Air resistance, not friction

[doG]

I repeat, however, that you need to distinguish between the rolling resistance of bowling balls (as asked by the OP) and vehicles, which is vastly more complicated.

Could you please highlight the phrase 'bowling balls' in the OP. I seem to be too dense to see it. All I could find there was the general term 'object' which would seem to include both vehicles and bowling balls since both are objects that roll.

[studiot]

 

endercreeper post#11

 

I meant an object such as a sphere rolling along a surface

 

Good evening doG

Edited August 11, 2013 by studiot

[doG]

 

Good evening doG

Thank you, missed that. It makes no difference though, Rolling resistance is rolling resistance. It is friction that results from elastic deformation. Be it a sphere or cylinder or a vehicle resting on such it is still the same. It is nothing to do with the drag caused by rolling through a fluid as it exists equally in a perfect vacuum. It is not dependent on speed. It depends only on the force between the surfaces and the materials they are made of. I simply made reference earlier to a table of coefficients of rolling resistance for vehicles as an example because I know of no reference for bowling balls, beach balls, ball bearings or other spheres. I routinely use the table for wheels in my calculations for hydraulic drive systems and knew that it would show the OP that their original hypothesis asserting no friction was involved was flawed.

[studiot]

 

Rolling resistance is rolling resistance. It is friction that results from elastic deformation. Be it a sphere or cylinder or a vehicle resting on such it is still the same

 

There are additional considerations.

 

Firstly , as I commented in post#8, a body the whole of which is rolling, has only one source of resistance, namely the contact between the body and the bearing surface.

Whereas a machine that has parts which move against each other during rolling has additional sources of dissipation.

 

Secondly the faster the motion the greater the rate of strain in the contact zone and therefore the greater the rate of doing dissipative work, as I noted in post#14.

 

However real machines are designed to minimise this effect, and your tables will reflect this so it can be ignored in normal circumstances.

 

Of course a body moving faster also has more kinetic energy to loose in the first place so there is also some offset there.

Edited August 12, 2013 by studiot

[Endercreeper01]

so then it does not depend on speed.

[John Cuthber]

Oh yes it does.

[Endercreeper01]

Oh yes it does.

how then?

[John Cuthber]

Because the energy loss on flexing is not constant and that in turn is because, if you flex it a lot it gets hot and that affects the visco-elastic properties.

[studiot]

I see you are still talking at cross purposes.

 

Here is a formula from the same linke dWiki article that clearly includes speed

 

 

Depends on applied torque The driving torque to overcome rolling resistance and maintain steady speed on level ground (with no air resistance) can be calculated by: where is the linear speed of the body (at the axle), and its rotational speed.

It is noteworthy that is usually not equal to the radius of the rolling body

 

 

However the article also says there are several definitions of the 'coefficient of rolling resistance'

 

It is also about vehicles and states that it includes driving mechanism losses.

 

I understood we were talking about hard bowling balls, with no motive power.

 

My sources suggest that the losses I refer to (they called them hysteresis losses) are less than 1% for metals, but may be up to 90% for elastomers, which is what I said earlier.

[John Cuthber]

Do you remember me saying this

"As you say, " Energy is lost flexing any flexible wheel." and that includes any rolling object with a finite Young's modulus."

and do you understand that a bowling ball's young's modulus is finite (and the hysteresis losses are, therefore, not zero.

 

The point I'm making is that not only are those losses not zero, but they depend on the speed of rolling.

 

Whether it's on an axle or not is entirely beside the point.

there is still energy loss, and it's still velocity dependent.

[doG]

Whereas a machine that has parts which move against each other during rolling has additional sources of dissipation.

And those additional sources have variables assigned to them. The have no bearing on the rolling resistance. The friction, i.e. rolling resistance, of a wheel rolling on a surface depends only on the force pushing the wheel against the surface it is rolling on, the materials of the wheel and surface. If the wheel has weight on an axle then that is additive to the weight of the wheel on the surface. Yes, there is also friction where the axle passes through the wheel but that has nothing to do with the resistance where the wheel contacts the surface it is rolling on. The rolling resistance of a 1000 pound steel wheel rolling on a steel rail is the same as a 1 pound steel wheel on a steel rail with 999 pounds resting on the wheel. Yes, there is additional friction where the axle goes through the wheel but that friction is not part of the friction at the surface that we call rolling resistance.

[studiot]

Why do you say this? I assume the pun was intended

 

 

And those additional sources have variables assigned to them. The have no bearing on the rolling resistance

 

When this extract from your own reference indicates the contrary.

 

 

In the broad sense, specific "rolling resistance" (for vehicles) is the force per unit vehicle weight required to move the vehicle on level ground at a constant slow speed where aerodynamic drag (air resistance) is insignificant and also where there are no traction (motor) forces or brakes applied. In other words the vehicle would be coasting if it were not for the force to maintain constant speed. An example of such usage for railroads is [3]. This broad sense includes wheel bearing resistance, the energy dissipated by vibration and oscillation of both the roadbed and the vehicle, and sliding of the wheel on the roadbed surface (pavement or a rail).

But there is an even broader sense which would include energy wasted by wheel slippage due to the torque applied from the engine. This includes the increased power required due to the increased velocity of the wheels where the tangential velocity of the driving wheel(s) becomes greater than the vehicle speed due to slippage. Since power is equal to force times velocity and the wheel velocity has increased, the power required has increased accordingly.

Edited August 15, 2013 by studiot

[doG]

The only thing I cited about that article was an example table of coefficients of rolling resistance to show that it is indeed friction, contrary to the OP's assertion and that the friction depends on the materials. When I personally calculate tractive effort for hydrostatic propulsion systems on off road machinery I add any other forces of friction that impede the propulsion system. The table in the article may include those other forces but it was only cited to show that it was friction and not simply air resistance alone. The table I use most often in my own work is in Mark's Standard Handbook for Mechanical Engineers and those values are simply the coefficients of rr for various wheels on various surfaces without any additional resistance added in.

[studiot]

I think the article section heading that the term rolling resistance means different things in different situations is worth noting.

[doG]

FWIW, now that I am at work and can reference my reference, Mark's Handbook - 9th Edition calls it rolling friction instead of rolling resistance, which was the term we used in school. Mark's Handbook says:

 

Rolling is substituted frequently for sliding friction, as in the case of wheels under vehicles, balls or rollers in bearings, rollers under skids when moving loads; frictional resistance to rolling motion is substantially smaller than to sliding motion. The coefficient of rolling friction is f=P/L where L is the load and P is the frictional resistance....

 

 

Mark's Handbook treats all of them the same. The OP's assertion is incorrect and it doesn't matter if the OP was simply asking about a ball on a surface or the wheel of a vehicle. The point of contact is subject to the resistance of rolling friction and the resistance is not that of air resistance alone, where it exists. The resistance would be the same in a perfect vacuum.

 

The article section heading may mean different things in other situations but in the context of an assertion that a rolling object is only affected by air resistance it matters not. It shows that rolling resistance is a factor and that the OP's assertion is wrong. That was the only point to be made.

[Endercreeper01]

FWIW, now that I am at work and can reference my reference, Mark's Handbook - 9th Edition calls it rolling friction instead of rolling resistance, which was the term we used in school. Mark's Handbook says:

 

 

Mark's Handbook treats all of them the same. The OP's assertion is incorrect and it doesn't matter if the OP was simply asking about a ball on a surface or the wheel of a vehicle. The point of contact is subject to the resistance of rolling friction and the resistance is not that of air resistance alone, where it exists. The resistance would be the same in a perfect vacuum.

 

The article section heading may mean different things in other situations but in the context of an assertion that a rolling object is only affected by air resistance it matters not. It shows that rolling resistance is a factor and that the OP's assertion is wrong. That was the only point to be made.

My point was that air resistance is what stops it because drag is proportional to velocity squared