static friction and kinetic friction force!!!

[peacefulheart]

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When a car is towed at constant velocity, the frictional force between the road and its tires is

A. static and in the direction of the motion of the car.

B. static and in the opposite direction to the motion of the car.

C. kinetic and in the direction of the motion of the car

D. kinetic and in the opposite direction to the motion of the car.

the answer is B.

1. I have no problem with the direction of friction force. I do not understand why it is static frictional force.

2. Could anyone explains how to tell static and kinetic friction. thanks a lot!!!

 

[Gauss44]

When a car is towed at constant velocity, the frictional force between the road and its tires is

A. static and in the direction of the motion of the car.

B. static and in the opposite direction to the motion of the car.

C. kinetic and in the direction of the motion of the car

D. kinetic and in the opposite direction to the motion of the car.

the answer is B.

1. I have no problem with the direction of friction force. I do not understand why it is static frictional force.

2. Could anyone explains how to tell static and kinetic friction. thanks a lot!!!

It's static because the tires are NOT skidding. If the tires are skidding or sliding, it's kinetic. If the tires are rolling, it's static. If the car is parked (obviously), it's static.

 

[LoLCareerGoals]

and i have a problem with direction actually. Static friction opposes sliding. When a tiring rolls its tires' surface "slides" left (assuming the car is moving right) against the ground. Wouldn't the friction force then be directed in the same direction as the motion of the car?

 

[peacefulheart]

It's static because the tires are NOT skidding. If the tires are skidding or sliding, it's kinetic. If the tires are rolling, it's static. If the car is parked (obviously), it's static.

1. Thanks for the reply.

2. I just can not visualize the difference between rolling and sliding. When the car is rolling and when it is sliding ?

thanks

 

[cheesier]

1. Thanks for the reply.

2. I just can not visualize the difference between rolling and sliding. When the car is rolling and when it is sliding ?

thanks

If you've driven in snow you know the difference. If you are trying to get your car moving in snow, sometimes the wheels will spin but the car won't move. This is because the coefficient of static friction in snow is too low. In this situation kinetic friction is acting on the wheels.

One way to think about this is that the wheel kind of pushes itself against the road to rotate. Let's say you are sliding a big bookshelf across the floor by pushing on it; if you are wearing socks, YOU might start to slide instead of the bookshelf. In this case, you are the point of contact between the tire and the road, and the bookshelf is the rest of the wheel. In order to move the bookshelf, you want it to slide while you stay stationary, just like you want the car tire to rotate while the small point of contact between it and the road stays stationary (due to a sufficiently high coefficient of static friction). If that was too confusing forget it, but it helps me conceptualize static vs kinetic friction

 

[LoLCareerGoals]

That makes sense. But can you still please explain why the direction is in the opposite direction of the car?
I thought friction was directed against the direction of 2 sliding (or attempting to slide) surfaces?
The tires are "trying" to skid and the static friction is not allowing them to. When I visualize skidding the friction should be in the direction of the car's desired movement! I am even visualizing little surface unevenness and still friction should be in the direction of the car's movement.
How am I misapplying a concept here?

Edit:
I think I understand where my confusion comes from. I was imagining a forced rotation which I guess would the case with active engine trying to rotate a tire. Here the fact that car is being towed is actually important. I would actually further argue that how it is being towed is also important. If the car front is elevated when towed, I think friction is still along the direction of the car's movement. If towing is perfectly horizontal then yes I would go with B since the force is trying to drag the tire along and not rotate it. Agree?

 

[HuskerMed1816]

That makes sense. But can you still please explain why the direction is in the opposite direction of the car?
I thought friction was directed against the direction of 2 sliding (or attempting to slide) surfaces?
The tires are "trying" to skid and the static friction is not allowing them to. When I visualize skidding the friction should be in the direction of the car's desired movement! I am even visualizing little surface unevenness and still friction should be in the direction of the car's movement.
How am I misapplying a concept here?

Edit:
I think I understand where my confusion comes from. I was imagining a forced rotation which I guess would the case with active engine trying to rotate a tire. Here the fact that car is being towed is actually important. I would actually further argue that how it is being towed is also important. If the car front is elevated when towed, I think friction is still along the direction of the car's movement. If towing is perfectly horizontal then yes I would go with B since the force is trying to drag the tire along and not rotate it. Agree?

Easiest answer: friction opposes motion. Thus it will act opposite of the direction the car is being towed. The car is not being dragged at all. Think about when you walk, you place a foot intending to move forward and static friction (assuming you aren't wearing socks and slide) acts opposite of your motion to allow you to actually move forward.

 

[cheesier]

That makes sense. But can you still please explain why the direction is in the opposite direction of the car?
I thought friction was directed against the direction of 2 sliding (or attempting to slide) surfaces?
The tires are "trying" to skid and the static friction is not allowing them to. When I visualize skidding the friction should be in the direction of the car's desired movement! I am even visualizing little surface unevenness and still friction should be in the direction of the car's movement.
How am I misapplying a concept here?

Edit:
I think I understand where my confusion comes from. I was imagining a forced rotation which I guess would the case with active engine trying to rotate a tire. Here the fact that car is being towed is actually important. I would actually further argue that how it is being towed is also important. If the car front is elevated when towed, I think friction is still along the direction of the car's movement. If towing is perfectly horizontal then yes I would go with B since the force is trying to drag the tire along and not rotate it. Agree?

If the car was not being towed, the static friction force would be in the same direction as motion, because without the force the car would not move. Applying the frictional force makes the car move forward, therefore the frictional force has to be in the forward direction.

I think it can best be summed up like this: if an outside force is acting on an object, friction opposes the force and is in the direction opposite the direction of motion. If an object is stationary and the only force acting upon it (parallel to its displacement) is friction, it must be in the direction of motion.

 

[HuskerMed1816]

If the car was not being towed, the static friction force would be in the same direction as motion, because without the force the car would not move. Applying the frictional force makes the car move forward, therefore the frictional force has to be in the forward direction.

I think it can best be summed up like this: if an outside force is acting on an object, friction opposes the force and is in the direction opposite the direction of motion. If an object is stationary and the only force acting upon it (parallel to its displacement) is friction, it must be in the direction of motion.

False. "If the object is stationary.. It is in the direction of its motion"?? Ummm..

If it is stationary, there is no friction. Note your MCAT prep or physics book where static friction is less than or equal to the coefficient times the normal force.

 

[LoLCareerGoals]

Easiest answer: friction opposes motion. Thus it will act opposite of the direction the car is being towed. The car is not being dragged at all. Think about when you walk, you place a foot intending to move forward and static friction (assuming you aren't wearing socks and slide) acts opposite of your motion to allow you to actually move forward.

I don't like this answer at all. Friction opposes motion??? The whole point of passage above was that there WAS NO MOTION between tires and the ground. A more precise statement is friction opposes the slide (or attempted slide for static) of 2 surfaces.

You are pushing a rolling barrel by applying a force to the top of the circular barrel. What is the direction of static friction between the surface of the barrel and the ground?
My analysis: by applying a force to the top of the barrel you are inducing rotation. By inducing this rotation you are attempting to force the barrel surface to slide against the surface of the ground in the direction opposite to the direction of the barrel's motion. This means friction is in the direction of the motion for this case!

 

[LoLCareerGoals]

If the car was not being towed, the static friction force would be in the same direction as motion, because without the force the car would not move. Applying the frictional force makes the car move forward, therefore the frictional force has to be in the forward direction.

I think it can best be summed up like this: if an outside force is acting on an object, friction opposes the force and is in the direction opposite the direction of motion. If an object is stationary and the only force acting upon it (parallel to its displacement) is friction, it must be in the direction of motion.

A bit confusing I agree. But in this case tires and the ground ARE stationary (hence the static friction), so see ^

I think it is important for the problem to specify that towing is basically a perfectly horizontal force acting on the center of the tire, which hardly the case for any real car being towed, so this q really threw me off.

 

[HuskerMed1816]

A bit confusing I agree. But in this case tires and the ground ARE stationary (hence the static friction), so see ^

I think it is important for the problem to specify that towing is basically a perfectly horizontal force acting on the center of the tire.

Ahhh I get what you are saying. However, friction ALWAYS opposes motion. Think, ideal I've rink.. No friction.. You don't stop til you hit something. In the case of the towed car the force come from the tow truck/tension on on pulling cable or whatever. That gives the car it's movement. In this case, constant velocity means the force acting to pull the car is counteracted by the friction this no acceleration.

 

[cheesier]

Ahhh I get what you are saying. However, friction ALWAYS opposes motion. Think, ideal I've rink.. No friction.. You don't stop til you hit something. In the case of the towed car the force come from the tow truck/tension on on pulling cable or whatever. That gives the car it's movement. In this case, constant velocity means the force acting to pull the car is counteracted by the friction this no acceleration.

It's not true that friction always opposes motion. If the car was not being towed, the static friction force would be in the same direction as the motion.

Here's a post on another forum about a similar problem:

http://www.physicsforums.com/archive/index.php/t-114755.html

False. "If the object is stationary.. It is in the direction of its motion"?? Ummm..

If it is stationary, there is no friction. Note your MCAT prep or physics book where static friction is less than or equal to the coefficient times the normal force.

In regards to the first sentence, yes, the portion of the tire in contact with the road is stationary and the car is moving. I could have worded it better but the point of this problem is that there is static friction force (which acts on stationary objects) that has a relationship with the displacement of a moving car.

In response to the bolded: no, individual vector forces have nothing to do with motion. Recall that NET force is important when determining displacement, but there can be individual force vectors resulting in no net displacement and a stationary object (you're probably stationary while you read this, but there are a multitude of individual force vectors acting on you). Generally speaking, whenever you see a static force vector, the object it is acting on is stationary (in the case of this problem, the portion of the tire in contact with the road was stationary).

When the coefficient of static friction is too large for an object to be moved by applying a force (like pushing a crate across a blacktop), that doesn't mean that there is no force being applied (and hence an equal and opposite frictional force), it just means that the force being applied isn't bigger than the frictional force. The two force vectors cancel each other out, but they are still there.

 

[HuskerMed1816]

If it wasn't being towed, the friction would still oppose the rotational motion of the wheel. As with many concepts in Physics the problem is defining your system and what motion you are talking about.

 

[cheesier]

If it wasn't being towed, the friction would still oppose the rotational motion of the wheel. As with many concepts in Physics the problem is defining your system and what motion you are talking about.

If the car is not being towed, the frictional force is in the same direction as the displacement. I provided a link with the exact same concept being discussed that would be a good thing to read if it's not clear.

 

[LoLCareerGoals]

Forget about the car. I just want to tow the tire. Car is nothing more than a tension force transducer that eventually transfers the tension to the tire.

`` _____________
/````````````````````````\ ```````` Ttow
|``````````O-------------|--------------------->
|`````````````````````````| <- tire
\_____________/
V V V V <----- friction causing uneven spots of the tire and the ground
____^_^_^__^_____________________ <- road


When the Ttow is applied to the center of the tire. Upper "V"s will move right (skid) some miniscule distance get caught on the lower "^"s so strongly that is it easier for the tire to rotate than slide. Now when they get caught upper "V"s want to move right, but don't so friction is left (opposing the motion). Fine.

-----------------------------------------------------------------------------------------------------------------------------
_____________
/````````````````````````\x-----------------------------------> Ttow
|`````````` O `````````` |
|``````````````````````````| <- tire
\_____________/
V V V <----- friction causing uneven spots of the tire and the ground
____^_^_^__^_____________________ <- road

Suppose now Ttow is acting OFF center of the tire. (I would expect this to be the case since most of the car's mass is above the tire and this is especially true if the car's front is elevating like in real towing). Ttow is inducing rotation! "V"s now want to move left because rotation is being induced clockwise. But they caught on lower "^"s and are prevented from doing so. Friction must then act to the right.

Your ALWAYS against motion statement is very imprecise imo. Friction acts against the slide (or attempted slide) of 2 surfaces. You insist on selecting proper system yet at no point you offer what you consider a system except vaguely suggesting the system is the car when it really should be the tire.

If I am off, propose 2 surfaces. Explain where 1st is moving with respect to the other and why.

 

[cheesier]

LOLcareergoals, I don't think it would make any difference whether the Ttow force is applied off center. It would make a difference in the torque, but I don't see why that would translate into a shifting in direction of the frictional force. I mean, you'll never be able to be perfectly centered, so would that mean that the static frictional force is always in the direction of motion when you're towing something? If not, how far off center would it have to be and what is happening mathematically that causes the direction of the static force to change?

 

[gettheleadout]

Ooooooh the direction of motion on this one really is tricky. The fact that it's towed throws you off; great question.


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[LoLCareerGoals]

LOL career goals, it should make no difference whether the towing is applied off center, because if the static friction force is in the direction of motion, the tire skids if it is a horizontal surface.

Imagine that the tire is moving ??? forward and the static friction is in the same direction as the displacement. Now imagine it is continuing to move but you connect a rope to it and begin to pull. If the tire continues rotating and all of a sudden you start pulling it, it will skid. Whether you pull it off center of in the middle wouldn't make a difference. It would skid either way. Now the reason it doesn't start to skid is that the static friction force opposes the tow cable.

Can you please use more precise words like "skid", "slide" or "rotate". "Move" can mean anything.

Let me give you an example where it matters and you can show me how it is different from the car example in the OP.
Tire. I attach rope to the center. Now a small, perfectly horizontal force is applied till the tire rolls.
Now attach same rope to the top of the tire, will it take more force to make it roll now?
Bottom line I think it is time we abandon the intuitive approach. Can you propose 2 surfaces and explain how they slide vs each other.

 

[gettheleadout]

LoLcareergoals has the more correct definition of friction, but I don't see why the car would be towed with force applies off center relative to the tires. The tow line is hooked under the axle, which is centered with the tires and that's where the force is applied. Even if it wasn't necessarily, speculating beyond that is just that, speculation. That's not something to bring into a discrete.


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[LoLCareerGoals]

LoLcareergoals has the more correct definition of friction, but I don't see why the car would be towed with force applies off center relative to the tires. The tow line is hooked under the axle, which is centered with the tires and that's where the force is applied. Even if it wasn't necessarily, speculating beyond that is just that, speculation. That's not something to bring into a discrete.
Sent from my iPhone using SDN Mobile

Ok if that was part of the problem definition, fine. I am down with B. All I was trying to say where the force is being applied to the tire is important.
Also when I am thinking towing, I am thinking: tow truck that lifts the front bumper or rope connected to the front of a truck. Socieo-cultural bias I guess 🙄

 

[cheesier]

Can you please use more precise words like "skid", "slide" or "rotate". "Move" can mean anything.

Let me give you an example where it matters and you can show me how it is different from the car example in the OP.
Tire. I attach rope to the center. Now a small, perfectly horizontal force is applied till the tire rolls.
Now attach same rope to the top of the tire, will it take more force to make it roll now?
Bottom line I think it is time we abandon the intuitive approach. Can you propose 2 surfaces and explain how they slide vs each other.

Yes, because the component of force parallel to displacement is smaller since presumably it it has a vertical component now. But that wouldn't make any difference in the direction of the static force vector which is what this question is asking. I edited my post above for clarity, but whether the towing force is centered or not, as long as the car is being towed on a horizontal surface, the static friction force is in the direction opposite that of displacement.

Whether two tires are in contact with the road vs 4 might makes no difference in the direction of the static friction vector. Mathematically, why would it? There would have to be an equation where plugging in a 2 instead of a 4 turns a vector form negative to positive. Similarly, whether the force is centered or not will change the torque, but again, I don't think that the presence of that torque turns a vector that was negative suddenly positive.

 

[HuskerMed1816]

Cheesier and I are really arguing the same point, we just fail to be able to clarify the details of our disagreement.

For the OP's question that was presented, the entire car (and the tires) are the system. The tow truck pulls the car say to the right, so for the sake of the MCAT, if there is no further detail added, the friction between the car/tires system and the road will oppose the (translational) motion of the system and act to the left. This is say system 1. In this scenario, the car is not what is causing the translational motion, the tow truck is. The tow truck would start to pull the car, but static friction acting opposite of this pull would create the torque that causes the wheels to rotate which is why we get static friction and no skid or sliding of the tire.

The specific rotational motion of the tire must be considered independent of the translational motion of the tire (and the entire car/tire system for that matter). To prevent the tire from skidding, friction much oppose the rotational motion of the tire (clockwise) and thus is to the right which is the same direction as the displacement of the vehicle. This is system 2. If the car was not being towed, it is the car creating the force to move forward. That means the car is causing the rotational torque and thus the static friction would act in the same direction the car would move (but is still opposite of the motion because now we have defined our system as the tire and the road and are discussing its rotational motion, not the translational motion).

This website provides some diagrams to help clarify the same point I think many of us are trying to make. http://www.phy.davidson.edu/fachome/dmb/PY430/Friction/rolling.html

Friction does always oppose motion. Translational and rotational motion however are entirely different types of motion and that is where the ability to have friction in the same direction as the movement of the vehicle arrises. I guess I apologize for not getting off my phone and making sure I stated everything with 100% accuracy, but honestly, I could give a **** less since I have already been accepted and have an entire bottle of Makers calling.

 

[gettheleadout]

Ok if that was part of the problem definition, fine. I am down with B. All I was trying to say where the force is being applied to the tire is important.
Also when I am thinking towing, I am thinking: tow truck that lifts the front bumper or rope connected to the front of a truck. Socieo-cultural bias I guess 🙄

Imagine the car is on ice and being towed with the front tires elevated by a truck with chains on its own tires. In this case the towed car just slides, right? The tires experience no friction on ice. On a road, the same thing applies, but now the tires roll forward because the road has friction to push their surface backward, in the direction opposite the car's motion.

 

[LoLCareerGoals]

Cheesier and I are really arguing the same point, we just fail to be able to clarify the details of our disagreement.

For the OP's question that was presented, the entire car (and the tires) are the system. The tow truck pulls the car say to the right, so for the sake of the MCAT, if there is no further detail added, the friction between the car/tires system and the road will oppose the (translational) motion of the system and act to the left.

What an amazingly trivialized/rigor free statement.

The specific rotational motion of the tire must be considered independent of the translational motion of the tire (and the entire car/tire system for that matter). To prevent the tire from skidding, friction much oppose the rotational motion of the tire (clockwise) and thus is to the right which is the same direction as the displacement of the vehicle. This is system 2. If the car was not being towed, it is the car creating the force to move forward. That means the car is causing the rotational torque and thus the static friction would act in the same direction the car would move (but is still opposite of the motion because now we have defined our system as the tire and the road and are discussing its rotational motion, not the translational motion).

so you agree with me?? Who cares about system 1. Friction is affecting system 2. Figure out how forces of system one translate into system 2 and analyze system 2 only.

Friction does always oppose motion. Translational and rotational motion however are entirely different types of motion and that is where the ability to have friction in the same direction as the movement of the vehicle arrises. I guess I apologize for not getting off my phone and making sure I stated everything with 100% accuracy, but honestly, I could give a **** less since I have already been accepted and have an entire bottle of Makers calling.

Ok now we are getting somewhere. You just want to wave some hands and not really get into the meaty part of the problem. Got it.

 

[LoLCareerGoals]

Imagine the car is on ice and being towed with the front tires elevated by a truck with chains on its own tires. In this case the towed car just slides, right? The tires experience no friction on ice. On a road, the same thing applies, but now the tires roll forward because the road has friction to push their surface backward, in the direction opposite the car's motion.

So basically due presence on an axis the force will eventually only be applied to the center of the tire after tension is transferred to the tire? So my 2 examples are correct but #2 is not applicable because of the presence of the axis?

 

[gettheleadout]

So basically due presence on an axis the force will eventually only be applied to the center of the tire after tension is transferred to the tire? So my 2 examples are correct but #2 is not applicable because of the presence of the axis?

The force can't be transferred anywhere but the center of the tire because it's fixed on an axle and no where else. You can't tow a car by tying a rope to the spokes above the axle when the tire is at rest, because towing doesn't work like that, the tire would just rotate til the point of attachment was closest to the towing force and then slide. So, in this case, there is no relevant factor of the angle of the towing affecting the direction of friction.

 

[LoLCareerGoals]

The force can't be transferred anywhere but the center of the tire because it's fixed on an axle and no where else. You can't tow a car by tying a rope to the spokes above the axle when the tire is at rest, because towing doesn't work like that, the tire would just rotate til the point of attachment was closest to the towing force and then slide. So, in this case, there is no relevant factor of the angle of the towing affecting the direction of friction.

Thank you. I guess those that truly understand the concept can word it just fine.