added to the wheel when it is attached to the axle which is attached to the truck. Which the wheel loses when it is not attached to the axle anymore. Weight and mass not being the same.

 

The wheel is carrying part of the truck, sure, but when wheel and truck part ways the wheel is 40kg (as it was before it parted from the truck) and the rest of the truck still has a mass of 2460kg. Momentum is a function of the mass. i.e. no change

 

As for the reduction in downward force (weight if you prefer), this will cause the tyre to bounce up (but not a great deal I suspect) as the strain energy is released from the elastic components of the wheel, but not forward i.e. 90? the original force. (remember equal and opposite reaction)

 

The above has got me to thinking of another possibility (again a stretch):

If the wheel bounced up and something rigid on the truck struck it, it could act like a ball getting hit by a bat.

[bruce]

One other possible scenario to consider is that the wheel is one of the driving wheels attached to a differential.

 

When the wheel detaches, the remaining driving wheel loses thrust because the axle of the detached wheel spins (no diff lock).  The vehicle loses forward thrust but the detached wheel continues at pretty much the same speed it was before, while the vehicle slows down.  This does not require any action from the vehicles driver, who could be unaware of what has happened.