EK Physics Lecture 2 Problem 41

[supras2kracer]

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The question shows a car it's just sitting there and has an arrow (on the rear wheel) pointing left and an arrow pointing right labeled A and B respectively. The front of the car is facing to the right.

If the rear wheels of the truck pictured below drive the truck forward then the frictional force on the rear tires due to the road is:

A. Kinetic and in the direction of A (left)
B. Kinetic and in the direction of B (right)
C. Static and in the direction of A
D. Static and in the direction of B

So the correct answer is D meaning it's static friction. I chose answer B because if the tire's moving it's kinetic friction right?

The answer explanation says: If we look at the point of the tires that make contact with the road, that point does not move relative to the road or else the tires would spin in place.

It's not really making sense to me. If a block slides along a surface it's kinetic friction. Static is only when you are getting up the force to get the object to move.

 

[Emerald84]

Remember Friction is a force opposing the direction of motion

We can make 2 assumptions about this problem:
(1) we are dealing with static friction. Their argument is that because the question stem does not state that the tires are spinning in place, the point on the tire of interest is always in the same position relative to the road.

(2) The direction the tires hit the ground is opposite to the direction it propels the car forward.
The tires are sliding on the road in the direction of A (backward)
I think this is a somewhat ambiguous question (Thanks EK!). There shouldn't be ambiguities like this on test day.

 

[supras2kracer]

There seems to be a ton of these super ambiguous questions in the EK books. Do they do that on purpose or is it just that they're poorly written?

EK gets good reviews though so I'm guessing it's the former.

Thanks for the help though!

 

[Emerald84]

I think the GC and Physics are poorly written. The Bio is great, although there is one ridiculously nonsense passage on the kidney I still can't figure out! Thought their verbal prep was phenomenal and probably because the questions are, yep AMBIGUOUS!

 

[Emerald84]

It's good prep though. I think they write questions to cover essential concepts🙂

 

[godcyning]

Tires spinning in place is one way to look at kinetic friction, but it's probably easier to imagine a skidding car as an example of kinetic friction. Like if you are braking hard, the wheels aren't turning, but you're still sliding along (like you are on ice or something). That's no different from a sliding block with 4 rubber patches for legs.

When the wheels are rolling, no patch of rubber in contact with the road will ever slide on the road, and so it's static friction that governs the interaction.

And actually, the answer being D means that the force on the car is forward. This makes sense in the sense that the force on the ground is backward, and Newtons 2nd law and stuff. The thing is, the point on the wheel in contact with the ground never moves backward relative to the ground; it moves forward.

http://en.wikipedia.org/wiki/Cycloid

 

[MLT2MT2DO]

My problem with this question is that it didn't state "the car is at a stopped position" I assumed the car was already moving this is why I chose "B".

If nothing else the poorly written wording of these questions has helped me pay really close attention to ALL questions.

 

[godcyning]

Even if the car is rolling, it's still static friction. If it helps, this is what "traction" is. The patch of wheel touching the road never moves relative to the road during the time it's in contact with the road. If it does, it means you're skidding.

 

[MLT2MT2DO]

Gotcha, thanks godc. I guess this is what the answer meant by "relative"...if I visualize it I guess it makes sense. My brain went straight from "car is moving=kinetic friction" instead of thinking about how the car was actually moving.

 

[supras2kracer]

Gotcha, thanks godc. I guess this is what the answer meant by "relative"...if I visualize it I guess it makes sense. My brain went straight from "car is moving=kinetic friction" instead of thinking about how the car was actually moving.

Yeah it's tricky, makes you really think before you answer these questions.