Physics Questions

[MaxFusion16]

f=ma
done

 

[jai6638]

damn friction.. did it last year in physics and now that ive changed schools and come to the US i have to do it again... :'(


btw dude for future reference, in case you need help with any other physics problems, in addition to ATOT u could probably brwose these forums


cheers

 

[Heisenberg]

Originally posted by: DrPizza
I just searched through a couple of college physics books. I couldn't find anything about rolling friction. But, now that I've thought about it, in problem 1, there is no coefficient of friction to calculate. The problem is too complex - it depends on where the friction comes from. Imagine that the tires are held onto the wagon with bolts and washers. If you tightened the bolts too much, it's going to create friction when you attempt to spin the wheels. The amount of friction between the washer/wheel/wagon is independent of the mass of the wagon (and therefore the normal force).

Pour some mass into your wagon - enough to double the mass, and I highly doubt you'll double the force of friction. The force will be somewhere between the original force of friction and twice the original force. I'm on a physics listserv - I'll ask that question of all the other physics teachers/profs on the same listserv tomorrow.

I think you're making this too complicated. You're right about friction being present in other places than just between the wheels and the pavement, but in these type of problems it's just assumed that the other mechanical parts are ideal and so have no friction. In that case it's easy to find whereas otherwise it'd be way too hard for intro physics classes.

 

[konakona]

Originally posted by: DrPizza
I just searched through a couple of college physics books. I couldn't find anything about rolling friction. But, now that I've thought about it, in problem 1, there is no coefficient of friction to calculate. The problem is too complex - it depends on where the friction comes from. Imagine that the tires are held onto the wagon with bolts and washers. If you tightened the bolts too much, it's going to create friction when you attempt to spin the wheels. The amount of friction between the washer/wheel/wagon is independent of the mass of the wagon (and therefore the normal force).

Pour some mass into your wagon - enough to double the mass, and I highly doubt you'll double the force of friction. The force will be somewhere between the original force of friction and twice the original force. I'm on a physics listserv - I'll ask that question of all the other physics teachers/profs on the same listserv tomorrow.

you have taken your reality check little too far... tis only a high school - intro level college physics question, so everything would be dumbed down and assumed to be ideal or whatever...

as for the 'rolling' friction.. maybe you are thinking of kinetic friction? if memory serves right, static friction is something that is exerted on a stationary object before it starts moving. once it is in motion, you only have to consider kinetic friction.

 

[DrPizza]

I'm not trying to take it too far. Finding the coefficient of friction in the first problem is inappropriate and a poor example of physics. If a high school teacher is going to make up physics problems, then perhaps they should make up problems that actually involve the physics principles that they are examining, rather than reinforce students' misconceptions about friction because the problem presents an "obvious" method to find the solution. Unfortunately, the intended method to find the solution is irrelevant to the actual solution.

In the case of a rolling wheel, there is NO, I repeat, NO kinetic friction, unless the tire is also skidding against the ground. When you are driving a car down the road, the type of friction existing between your tires and the ground is STATIC friction. But, even that's incorrect as traction with tires is more complicated than the simple physics principles being taught with F=uF. Note: the most common misconception about friction is that it is dependent upon the surface area. This is incorrect. However, in the case of car tires, the amount of traction *is* dependent on the surface area. Traction is too difficult for high school, or college physics. If it wasn't, then it would be a piece of cake to develop the most ideal tire tread and all the engineers at goodyear, michelin, etc. could retire and go home.

 

[sciencewhiz]

Originally posted by: DrPizza
I'm not trying to take it too far. Finding the coefficient of friction in the first problem is inappropriate and a poor example of physics. If a high school teacher is going to make up physics problems, then perhaps they should make up problems that actually involve the physics principles that they are examining, rather than reinforce students' misconceptions about friction because the problem presents an "obvious" method to find the solution. Unfortunately, the intended method to find the solution is irrelevant to the actual solution.

In the case of a rolling wheel, there is NO, I repeat, NO kinetic friction, unless the tire is also skidding against the ground. When you are driving a car down the road, the type of friction existing between your tires and the ground is STATIC friction. But, even that's incorrect as traction with tires is more complicated than the simple physics principles being taught with F=uF. Note: the most common misconception about friction is that it is dependent upon the surface area. This is incorrect. However, in the case of car tires, the amount of traction *is* dependent on the surface area. Traction is too difficult for high school, or college physics. If it wasn't, then it would be a piece of cake to develop the most ideal tire tread and all the engineers at goodyear, michelin, etc. could retire and go home.

So, do you not teach electricity or magnetism, because maxwell's equations are to difficult for someone who hasn't had 3 dimensional calculus?

Do you not teach friction at all, because it's too difficult? people are better off thinking that everything glides smoothly until they become real engineers and work at goodyear?

Do you never teach statics, because it is a limited case of dynamics?

Do you never teach optics, because we still aren't sure whether light is a wave or a particle?

Is it better to give up on a problem, then to solve a simplified model?

 

[silverpig]

Originally posted by: DrPizza
I'm not trying to take it too far. Finding the coefficient of friction in the first problem is inappropriate and a poor example of physics. If a high school teacher is going to make up physics problems, then perhaps they should make up problems that actually involve the physics principles that they are examining, rather than reinforce students' misconceptions about friction because the problem presents an "obvious" method to find the solution. Unfortunately, the intended method to find the solution is irrelevant to the actual solution.

In the case of a rolling wheel, there is NO, I repeat, NO kinetic friction, unless the tire is also skidding against the ground. When you are driving a car down the road, the type of friction existing between your tires and the ground is STATIC friction. But, even that's incorrect as traction with tires is more complicated than the simple physics principles being taught with F=uF. Note: the most common misconception about friction is that it is dependent upon the surface area. This is incorrect. However, in the case of car tires, the amount of traction *is* dependent on the surface area. Traction is too difficult for high school, or college physics. If it wasn't, then it would be a piece of cake to develop the most ideal tire tread and all the engineers at goodyear, michelin, etc. could retire and go home.

There's kinetic friction on a moving wheel. That's why they grease the bearings... to reduce that. Having a problem like that (what is the frictional force) is just fine. Finding the coefficient of friction is a simplification, but it's not that big a deal.

 

[BigPoppa]

Just assume the wagon doesn't have wheels.

Dr. Pizza is right in that there is no (rather, limited) kinetic friction between a rolling wheel and the surface it travels on. The teacher should have just made the wagon a simple box.

I'd appreciate it if physics profs. would take the time to make better examples and problem sets. Lord knows I had one hell of a time learning college physics. It doesn't help when your last test has a class average of 53.

 

[Gibson486]

Originally posted by: BigPoppa
Just assume the wagon doesn't have wheels.

Dr. Pizza is right in that there is no (rather, limited) kinetic friction between a rolling wheel and the surface it travels on. The teacher should have just made the wagon a simple box.

I'd appreciate it if physics profs. would take the time to make better examples and problem sets. Lord knows I had one hell of a time learning college physics. It doesn't help when your last test has a class average of 53.

Yeah, low grades suck, but it is worse when you ar eodne teh class and realize how easy teh stuff was to begin with.

Lesson learned: If class avg. was below 60, the prof did his job.

 

[DrPizza]

Originally posted by: sciencewhiz

Originally posted by: DrPizza
I'm not trying to take it too far. Finding the coefficient of friction in the first problem is inappropriate and a poor example of physics. If a high school teacher is going to make up physics problems, then perhaps they should make up problems that actually involve the physics principles that they are examining, rather than reinforce students' misconceptions about friction because the problem presents an "obvious" method to find the solution. Unfortunately, the intended method to find the solution is irrelevant to the actual solution.

In the case of a rolling wheel, there is NO, I repeat, NO kinetic friction, unless the tire is also skidding against the ground. When you are driving a car down the road, the type of friction existing between your tires and the ground is STATIC friction. But, even that's incorrect as traction with tires is more complicated than the simple physics principles being taught with F=uF. Note: the most common misconception about friction is that it is dependent upon the surface area. This is incorrect. However, in the case of car tires, the amount of traction *is* dependent on the surface area. Traction is too difficult for high school, or college physics. If it wasn't, then it would be a piece of cake to develop the most ideal tire tread and all the engineers at goodyear, michelin, etc. could retire and go home.

So, do you not teach electricity or magnetism, because maxwell's equations are to difficult for someone who hasn't had 3 dimensional calculus?

Do you not teach friction at all, because it's too difficult? people are better off thinking that everything glides smoothly until they become real engineers and work at goodyear?

Do you never teach statics, because it is a limited case of dynamics?

Do you never teach optics, because we still aren't sure whether light is a wave or a particle?

Is it better to give up on a problem, then to solve a simplified model?

wtf are you talking about?
saying that the problem is a simplified model is incorrect.
The professor shouldn't have used wheels in the first place.
I spent about 2 hours this afternoon discussing the problem with other physics teachers and professors.

Believe it or not, I've dedicated at least 2 hours to this problem to get a correct answer. I do NOT teach my students incorrect physics. I *DO* simplify problems i.e. "ignore air resistance." But I do NOT teach them how to do problems wrong because it's easier to do them that way.

1. Unless the wheel is slipping on the ground, there is ABSOLUTELY NO kinetic friction between the wheel and the ground.
2. The coefficient of static friction is NOT found
3. The coefficient of rolling friction is NOT found either.
Yes, there *IS* a coefficient of rolling friction, but that's not what the problem is finding, especially since the "coefficient of friction" that this problem finds is an order of magnitude or so above the coefficient of rolling friction for a tire.

Hopefully we all realize that the majority of the friction is due to where the wheel attaches to the axle. Ultimately, this force is expressed through static friction with the ground. But, any high school physics student should know that to calculate the coefficient of static friction, you need to find the amount of force to "break loose" the two surfaces and begin to cause slipping between the surfaces. That is not what the force is in this case.

Now, can we calculate a "coefficient of friction" for whatever this 2N force is? Well, first of all, what is a coefficient of friction? It's a constant that we can use to calculate the force of friction between two certain materials, as long as we know the normal force between the two surfaces. (or calculate the normal force from the force of friction.) Having one in this case implies that if we were to double the normal force, we'd double the frictional force. That is NOT the case unless all of the frictional forces were acting parallel to the direction of motion and perpendicular to the normal force. Something that's unfortunately not true.

Think of it this way: Overtighten the nuts on a bicycle hub - that increases the frictional force. Do so until you can barely spin the wheel. Now, attach it to the bike and sit on the bike. If a coefficient of friction applied, increasing the amount of force acting on that hub should greatly increase the frictional force to the point where it's virtually impossible to spin the wheel. Doesn't happen.

Yes, I do teach my students about friction.
I teach them that the coefficient of friction can only be calculated experimentally (unless things have changed in the past few years, I haven't heard of any successful attempts to accurately predict the coefficient of friction other than by experimentally)
I teach my students what kinetic friction is... and how the coefficient of kinetic friction is calculated.
I teach my students what static friction is... and how the coefficient of static friction is calculated.
I teach my students what rolling friction is. I've never taught about the coefficient of rolling friction. I'll probably mention it this year however, due to this problem. I spend quite a bit of time explaining where rolling friction comes from - deformations between the two surfaces as well as some chemical bonding that occurs.

I also show them that in the case of tires on pavement, it's not a simple case of friction. Very few of my students would ever make the error and say that when a car is driving down the road, the type of friction between the wheels and the ground is kinetic friction.