How do you know which way friction points?

[starstruck_]

I was working on a centripetal question where it said that on glare ice, any car needs to travel 60 km/h to successfully make it through the banked highway, and in good road conditions, a car travels at 90 km/k, I had to calculate the coefficient of static friction.

When drawing my FBD, I made my friction point into the curve (so it" helped" with the centripetal acceleration). My reasoning for this was that if the speed required for the car to make it through is 60 km/h and the car is going 90km/h in good road conditions, then the friction would have to point inwards because if it didn't, then the car would go off on a tangent. I don't think this is the right reasoning though, so could someone explain why the friction would point into the curve in a better way?

 

[andrewkirk]

Yes, the friction will point inwards towards the centre of the curve. As you say, it is to provide the centripetal acceleration. Your reasoning is correct.

The friction force vector will not point exactly to the centre of the curve because there will also be a component pointing in the direction of travel, which is tangent to the curve. That component will point backwards if the car is slowing significantly and forwards otherwise (if the car's speed is constant, some level of forward 'push' is still needed off the road in order to counter air resistance)..

 

[starstruck_]

Yes, the friction will point inwards towards the centre of the curve. As you say, it is to provide the centripetal acceleration. Your reasoning is correct.

The friction force vector will not point exactly to the centre of the curve because there will also be a component pointing in the direction of travel, which is tangent to the curve. That component will point backwards if the car is slowing significantly and forwards otherwise (if the car's speed is constant, some level of forward 'push' is still needed off the road in order to counter air resistance)..

Ohhh that makes sense! Forgot air resistance was a thing since we've been ignoring it for so long XD

 

[FactChecker]

You should not call it "static friction" in the OP. Static friction is only for friction between a stationary object and the surface it rests on. It is always equal and opposite to the total of the components of all other forces that are tangent to the surface. Because the car is moving, it is not static friction. The friction of a rolling tire involves the scrub angle of the tire. That being said, for the car to make a turn without moving inward or outward would still require the friction (not static) to be equal and opposite to the component of centrifugal force that is tangent to the surface. If the curve is banked, there is also a component of force from the ground constraint that is not friction at all.

PS. I do not think that the force from the scrub angle of a rolling tire is called "friction" either, but I could be wrong.

 

[A.T.]

Static friction is only for friction between a stationary object and the surface it rests on.

Static friction is when the contact patches are at relative rest.

Because the car is moving, it is not static friction.

What the car does as a whole is irrelevant for whether it is static friction.

 

[sophiecentaur]

"Which way" can be confusing if you use intuition but I think you can always get the right answer if you consider which in direction the motion would happen with no friction. Friction is in the opposite direction to this.
Driven tyres would move backwards with no grip so the friction force must be forwards. Distance from the centre of the car's path would increase without tyre friction so friction must be directed inwards. And so on.
Car tyres are a problem because you can quote 'rolling resistance' as being due to friction with the road - but is isn't simply that. Rolling resistance is due to work done in deforming the tyres and road surface and, as the car would go faster without it, rolling resistance must actually be directed backwards. It will, of course, be less than the friction force that is pushing the car forwards. So the car requires forward power to keep it on a strictly circular path, if the wheels are held in a steady direction because the scrub angle (above) changes with speed and angle of turn.

 

[FactChecker]

Static friction is when the contact patches are at relative rest.

What the car does as a whole is irrelevant for whether it is static friction.

The forces of a rolling tire are not static friction.

 

[A.T.]

The forces of a rolling tire are not static friction.

Static friction can be one of the forces on a rolling tire.

 

[starstruck_]

Static friction can be one of the forces on a rolling tire.

We didn't get a definite answer during our lesson for this topic this year, but in grade 11/12 physics, our teacher said it would be static friction that acts between the tires and the road because of the little grooves between the tires. Not sure if that holds true when it's in centripetal motion but the question itself also used static friction.

 

[A.T.]

We didn't get a definite answer during our lesson for this topic this year, but in grade 11/12 physics, our teacher said it would be static friction that acts between the tires and the road because of the little grooves between the tires. Not sure if that holds true when it's in centripetal motion but the question itself also used static friction.

That is okay assuming there is no slippage between tire and road.

 

[jbriggs444]

We didn't get a definite answer during our lesson for this topic this year, but in grade 11/12 physics, our teacher said it would be static friction that acts between the tires and the road because of the little grooves between the tires. Not sure if that holds true when it's in centripetal motion but the question itself also used static friction.

Fair warning: I am neither an enthusiast nor an automotive engineer.

My understanding is that the groves between the treads are there for several purposes.

1. To allow a path for water to escape from under the tire and thereby reduce the likelihood and severity of hydroplaning
2. To interpenetrate with a loose road surface (e.g. snow tires or lugs on tractor tires), thereby improving grip.
3. To allow individual treads to lie flat independently of one another rather than deforming and "squirming" against the road surface as they enter, move through and leave the contact region.

It is this last feature that I imagine that your instructor had in mind when explaining that groves are what allows for static friction.

Tires are far from an ideal realization of the distinction between kinetic and static friction. @FactChecker has alluded to this with the idea of a "scrub angle". Real tires exhibit neither pure kinetic friction (squealing to a stop with brakes locked) or pure static friction (cornering on rails). They will provide no lateral resistance without some non-zero lateral slippage. [Rubber is... rubbery]

Per my understanding, consumer tires are engineered to make the transition from nearly pure static friction to nearly pure kinetic friction gradually as the lateral slip rate increases. The goal is gradually increasing friction with increasing slip rate. As slip rate becomes more extreme and traction inevitably falls off, the goal is a gradual drop-off rather than a sudden and surprising "break loose" event. This real world behavior goes counter to the simplistic textbook characterization of a single coefficient of kinetic and of static friction.

 

[FactChecker]

If the goal of the thread is to clarify first principles of static and dynamic friction, then it is better to ignore the details of tire forces. We should probably assume that the lateral force is countered by static friction. Post #9 seems to indicate that this is the goal.

If the goal is to clarify first principles of steering with a rolling rubber tire, then it is better to worry about the linear relation between the scrub angle of the tire and the forces. That is adequate for a crude simulation of steering a vehicle.

If the goal is to really study rubber tire forces, then it would require real knowledge of tire tread, tread distortion, and tire engineering. That is far beyond my knowledge base.

 

[CWatters]

I was working on a centripetal question where it said that on glare ice, any car needs to travel 60 km/h to successfully make it through the banked highway, and in good road conditions, a car travels at 90 km/k, I had to calculate the coefficient of static friction.

When drawing my FBD, I made my friction point into the curve (so it" helped" with the centripetal acceleration). My reasoning for this was that if the speed required for the car to make it through is 60 km/h and the car is going 90km/h in good road conditions, then the friction would have to point inwards because if it didn't, then the car would go off on a tangent. I don't think this is the right reasoning though, so could someone explain why the friction would point into the curve in a better way?

It seems a reasonably good explanation to me.

If it doesn't slide up or down the banking at 60kph on ice then at 90kph it will tend to slide up the banking so friction will point more or less down the banking.