Question on forces/friction acting on wheel

[Thevanquished]

i have this burning question which i have thought about for a long time and yet i could not seem to figure it out. here it goes.

when a wheel rotates to move a car or any vehicle, the friction would act on the opposite direction and so the friction (red arrow) would act in the same direction as the motion (black arrow), therefore propelling the car. However, given that this explanation is correct, how would the car travel at constant speed since there's no backward friction and there is always a resultant force (friction acting to the front) acting on the car? If you might say that the air resistance would oppose this force thereby allowing the car to travel at constant speed, i do not think that the air resistance is so huge to match the force, even if so, how then would the car be able to accelerate since the forces are always cancelled?

 

[Doc Al]

Let's ignore air resistance, rolling friction, and similar complications. To accelerate the car (assume no slipping), the ground must exert a forward static friction force on the tires. But when the car is moving at constant velocity, there's no need for a friction force from the ground.

 

[Thevanquished]

hmm... i don't get what u mean exactly... i mean u can't control the friction acting on the wheel can you? whether moving in constant velocity or not the wheel rotates in the same manner so won't the friction acting on it be the same?

 

[rcgldr]

i mean u can't control the friction acting on the wheel can you? whether moving in constant velocity or not the wheel rotates in the same manner so won't the friction acting on it be the same?

Friction just keeps the objects from sliding with respect to each other, there's no set force. If the car is going at a constant speed, then the net force is zero (otherwise the car would be changing speed).

If there's a block resting on the ground, friction keeps the block from sliding, but if nothing is pushing the block the friction force is zero. If you push the block with 10 lbs of force and the block doesn't slide, the friction force is 10lbs. If you push with 20 lbs of force and the block doesn't slide the friction force is 20 lbs. At some point if you push the block hard enough it will start sliding, and that is the maximum force that friction can provide (for the given conditions).

 

[russ_watters]

hmm... i don't get what u mean exactly... i mean u can't control the friction acting on the wheel can you? whether moving in constant velocity or not the wheel rotates in the same manner so won't the friction acting on it be the same?

No. The static friction force between the wheel and the ground is determined by the output of the engine. The greater the engine output, the greater the torque on the wheels and the greater the force between the wheels and ground.

 

[Thevanquished]

hmmm ok... but all of you seem to be answering only one part of my question.. for the wheels right... the friction seems to be the one propelling the car forwards and so if like you stated that the greater the torque the greater the friction, won't both forces cancel out and the resultant force acting on the wheel would be 0 and therefore the car would be moving at constant speed. How then does the car accelerate? can someone please answer both of the question (how does it move in constant speed and how does it accelerate) at the same time?

 

[Doc Al]

As I said earlier, if you ignore the complications due to air resistance and other dissipative forces, no tire friction is required for a car moving at constant speed. Of course, to overcome those dissipative forces some friction is required.

To accelerate the car, there must be a friction force on the tires pushing forward. That is produced as a result of the engine creating torque on the wheel. No, the torque due to the engine and the torque due to tire friction do no cancel out--the tire must also accelerate and thus requires a net torque. More importantly, the tires exert an external force on the car, driving it forward.

 

[Thevanquished]

u say no friction is required but no matter what it will still be produced..

 

[Doc Al]

u say no friction is required but no matter what it will still be produced..

Why do you think that?

 

[Thevanquished]

because when 2 surfaces meet there will definitely be friction

 

[Doc Al]

because when 2 surfaces meet there will definitely be friction

Really? I have a book lying on my desk. What's the force of friction on it?

Realize that (in the absence of skidding or deformation) the friction between tire and road is static friction. Static friction will be whatever is required to prevent slipping, from zero up to the maximum value the surfaces can generate for a given normal force.

Let's forget the car for a minute. Imagine just a wheel rolling without slipping along a perfectly horizontal surface at some speed. Assume the usual idealizations (no air resistance or deformation). In this case, there is no tendency to slip, thus the static friction is zero.

 

[Thevanquished]

yes the static friction may be zero but there would still be kinetic friction

 

[Doc Al]

yes the static friction may be zero but there would still be kinetic friction

Nope. Kinetic friction would only exist if the tire slipped (skidded) against the ground. As long as it rolls without slipping, the contact patch of the tire is always at zero speed with respect to the ground. No relative motion of the surfaces means no kinetic friction.

 

[Thevanquished]

ok... if there's no static friction and there's no kinetic friction... how is the care even supposed to move??

 

[Doc Al]

ok... if there's no static friction and there's no kinetic friction... how is the care even supposed to move??

Movement (constant velocity) does not require a force. (Review Newton's first law.)

Acceleration (change in velocity) requires a net force.

 

[Thevanquished]

i know... but from rest how does it even start moving?

 

[Doc Al]

i know... but from rest how does it even start moving?

As stated already in this thread, to accelerate the car requires a friction force on the tires.

 

[Thevanquished]

ok man do u have msn or something? so u can explain better

 

[Dadface]

Your question can be turned around...but from motion how does it even stop moving?The point being made here is that if there is no resultant force the object will carry on in the same way..if it is at rest it will remain at rest,if it is moving at a constant velocity(straight line and steady speed)it will continue moving at a constant velocity.The object will only change its velocity(accelerate) if there is a resultant force.

 

[Chuck St. Lou]

Your original Question;

However, given that this explanation is correct, how would the car travel at constant speed since there's no backward friction and there is always a resultant force.


With out some friction nothing will roll.
Like a brick sitting on concrete, gravity is pushing down on the tire and the normal force is straight up and down and not backwards or forwards.

 

[Thevanquished]

hmm... can anyone explain it in the sense of how forces are acting on it which causes it to accelerate and move at a constant speed instead of the other way round like "because it accelerates that's why it has resultant force and forward friction" and "because it is moving in constant speed that's why it has no friction"

 

[Doc Al]

hmm... can anyone explain it in the sense of how forces are acting on it which causes it to accelerate and move at a constant speed instead of the other way round like "because it accelerates that's why it has resultant force and forward friction" and "because it is moving in constant speed that's why it has no friction"

I'm still unclear as to what you are looking for. Before going any further, do you understand these two statements:
(1) If the tire is accelerating, there must be a net force on it. And the converse: If there's a net force on the tire, it is accelerating.
(2) If the tire is moving at constant velocity, the net force on it is zero. And the converse: If there's no net force on the tire, it's velocity cannot change.

These, of course, are just statements of Newton's laws of motion.

Try this. Imagine a car on a horizontal surface. (Forget air resistance and such complications.) The car is at rest and you want to get it moving. You arrange for the engine to exert a torque on the wheels. If the surface could produce no friction (say you were on wet ice), then the tires would just spin and the car would just sit there.

But friction from the ground acts to prevent the tires from slipping. The tires "try" to slip backward; friction prevents that by pushing forward. That forward push accelerates the car.

Once the car is moving at the speed you want, the tires no longer need to accelerate. They are moving (and spinning) at the exact rate needed to maintain that speed. Thus there is no tendency to slip, and no friction force is generated. (Of course, in real life you know that other forces act to slow the car down, so you have to keep giving it gas.)

 

[Dadface]

Friction plays a major part in all of this.If the car was stationary and there was no friction the driving wheels would simply spin and the car would go nowhere.If the car was moving and hit an ice patch it would skid.The driving force from the engine is transmitted to the driving wheels the tyres of which put a backward force on the road surface this resulting in an equal forward force on the car.If the car reaches a constant velocity the driving force does not disappear it is simply that the other forces ,mainly air resistance,become equal to the driving force making the resultant force zero.If the wheels were considered separately when they reach a constant angular velocity the torque transmitted by the engine is equal in size to the torque caused by friction.

 

[gmax137]

I'm not sure if this matters, but all of the posts that include "assuming no slippage" are apt to reach meaningless conclusions, because the tires *do slip* when the car is accelerating. You see this clearly whenever the driver "burns rubber" but it happens whenever you pull away from a stop (it's just to a lesser extent and therefore less obvious). Otherwise, our tires would last forever.

 

[russ_watters]

"start moving" is acceleration... go back to my post: the engine produces a torque on the tire, creating a static friction force between the tire and the ground.

Perhaps you are just under the misunderstanding that a pair of forces cancells to zero. Not true. Imagine you are pushing an object across a perfectly smooth surface (for example, a plastic puck on an air hockey table). You can say the puck is pushing against your finger or that your finger is pushing against the puck, but really, that's just two ways of looking at the same force. You don't subtract one from the other.

 

[sganesh88]

I guess the main difficulty faced by the original poster has got to do with friction remaining zero when the wheel is in uniform rolling. I would like to put it this way. The road surface "programs" the wheel such that it never tries to slip past it. For such a situation, the solution comes out to be Vcom = R*W(omega). It corresponds to the curve cycloid.
http://en.wikipedia.org/wiki/File:CycloidAnim04.gif
Does any point on the curve try to slip past the surface? Watch the animation again.

 

[Dadface]

I think that a misunderstanding of Newtons third law is a main reason why some are finding this topic difficult.The third law is deceptive, it might look simple at first glance but in fact it is conceptually quite difficult.I will express the law in a way which may be unfamiliar to some.
Forces occur in pairs which...
1. are equal in size
2. opposite in direction.
3. forces of the same type(eg gravitational)
4. act on different bodies.
It seems to be point number 4. that is causing the confusion.If the tyre puts a backward force on the road, the road puts an equal sized forward force on the tyre.Although these two forces are equal in size etc they do not cancel out because they act on different bodies namely the tyre and the road.

 

[Thevanquished]

if they do not cancel out... a car/wheel can never move at a constant speed so yes this is my question. i would like someone to use the same theory which will be able to explain both phenomenon (i.e. moving at constant speed AND accelerating) at the same time and not just one at a time which makes this confusing.

Does any point on the curve try to slip past the surface?

hmm i don't get what u mean by the curve slipping past the surface

 

[Doc Al]

if they do not cancel out... a car/wheel can never move at a constant speed so yes this is my question.

Is your question really about Newton's 3rd law? And why the forces don't cancel out?

If so, then realize that 3rd-law force pairs act on different objects thus never directly cancel. For example: For the car to accelerate, the road must exert a friction force on the tires. Accordingly, the tires will exert an equal and opposite force on the road.

Only when two equal and opposite forces act on the same object will they cancel. That's not the case here.

i would like someone to use the same theory which will be able to explain both phenomenon (i.e. moving at constant speed AND accelerating) at the same time and not just one at a time which makes this confusing.

What do you mean "at the same time"? The car cannot move at constant speed and accelerate at the same time.

I think part of the problem is that you have the idea that the friction force is fixed and unchanging. That's not the case. It varies as needed to prevent slipping.

 

[Thevanquished]

i mean explaining both at the same time... not that it happens at the same time... as in explaining both phenomenon using the same concept/theory which does not contradict each other at the same time

Is your question really about Newton's 3rd law? And why the forces don't cancel out?

If so, then realize that 3rd-law force pairs act on different objects thus never directly cancel. For example: For the car to accelerate, the road must exert a friction force on the tires. Accordingly, the tires will exert an equal and opposite force on the road.

Only when two equal and opposite forces act on the same object will they cancel. That's not the case here.

yes i know that they will only cancel out when they act on the same object... so if in this case they do NOT cancel out, how can a wheel even maintain a constant speed since there is always a resultant/net force acting on the wheel (friction)