Questions on Wheels: Reduce Friction & Traction

[aspodkfpo]

Homework Statement

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Relevant Equations

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Why do rollers/wheels reduce friction?

When a car drives forward does friction act backwards on the car, or only forwards in the form of traction? I don't see how friction would act backwards.

 

[haruspex]

Homework Statement:: n/a
Relevant Equations:: n/a

Why do rollers/wheels reduce friction?

When a car drives forward does friction act backwards on the car, or only forwards in the form of traction? I don't see how friction would act backwards.

Don't confuse friction with rolling resistance. Unless the car is skidding, friction is static. Whether that accelerates the car, decelerates it (e.g. braking), keeps it going at constant speed (uphill maybe), or does almost nothing (coasting) depends on circumstances.
So when a car accelerates forwards on a level surface, friction acts forwards on the car.

 

[etotheipi]

One way of thinking about it is that if we ignore air resistance and rolling resistance, the only external force acting on the car is the contact force from the ground. If the car is accelerating horizontally, it must be due to the horizontal component of this contact force (the friction) in the necessary direction! If the car is rolling at constant velocity, then there is no friction force.

@haruspex also mentioned rolling resistance, which is slightly different in origin, and constitutes both an external torque in the opposite direction to the rotation of the wheel and an external force in the opposite direction to the motion of the car.

If you want to analyse the acceleration of the car fully, you will need to consider both the forward friction and the backward rolling resistance!

 

[aspodkfpo]

Don't confuse friction with rolling resistance. Unless the car is skidding, friction is static. Whether that accelerates the car, decelerates it (e.g. braking), keeps it going at constant speed (uphill maybe), or does almost nothing (coasting) depends on circumstances.
So when a car accelerates forwards on a level surface, friction acts forwards on the car.

What exactly is rolling resistance, where does it come from, how does it act, and at what point does it equal to friction, thereby causing constant speed?

As I understand from your statement, friction doesn't act backwards, but it's rolling resistance instead.

 

[etotheipi]

What exactly is rolling resistance

The effect arises because of an uneven distribution of normal forces acting on the wheel due to deformation of the surface on which the wheel is rolling. The following diagram omits the tangential components of the contact forces (the static friction), but does a good job of showing why the normal forces constitute a backward force and torque:

https://lockhaven.edu/~dsimanek/scenario/rolling.htm

 

[aspodkfpo]

The effect arises because of an uneven distribution of normal forces acting on the wheel due to deformation of the surface on which the wheel is rolling. The following diagram omits the tangential components of the contact forces (the static friction), but does a good job of showing why the normal forces constitute a backward force and torque:

View attachment 267175

https://lockhaven.edu/~dsimanek/scenario/rolling.htm

So, I assume that it's because the net sum of the normal forces point backwards. And since the front of the wheel is pressed harder on the ground, this sum always point backwards? And the front of the wheel presses harder due to its momentum when going downwards, causing the front part to have more normal force stored like in a spring?

 

[etotheipi]

And the front of the wheel presses harder due to its momentum when going downwards, causing the front part to have more normal force stored like in a spring?

Remember that you cannot 'store' a force, but you can store energy.

What happens is that the material just in front of the wheel is being compressed, and the material just behind the wheel is relaxing. Deformable materials exhibit hysteresis (i.e. not all energy is recovered when the material returns to its original state), and accordingly the work done by the material on the wheel when it relaxes is slightly less than when it is being compressed. The result is that the force during compression (at the front end of the wheel) is slightly greater than the force during relaxation (at the rear end of the wheel).

 

[haruspex]

What exactly is rolling resistance, where does it come from, how does it act, and at what point does it equal to friction, thereby causing constant speed?

As I understand from your statement, friction doesn't act backwards, but it's rolling resistance instead.

See section 4.1 of https://www.physicsforums.com/insights/frequently-made-errors-mechanics-friction/
As mentioned there, it can arise from deformation of the surface and/or deformation of the wheel. @etotheipi's diagram shows the first case, but the other works a little differently. I had intended to add diagrams to the article. I have them somewhere but it looks like I forgot to post them.
There can also be a kinetic frictional torque in the axle; I think that counts as rolling resistance too.