Air resistance, not friction

[Endercreeper01]

When you have a object rolling across a surface, then it is not friction slowing it down, It is air resistance.

Here is why. When you have an object rolling across a surface, then the force of friction is μN, where μ is the kinetic friction coefficient and N is the normal force. In this case, it is on earth, so then N=mg and so F=mgμ. When you put a force on an object greater then that force, then friction does not stop it because friction does not depend on the object's velocity. What really stops it should be the air because air resistance is proportional to v^2, so as it accelerates, air resistance gets bigger and bigger, slowing it's acceleration more and more until a=0 and then it slows it down until it stops, so air is what stops it. Is this correct?

[doG]

No, it is not correct. Here's an experiment that will help you to see this. Go to a sandy beach with a bicycle that has nice fat tires and ride it across the sand. Now fit the bicycle with thin steel plates as tires and try riding it across the sand. You will learn that one has much more rolling resistance than the other when both move at the same speed. Rolling resistance is a variable used when calculating propulsion systems for vehicles and varies with the wheel type and surface. The wiki article on rolling resistance has a list of coefficients that are frequently used.

[Endercreeper01]

No, it is not correct. Here's an experiment that will help you to see this. Go to a sandy beach with a bicycle that has nice fat tires and ride it across the sand. Now fit the bicycle with thin steel plates as tires and try riding it across the sand. You will learn that one has much more rolling resistance than the other when both move at the same speed. Rolling resistance is a variable used when calculating propulsion systems for vehicles and varies with the wheel type and surface. The wiki article on rolling resistance has a list of coefficients that are frequently used.

Yes, but does rolling resistance depend on speed?

[John Cuthber]

It seems that it does depend on speed.

http://en.wikipedia.org/wiki/Low-rolling_resistance_tires

Rolling Resistance ( N / Lbs) = Pα x Zβ x (a + bxV + cxVxV)

 P is the tire inflation pressure ( kPa / psi)       Z is the applied load for vehicle weight ( N /Lbs)       V is the vehicle speed ( km/h / mph)       alpha, beta, a, b, c are the coefficients for the model.

A rolling object will be slowed down by air resistance as well as by rolling resistance.

[doG]

Yes, but does rolling resistance depend on speed?

No, rolling resistance depends on the friction between the materials. Speed would effect the coefficient of drag which additionally applies to the total resistance of a body's motion in a fluid environment but not the rolling resistance itself.

[imatfaal]

Further to above responses - and with respect to the details of your OP. In ideal circumstances - ie a mechanics problem - it is worth bearing in mind that if an object is rolling (often called pure roll) without slipping (akin to wheel spin in a car) or skidding then the (maximal) frictional force would be calculated using the static coefficient rather than the kinetic. One should also remember that frictional force as calculated by N.mu_s is the maximum value it can hold - below this maximum the friction can hold any value in order to balance the motive force and prevent motion. In ideal circumstances for a driven wheel that is in pure roll then you need to consider the torques on the wheel and find the firctional force that way

[John Cuthber]

No, rolling resistance depends on the friction between the materials. Speed would effect the coefficient of drag which additionally applies to the total resistance of a body's motion in a fluid environment but not the rolling resistance itself.

As far as I can see from the cited article, the drag coefficient only considers the air resistance.

"The drag coefficient of any object comprises the effects of the two basic contributors to fluid dynamic drag: skin friction and form drag. "

The rolling resistance is not included, but something separate.

It seems that the society of automotive engineers have seen fit to model that by an equation which is quadratic in velocity

(Their eqn is also proportional to the vehicle weight- drag wouldn't be.

[studiot]

Well I think everyone here is talking about different situations. The Original Post

.

 

When you have a object rolling across a surface,

 

There is no mention of an axle or other connection, it is implied that the whole object is rolling eg a round or constant rolling profile object, like a bowling ball.

 

If a wheel plus axle and drive is intended than there is a whole raft of aditional sources of retarding friction.

 

Secondly I think the OP was not considering objects such as wheels because the rolling resistance of a tyred wheel is affected by tyre pressure, profile, flexibility and many other complicated factors. Energy is lost flexing any flexible wheel.

[John Cuthber]

A bowling ball has rolling resistance in the same way that a tire does.

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

Edited August 11, 2013 by John Cuthber

[doG]

As far as I can see from the cited article, the drag coefficient only considers the air resistance.

"The drag coefficient of any object comprises the effects of the two basic contributors to fluid dynamic drag: skin friction and form drag. "

The rolling resistance is not included, but something separate.

It seems that the society of automotive engineers have seen fit to model that by an equation which is quadratic in velocity

(Their eqn is also proportional to the vehicle weight- drag wouldn't be.

Correct. Rolling resistance and drag are two different variables of friction therefore rolling resistance would not be included in the evaluation of drag caused by air resistance. Both are necessary in calculating the required propulsion of motor vehicles here on Earth but not so for a lunar rover. Rocket propulsion equations require a consideration of drag for rockets inside the atmosphere but not for those in space.

 

The OP asserted that it is not friction that slows down a rolling body but air resistance and that is incorrect. Air resistance plays a part when it is present but a rolling body in a perfect vacuum would still slow down and that is because of the rolling friction that affects it.

[Endercreeper01]

I meant an object such as a sphere rolling along a surface. It does not seem to depend on velocity, according to the Wikipedia article.

[doG]

There is no mention of an axle or other connection, it is implied that the whole object is rolling eg a round or constant rolling profile object, like a bowling ball.

So. Rolling resistance is still there and still has the same affect.

[Endercreeper01]

Correct. Rolling resistance and drag are two different variables of friction therefore rolling resistance would not be included in the evaluation of drag caused by air resistance. Both are necessary in calculating the required propulsion of motor vehicles here on Earth but not so for a lunar rover. Rocket propulsion equations require a consideration of drag for rockets inside the atmosphere but not for those in space.

 

The OP asserted that it is not friction that slows down a rolling body but air resistance and that is incorrect. Air resistance plays a part when it is present but a rolling body in a perfect vacuum would still slow down and that is because of the rolling friction that affects it.

According to the wiki article, it does not depend on speed

[studiot]

Why should it depend upon speed?

 

Rolling resistance of balls is due to the very slight deformation of the contact surfaces.

This, in turn, is due to the force pressing the two together.

This force is independent of velocity.

 

This is why very high strength steel is used for ball bearings.

It leads to minimal contact deformation.

 

You should further note that ball bearings usually run in an oil environment that offers significantly greater viscous drag than air, without ill effect.

Why do you think we use oil not air?

Edited August 11, 2013 by studiot

[John Cuthber]

According to the wiki article, it does not depend on speed

And according to the other WIKI article it does, and they seem to be citing people who measure it.

 

Probably because visco-elastic forces are a bit odd- and time dependent.

 

http://en.wikipedia.org/wiki/Viscoelasticity#Elastic_behavior_versus_viscoelastic_behavior

[Endercreeper01]

And according to the other WIKI article it does, and they seem to be citing people who measure it.

 

Probably because visco-elastic forces are a bit odd- and time dependent.

 

http://en.wikipedia.org/wiki/Viscoelasticity#Elastic_behavior_versus_viscoelastic_behavior

Yes, but as studiot said, why should it depend on velocity?

[studiot]

If you think about it there must be a weak dependence upon speed.

 

Deformation is a dissipative action, the energy is not recovered into the motion it is lost as heat.

 

So the greater the rate of deformation the greater the heat loss, and one of the functions of the oil is cooling.

 

Engineers ensure however, that this loss is very very small compared to the energy of motion.

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.

Edited August 11, 2013 by studiot

[Endercreeper01]

There is a dependance on speed, but it is too small to consider

[John Cuthber]

There is a dependance on speed, but it is too small to consider

Not according to the people who measure it.

 

And, re "Yes, but as studiot said, why should it depend on velocity? "

 

Probably because visco-elastic forces are a bit odd- and time dependent.

[Endercreeper01]

Not according to the people who measure it.

 

And, re "Yes, but as studiot said, why should it depend on velocity? "

 

Probably because visco-elastic forces are a bit odd- and time dependent.

it is time dependent, but it is not a huge effect for this.

[Endercreeper01]

Not according to the people who measure it.

 

And, re "Yes, but as studiot said, why should it depend on velocity? "

 

Probably because visco-elastic forces are a bit odd- and time dependent.

it is time dependent, but it is not a huge effect for this.

[Endercreeper01]

Not according to the people who measure it.

 

And, re "Yes, but as studiot said, why should it depend on velocity? "

 

Probably because visco-elastic forces are a bit odd- and time dependent.

it is time dependent, but it is not a huge effect for this.

[Endercreeper01]

Not according to the people who measure it.

 

And, re "Yes, but as studiot said, why should it depend on velocity? "

 

Probably because visco-elastic forces are a bit odd- and time dependent.

it is time dependent, but it is not a huge effect for this.

[Endercreeper01]

Not according to the people who measure it.

 

And, re "Yes, but as studiot said, why should it depend on velocity? "

 

Probably because visco-elastic forces are a bit odd- and time dependent.

it is time dependent, but it is not a huge effect for this.

Not according to the people who measure it.

 

And, re "Yes, but as studiot said, why should it depend on velocity? "

 

Probably because visco-elastic forces are a bit odd- and time dependent.

it is time dependent, but it is not a huge effect for this.

[studiot]

I have tried to offer a simple analysis and explanation.

 

If you want the definitive text it is to be found in the Cambridge University book

 

Contact Mechanics by K L Johnson.

 

He decomposes the motion of contacting surfaces into three parts

 

Sliding, Rolling and Spin

 

This introduces both linear forces and moments acting upon the bodies and contact area.

 

It should also be noted that, although the velocity of the contact point in rolling is zero, the acceleration is non zero so Newton's laws require a force to be acting.