EK 1001 Physics Question 224/225

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badmintondr

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Can someone clarify why exactly the answers are as such. The answer given in the book don't really explain it for me.

Is there a specific difference between having the car towed at a constant velocity versus a car moving from its own power at constant velocity that I'm overlooking?

thanks
 
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badmintondr said:
Can someone clarify why exactly the answers are as such. The answer given in the book don't really explain it for me.

Is there a specific difference between having the car towed at a constant velocity versus a car moving from its own power at constant velocity that I'm overlooking?

thanks
Click to expand...

There is a huge difference. If the car is under it's own power then force static acts opposite to the wheels motion. If the car is moving to the right, then the tire is moving to the left on the ground and so force friction will move to the right. Now this actually depends on whether the car is all wheel or front wheel but that isn't relevant. I only point that out that in some cars the static will be the left instead of the right as in this EK example.

Now, if it is being pulled, the assumption is that the tires AREN'T MOVING. so, when the tow truck first starts to pull the car to the right, the tire is trying to move to the right so friction is acting to the LEFT. Now, once the tire starts to move, this will no longer be valid. It will be as if the car's tires were moving on their own.

You'll notice that they claim if it were on ice it would be easier to tow. This is assuming that the car is stopped and you're getting it to move. Once the tires move then static friction will be to the left again. EK is only talking about the beginning.
 
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TheBoondocks said:
You'll notice that they claim if it were on ice it would be easier to tow. This is assuming that the car is stopped and you're getting it to move. Once the tires move then static friction will be to the left again. EK is only talking about the beginning.
Click to expand...

I thought ice example was only comparative. In this case with ice as surface, static force is always to the left. How is it shifting the direction?

Maybe I am reading this wrong....
 
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Awesomeness. Beautiful and clear answer. I totally get it. I was under the impression that the tires will rolling along lol. Like how tow trucks lift one end of the car and have the other 2 freely spinning. Thanks
 
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It seems there is some confusion here. This is what I undertand.


1. When a car is moving right by its own power

Imagine this by zooming in your image where tires interact with the surface. When car moves/ is moving to right, tires need to move in clockwise direction. This, at tire-surface interface, means that force is applied in the LEFT direction. According to the Newton's 3rd Law, this force has to be compensated by opposite force - which is the frictional force. Thus, frictional force is to the RIGHT.

2. When a car is being pulled

Zoom out of the previous image and imagine the car being pulled by a force to the right. It is this RIGHTward pulling force that is moving the car. Wheels are simply rolling over (not skidding). There is no force applied by tires and thus there is opposing frictional force. Only frictional force present is due to the rightward pulling force. This frictional force has to be in LEFT direction.


Ice example simply showed that if coefficient is less, then frictional force would be less and thus it would be easier to pulled the car.
 
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unique135 said:
It seems there is some confusion here. This is what I undertand.


1. When a car is moving right by its own power

Imagine this by zooming in your image where tires interact with the surface. When car moves/ is moving to right, tires need to move in clockwise direction. This, at tire-surface interface, means that force is applied in the LEFT direction. According to the Newton's 3rd Law, this force has to be compensated by opposite force - which is the frictional force. Thus, frictional force is to the RIGHT.

2. When a car is being pulled

Zoom out of the previous image and imagine the car being pulled by a force to the right. It is this RIGHTward pulling force that is moving the car. Wheels are simply rolling over (not skidding). There is no force applied by tires and thus there is opposing frictional force. Only frictional force present is due to the rightward pulling force. This frictional force has to be in LEFT direction.


Ice example simply showed that if coefficient is less, then frictional force would be less and thus it would be easier to pulled the car.
Click to expand...

You are smart and correct based off what EK said. However EK is wrong in saying that it would be easier to tow a car on ice. It would be harder to pull it by having the TIRES ROLL. This is what confused me.

In towing, the car has no power and so the wheels aren't turning clockwise on their own. If it is towed to the right the wheel must move clockwise. If they are rolling it means they are not sliding. In order for them to move clockwise, the static friction force must act to the left. It is force static that gets the tire to roll. So, if the force static were reduced ala on ice, the car tire wouldn't roll as the static friction wouldn't be enough to get it to move clockwise and drive it forward.

Normally, the force of the car moves it's tires clockwise. So, friction will act to the right to oppose this and so the car moves. In towing, the truck provides the force without power to the wheels. So, without friction, the wheels wouldn't move.

So, friction is necessary in both cases. I was wrong in saying it switches, but I knew that friction had to act to turn the wheel in the towing case which would mean that even on ice, it is relevant.

To OP, the tires are moving clockwise but that is due to static friction acting leftward against the rightward force of the truck. Remember, in order for something to move against a surface without slipping, static friction must be acting. I love EK 1001, but a decent amount of their explanations are terrible and contradict the answers.

If there were no friction as EK implies, the car would still MOVE. However, the wheels would NOT turn. So I guess EK maybe correct in that it would be easy because then you would be up against minimal kinetic friction. However, their comment about static friction acting would be incorrect. The tires would be SLIDING and not ROLLING which was the whole POINT of the question.
 
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Sorry but I am still confused with the answers that you gave. Maybe you can help me clear out this confusion and my apologies if I am creating confusion for you and others.


TheBoondocks said:
It is force static that gets the tire to roll. So, if the force static were reduced ala on ice, the car tire wouldn't roll as the static friction wouldn't be enough to get it to move clockwise and drive it forward.
Click to expand...
Understanding that kinetic friction force appears only after static friction force, if there is no static friction force present then why would the car move then? and that too easily?


TheBoondocks said:
However, their comment about static friction acting would be incorrect. The tires would be SLIDING and not ROLLING which was the whole POINT of the question.
Click to expand...
This is where I totally get lost.


PLEASE DON'T ABSOLUTELY RELY ON CONTENT BELOW. THIS IS WHAT "I" UNDERSTAND

Static Rolling Friction Force Example
Car attempting to move.

Kinetic Rolling Friction Force Example
Car is moving.

Static Sliding Friction Force Example
Block is pushed.

Kinetic Sliding Friction Force Example/s
Block is moving.
Car is moving with brakes on.
Car is accelerating above maximum acceleration


Little Detailed

This is what "I" understand. If a very minute force (0.0000000001 N) is applied, it will not overcome the opposing static rolling frictional force; thus the tires will not rotate and the car will not move. Now, if I increase this force that it overcomes static rolling frictional force and the tires will roll. What is the static rolling frictional force? What frictional force is acting on the rolling tires?

Static Rolling Friction Force seems to be non-existent on the literature (I was able to retrieve only few literatures but their definition was unclear). This is the friction force that resists the rolling of the wheels.

Kinetic Rolling Friction Force is the friction that resist the motion by reducing velocity. It responsible for slowing down the moving car and even stopping the rolling ball. This force would/should have the minimum magnitude of all four friction forces discussed.

Static Sliding Friction Force - It is the friction force most talked about. Again, it resists the motion of the car. In rolling objects such as cars, only at two instances - the static sliding friction force is achieved. Otherwise, the regular normal non-skidding cars never overcome this static sliding force. What are these two instances?

1. The car moving at a velocity suddenly brakes. Static sliding friction force is achieved and kinetic sliding friction force acts. Tires slides over the surface.

2. The wheels of the car accelerates at much higher rate. One where acceleration is higher than maximum acceleration (coefficient of static sliding friction force x gravity). Same thing happens here, tire slides over ther surface.


Maybe I am right. Maybe I am wrong. You guys say.
 
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unique135 said:
Sorry but I am still confused with the answers that you gave. Maybe you can help me clear out this confusion and my apologies if I am creating confusion for you and others.



Understanding that kinetic friction force appears only after static friction force, if there is no static friction force present then why would the car move then? and that too easily?



This is where I totally get lost.


PLEASE DON'T ABSOLUTELY RELY ON CONTENT BELOW. THIS IS WHAT "I" UNDERSTAND

Static Rolling Friction Force Example
Car attempting to move.

Kinetic Rolling Friction Force Example
Car is moving.

Static Sliding Friction Force Example/s
Block is pushed.

Kinetic Sliding Friction Force Example/s
Block is moving.
Car is moving with brakes on.
Car is accelerating above maximum acceleration


Little Detailed

This is what "I" understand. If a very minute force (0.0000000001 N) is applied, it will not overcome the opposing static rolling frictional force; thus the tires will not rotate and the car will not move. Now, if I increase this force that it overcomes static rolling frictional force and the tires will roll. What is the static rolling frictional force? What frictional force is acting on the rolling tires?

Static Rolling Friction Force seems to be non-existent on the literature (I was able to retrieve only few literatures but their definition was unclear). This is the friction force that resists the rolling of the wheels.

Kinetic Rolling Friction Force is the friction that resist the motion by reducing velocity. It responsible for slowing down the moving car and even stopping the rolling ball. This force would/should have the minimum magnitude of all four friction forces discussed.

Static Sliding Friction Force - It is the friction force most talked about. Again, it resists the motion of the car. In rolling objects such as cars, only at two instances - the static sliding friction force is achieved. Otherwise, the regular normal non-skidding cars never overcome this static sliding force. What are these two instances?

1. The car moving at a velocity suddenly brakes. Static sliding friction force is achieved and kinetic sliding friction force acts. Tires slides over the surface.

2. The wheels of the car accelerates at much higher rate. One where acceleration is higher than maximum acceleration (coefficient of static sliding friction force x gravity). Same thing happens here, tire slides over ther surface.


Maybe I am right. Maybe I am wrong. You guys say.
Click to expand...

OK, I think you are confused about what I said.

The OP wondered why friction was in the direction it was. In order for a tire to roll AND the car move forward, static friction is required.

EK asks the following: When a car is towed at a constant velocity, the frictional force between the road and tires is: the answer is static and in the opposite direction to the motion of the car.

In order for this to be true, that is the car is moving forward AND the tires are rolling. Static friction must act. We know that the tow truck is directing a rightward force on the truck. So, the static frictional force must be acting to the left. However. At the point the tires make contact WITH the road, they can't be slipping. This is where force static acts. Without this force, the tires wouldn't roll, they would slide. The Car would still move, but it would be sliding.

On Ice, why can't you go anywhere? This is because there is no force static to drive the car forward. In towing the car, the car will MOVE. The question is whether it will move and the tires ROLL or if it will move while the tires SLIDE ACROSS THE ICE. This is assuming the Tow Truck isn't on Ice as well obviously.
 
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This is what Crayton, EK's expert wrote on this very question. This first part will eliminate some confusion. I will then post another of his responses to the same question. It helped make sense of it for me.

Here's how you can figure out if the friction is static or kinetic:

If neither surface is moving, then you can be sure it's static friction.

When there is motion, the friction may be kinetic OR static, you have to figure out which.

If you want to know if friction is kinetic or static, the key is whether the two surfaces are sliding RELATIVE to each other. When a tire is turning, you can picture it was two gears turning together, each tooth fits into a space on the other gear and as the gears turn, every single tooth touches only one other space on the other gear. In this way as the tire spins, if your face is a point on the tire, you roll around, make contact with one point on the road, then roll around again, make contact with one point on the road, etc. Since each point on the tire only contacts a single point on the road (and vice versa) the two surfaces are not sliding relative to each other, and this is static friction.

I can tell you the only two ways you get kinetic friction with a tire. One would be the road is moving underneath the tire but the tire is locked still (so imagine locking your brakes and skidding across the road or putting your car in park, putting the brakes on and having a tow truck drag it down the road). In that case if your face is a point on the tire, you slide across multiple points on the road.

The other would be the tire is turning but the road surface is "locked". This is what happens if you're at a stop and gun it in first gear or you get stuck in the mud or the ice. The tires spin in place for a second before you get traction and start to move. In this case a single point on the road sees multiple points on the tire sliding past it.

So for this problem, since the back wheels are not skidding, the frictional force between the road and tire is static. This problem, does presume that you know when a car is being towed the back wheels will roll. (No one is sitting the car and putting the brakes on when the car gets towed).


As for the direction of friction, it acts to oppose the force that leads to it.

So for the problem 41 on page 35 of the physics book, where the car is driving itself, the back wheels are generating the force, which turns them clockwise (leftward at the surface of the road). So the engine makes the tires push leftward on the road, and frictional force is the opposite direction (a rightward force) on the tires, which makes the car go rightward.

For this problem you're asking about, the tires are not under power (the are turning passively not actively). Let's assume the tow truck is pulling the car rightward. What is happening is the tow truck is applying a rightward force on the car (and the tires). SO the frictional force is in the opposite direction (a leftward force) on the tires.
 
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No. What would happen if you towed a car across a frictionless surface (say tow it across ice). As you drag the wheels across the ice, the wheels would not turn at all. The only reason the wheels would rotate at all is because friction acts between the tire and the road to create a torque on the tire that makes it rotate. As you tow the car to the right, the wheels will turn clockwise. A clockwise turning wheel represents making the bottom of the wheel move leftward. Since friction is what is causing the rotation at all, the friction is acting leftward, which is opposite to the motion of the car.

When a car drives normally to the right, the engine provides the torque to turn the wheel clockwise. So the wheel will turn clockwise regardless of whether there is friction on the road or not. But, just getting the wheel to rotate doesn't mean the car is going to move forward. If the road were frictionless, the tire wouldn't get any traction and the wheel would just spin in place. So in this case, the only reason the car moves foward is becuase of friction. Since the car is moving rightward, and friction is causing the movement, friction must be rightward (same direction as the car).
 
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unique135 said:
It seems there is some confusion here. This is what I undertand.


1. When a car is moving right by its own power

Imagine this by zooming in your image where tires interact with the surface. When car moves/ is moving to right, tires need to move in clockwise direction. This, at tire-surface interface, means that force is applied in the LEFT direction. According to the Newton's 3rd Law, this force has to be compensated by opposite force - which is the frictional force. Thus, frictional force is to the RIGHT.

2. When a car is being pulled

Zoom out of the previous image and imagine the car being pulled by a force to the right. It is this RIGHTward pulling force that is moving the car. Wheels are simply rolling over (not skidding). There is no force applied by tires and thus there is opposing frictional force. Only frictional force present is due to the rightward pulling force. This frictional force has to be in LEFT direction.


Ice example simply showed that if coefficient is less, then frictional force would be less and thus it would be easier to pulled the car.
Click to expand...

1. Your statement here is perfect.
2. Here, you are almost correct. The pulling force WILL not ALONE get the tires to roll. It will MOVE THE CAR TO THE RIGHT. What determines whether the car MOVES BY SLIDING or MOVES BY ROLLING is static friction. When I capitalize I'm not belittling, it's hard to emphasize stuff online. Finally, I love the depth you went through this. I gained an even better understanding and realize my mistake.

I WAS WRONG IN SAYING IT SWITCHES. I think you thought I still think that it does. I stated this in my previous post made on Monday Morning. What I was saying is that EK's statement that on Ice it would be easier to pull is MISLEADING. If there were no friction because that basically is what ice is, the tires WOULDN'T roll.

Why is this significant. On ice, the answer is no longer valid. What you need to remember is that the question assumed CONSTANT VELOCITY. If it were being pulled on ice, the force friction would be KINETIC AND OPPOSITE instead of STATIC AND OPPOSITE. So, on Ice the static/kinetic frictional force are basically zero.

In closing, I see your point regarding the initial beginning. However, IGNORE WHAT I SAID EARLIER ABOUT IT SWITCHING. Ignore that and that'll eliminate most of your confusion. If it's moving at constant velocity, it could be doing this by either sliding due to non-rolling tires, or moving due to rolling tires. On ice, it would slide.

I foolishly saw this as switching which is wrong. It just that the tires wouldn't roll on Ice as it's being Told. If you read the post "Creyton's illucidation," you'll see he says this better than I do.
 
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Lol, we both had our concepts right. We just started with different facts/interpretation and that is:

Is ice truly frictionless?
I went with ice as nearly frictionless but not completely frictionless and expected tires to roll. I am pretty sure this is what EK text was talking about. On the other hand, you went with ice as completely frictionless and expected tires to slide.


But, if you read my last post it will provide you strong and clear insights related to friction.

Thanks for discussing and equally participating on this discussion. I think lot was learnt here 🙂
 
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unique135 said:
Lol, we both had our concepts right. We just started with different facts/interpretation and that is:

Is ice truly frictionless?
I went with ice as nearly frictionless but not completely frictionless and expected tires to roll. I am pretty sure this is what EK text was talking about.

Brilliant. I never considered this and this is clearly what EK meant. Excellent catch and insight. If you have this mantra to thoroughly understand, you will kill it.

On the other hand, you went with ice as completely frictionless and expected tires to slide.


But, if you read my last post it will provide you strong and clear insights related to friction.

Thanks for discussing and equally participating on this discussion. I think lot was learnt here 🙂
Click to expand...

Brilliant. I never considered this and this is clearly what EK meant. Excellent catch and insight. If you have this mantra to thoroughly understand, you will kill it. 🙂
 
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