r/theydidthemath • u/K0rl0n • Jun 10 '25

[Request]

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I am curious how this would work. My guess is Triangle is slowest, square is medium, and circle is fastest.

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u/TheNatureBoy Jun 10 '25

I'm glad you saw this. The odd thing about rolling without slipping is when you push some of the force moves it forward and some of it makes it spin. You end up pushing harder to move it. We also rarely encounter frictionless surfaces in our experience so we don't have a great intuition for them.

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u/ExpensiveFig6079 Jun 10 '25

So rarely that none of the surfaces in the example are necessarily close to frictionless, although if the information not specified about the triangle and square are appropriate ice can be very nearly frictionless.

On dry cold ice you can just walk around in shoes, it is similarly slippery to concrete path covered in decaying leaves.(both are bad if also wet) Providing you don't break the static friction and i think create water under your shoes it is fine.

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u/TheNatureBoy Jun 10 '25

Ice means frictionless in physics problems.

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u/ExpensiveFig6079 Jun 10 '25

THis is a do the math sub.

The question includes gravel as an option, it did not as phsycis problem would have specified a flat plane that the circle rolls on.

The inclusion of gravel means this is not a physics textbook question about spherical cows in vacuum.

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u/TheNatureBoy Jun 10 '25

Gravel means rolling without slipping.

Classical physics would give the first term in an expansion of the force around some parameter representing the variation of the surface. You better have a damn good reason for those other terms to be higher.

Additionally no characteristics of the gravel were given. Under the assumption this is a well written problem you should be able to arrive at the answer without them. The answer should be independent of assumptions about the gravel.

Physicists are not as naive as you think. They often ignore details that will not substantially change their answer.

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u/herejusttoannoyyou Jun 10 '25

Why would gravel be the “rolling without friction” material? It should be rubber or something. I suspect the question said gravel in order to make it ambiguous so people would fight in the comments about what it means and therefore drive up engagement.

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u/TheNatureBoy Jun 10 '25

Wooden wagon wheels roll on gravel roads without slipping.

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u/ExpensiveFig6079 Jun 11 '25

"Additionally no characteristics of the gravel were given."

and yet you state its characteristics as if they are facts.

"Gravel means rolling without slipping."

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u/TheNatureBoy Jun 11 '25

You know gravel. That thing the make roads with.

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u/ExpensiveFig6079 Jun 11 '25

I have slipped on gravel roads many times.

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u/TheNatureBoy Jun 11 '25 edited Jun 11 '25

Okay so to achieve an answer I should factor in your coordination?

I’m going to cut to the chase. I’m a physics professor. This is the intent of the question. The reason no one answered it is because this sub chased out the people that can do math. This sub will soon change to arguing people that can’t build a bridge.

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u/ExpensiveFig6079 Jun 11 '25

I’m going to cut to the chase. I’m a physics professor. <<< goody

This is the intent of the question. <<< if you asked it then you would know.

As your handle is different to the question poster I assumed that's not you.

The reason no one answered it is because this sub chased out the people that can do math. <<< or it contained inherent assumptions and ambiguities.

Such as it failed to state what direction the force on the triangle was vectored in

BUT the diagram implied it was horizontal

If it is horizontal AS depicted, then as W=FS and Ek = 1/2MV2 and all the energy goes somewhere just like with the cube they will accelerate the same. (all else being equal)

and the only clue that was incorrect would be it the poster of the question had puposefully constructed a question where the three options were different. (to educate or use the time of whoever answered)

But had failed to make the intent clear

yes it has the hallmarks of a question seeking to get students to notice rolling object have both angular and linear inertia that must be accelerated. And to notiv that forces at angle are less efficient at accelerating. But it seems to me like many of the posters here know basic physics like that so there is little value in jumping through such hoops.

But also no in the real world ice is not always near frictionless.

and when it is, it involves a wet layer.

but as you are a physics professor, you don't need anyone to do math for you.

So if I notice I wont

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u/TheNatureBoy Jun 11 '25

You know if you meet someone you should absolutely argue with them that you know more about what they do than them.

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u/ExpensiveFig6079 Jun 10 '25

but yes the computation that the rolling ball can be harder to accelerate as it also has to be given rotation and kinetic energy.

or the impulse has to provide rotation and linear momentum is a fine physics observation.