If a wheel gets rid of friction, how does it move?

[iVenky]

I have a fundamental question that's bothering me.

We need rebound force due to friction in order to move forward while walking.

On the other hand, we say wheels reduce friction. Then how does it help move forward in the first place? I am confused here

 

[Drakkith]

Wheels don't get rid of friction. They work in such a manner as to always be using static friction, not sliding friction. When the engine in your car turns the wheels, the wheels apply a force on the ground, and the force-pair associated with this applied force is the static friction from the ground on the tires. As the wheel turns the points of the wheel in contact with the ground are lifted upwards instead of sliding, and new points are lowered into contact. So unless your wheel is slipping the contact area is always stationary relative to the ground and you have a force-pair that is static friction.

 

[iVenky]

Wheels don't get rid of friction. They work in such a manner as to always be using static friction, not sliding friction. When the engine in your car turns the wheels, the wheels apply a force on the ground, and the force-pair associated with this applied force is the static friction from the ground on the tires. As the wheel turns the points of the wheel in contact with the ground are lifted upwards instead of sliding, and new points are lowered into contact. So unless your wheel is slipping the contact area is always stationary relative to the ground and you have a force-pair that is static friction.

Thanks for the reply. So sliding friction is the one that makes a moving object stop, right? And, it's not clear to me the contact area is always stationary relative to the ground. Can you help me with that explanation? Even if I look at one specific point on the wheel and it goes in a circle, isn't periodically touching the ground (but it's not sliding you mean?)

 

[Lnewqban]

... On the other hand, we say wheels reduce friction. Then how does it help move forward in the first place? I am confused here

The reduction of friction happens at the contact surface between bearing and axle.
Friction is desired at the contact surface between wheel and ground for propulsion, for stopping and for steering.

Please, see this good tutorial:
https://www.physicsforums.com/insights/explaining-rolling-motion/

 

[Drakkith]

And, it's not clear to me the contact area is always stationary relative to the ground. Can you help me with that explanation? Even if I look at one specific point on the wheel and it goes in a circle, isn't periodically touching the ground (but it's not sliding you mean?)

Just imagine your car tire as you are driving. If the contact points were not stationary with respect to the ground, then your car would be sliding around, right? Try it yourself. Get some paint or ink and paint a big dot on your tire tread. Then drive forward or reverse. You should see a mark on the ground that's the same shape and size as the dot on your tire, with no smears.

 

[iVenky]

Ok now I understand, basically, it's just touching at a point but not sliding thereby you just have static friction (equivalent to how any object would be when it's not moving) and the sliding effect is minimized.

It's like saying that you get the initial push from the static friction by applying the torque but you minimized the friction that caused by the speed of the car (& proportional to the speed ) when it's moving thereby helping it to move smoothly without getting stopped (or any wheel for example).

Is my understanding, right?

 

[PeroK]

Ok now I understand, basically, it's just touching at a point but not sliding thereby you just have static friction (equivalent to how any object would be when it's not moving) and the sliding effect is minimized.

It's like saying that you get the initial push from the static friction by applying the torque but you minimized the friction that caused by the speed of the car (& proportional to the speed ) when it's moving thereby helping it to move smoothly without getting stopped (or any wheel for example).

Is my understanding, right?

You have to distinguish between the magnitude of a force and what it does. Static friction, because it acts over zero distance, does not take any energy out of the system. Instead it can be used to accelerate or decelerate a system in a controlled way.

From this point of view, you want high friction on your tyres or shoes. It's much easier to walk in a pair of grippy rubber soled shoes, than in a pair of smooth soles on ice. If you have somewhere to take a run you can slide on ice, but you can't easily walk on it.

In the same way, you want high-friction tyres on a vehicle. The engine rotates the wheels one way. If you had smooth tyres on ice, the wheels would spin without the vehicle moving anywhere. But, the friction between the tyres and the surface a) opposes the rotation and b) provides a forward accleration to the whole vehicle.

Braking is quite complicated, but it's the kinetic friction in the braking system (not between the tyres and the surface) that dissipates the energy. But, you need static friction between the tyres and the surface to turn the internal braking forces into an external force on the vehicle.

 

[etotheipi]

Neglecting the air and rolling resistance, the only position where momentum can flow into or out of the system are the points of contact of the wheels with the ground. If the centre of mass of the car is accelerating it must be due to frictional forces acting here in the necessary direction. If the car is moving at a constant velocity, then there is no frictional force (and theoretically it would carry on forever, in contrast with a sliding object that would eventually be brought to rest by friction).

In reality rolling resistance exerts both a force and a torque on the wheel; but the resistive force (and respective torque) due to rolling resistance is much lower than that due to sliding friction.

 

[sophiecentaur]

So sliding friction is the one that makes a moving object stop, right?

Ok now I understand, basically, it's just touching at a point but not sliding thereby you just have static friction

But somewhere in the car's drive system, there needs to be sliding friction (or maybe viscosity, electrical resistance or charging a battery, depending on the brake design). Braking involves transferring Energy somewhere but not necessarily at the contact with the ground.

 

[jack action]

I have a fundamental question that's bothering me.

We need rebound force due to friction in order to move forward while walking.

On the other hand, we say wheels reduce friction. Then how does it help move forward in the first place? I am confused here

Using walking as an example, they are two cases to consider:

1. You can push forward your foot and drag it on the ground, and the kinetic friction creates a sliding force you must fight back;
2. Or you can push rearward on the ground with your foot - via static friction - to propel yourself forward. That is the same as rolling.

The following image should help you visualize the link between rolling and walking:

 

[berkeman]

On the other hand, we say wheels reduce friction.

Another way to answer your question is to ask how much friction would increase if you took off these wheels and tried to keep pulling the wagon? (I know that may not be your question, but I still had to comment on your literal question)

https://image.shutterstock.com/image-photo/father-pulling-his-young-son-260nw-1312125503.jpg

 

[sophiecentaur]

"Friction" is just one of many words that are treated as but not a value judgement. Friction is just the name of a vaguely tangential contact force. It doesn't either signify either help or hindrance. It's the same as when someone says they "have a temperature". There is an unspoken implication in everyday language when such words are used. There is no possibility of a fruitful conversation about them except to warn people to think beyond their daily use by non-Scientists.

 

[iVenky]

Using walking as an example, they are two cases to consider:

1. You can push forward your foot and drag it on the ground, and the kinetic friction creates a sliding force you must fight back;
2. Or you can push rearward on the ground with your foot - via static friction - to propel yourself forward. That is the same as rolling.

The following image should help you visualize the link between rolling and walking:

View attachment 266655​

Got it thanks. Can I look at it as it's hard to slide using legs and move forward, rather than walking and moving forward?

 

[anorlunda]

Can I look at it as it's hard to slide using legs and move forward, rather than walking and moving forward?

Have you ever walked on ice? Have you ever ice skated or roller skated? Questions such as yours do not always have yes/no answers.

 

[jack action]

Got it thanks. Can I look at it as it's hard to slide using legs and move forward, rather than walking and moving forward?

When you slide your foot, you're going against [kinetic] friction, therefore adding a moving force that requires energy to fight back.

When you walk (or roll), you're using [static] friction to brace yourself and propel your body forward. But since the friction force doesn't move with respect to the ground, it doesn't require any energy from your part.

 

[sophiecentaur]

Have you ever ice skated or roller skated?

The technique is to press away from the path of least resistance of the skates, putting your foot at an angle to the wanted direction of travel. Hard to describe but it is easy to learn,

 

[sophiecentaur]

When you slide your foot, you're going against [kinetic] friction, therefore adding a moving force that requires energy to fight back.

When you walk (or roll), you're using [static] friction to brace yourself and propel your body forward. But since the friction force doesn't move with respect to the ground, it doesn't require any energy from your part.

Yes. You got the idea. It's a combination of useful and wasted work and of matching your effort to the job. Wheelspin needs to be avoided if you want a quick getaway (unless you are drag racing when you don't care about efficiency. There are tricks to avoid wasting energy - like in a rack and pinion section of steep railway track. R and P is an extreme form of 'friction'.