Basic science question...

Post by **tnf** » Thu Oct 27, 2005 12:44 am

so what are you asking?

Guest · Post by **Guest** » Thu Oct 27, 2005 12:45 am

A ring, a full sphere, and a coin are rolling without slipping down a smooth incline. If they start at the same time, which one would be the first one to reach the bottom?

Post by **tnf** » Thu Oct 27, 2005 12:48 am

smooth incline with enough friction to allow rolling without slipping.

Guest · Post by **Guest** » Thu Oct 27, 2005 12:48 am

yes
they all have the same mass and radius.

Post by **tnf** » Thu Oct 27, 2005 12:48 am

do you have the coefficients of rolling friction for each?

Guest · Post by **Guest** » Thu Oct 27, 2005 12:48 am

Post by **werldhed** » Thu Oct 27, 2005 12:49 am

If there is friction, which I think we've established there IS in this problem, and all objects have the same circumfrence...
I believe it will be the sphere, which has the smallest surface area in contact with the slope.

edit: although I think i remember something about the empty middle of a ring playing a roll, but I can't fathom what.

Guest · Post by **Guest** » Thu Oct 27, 2005 12:49 am

werldhed wrote:If there is friction, which I think we've established there IS in this problem, and all objects have the same circumfrence...
I believe it will be the sphere, which has the smallest surface area in contact with the slope.

you're right but your explanation is completely off.

Post by **R00k** » Thu Oct 27, 2005 12:50 am

Discounting air resistance and everything?

Guest · Post by **Guest** » Thu Oct 27, 2005 12:51 am

Guest · Post by **Guest** » Thu Oct 27, 2005 12:54 am

lol I remember when I used to fuck people up asking questions like this... those were the days...

Post by **shiznit** » Thu Oct 27, 2005 12:55 am

He will reach the speed of light and fall of the train, jeez.

Post by **werldhed** » Thu Oct 27, 2005 12:55 am

ToxicBug wrote:
werldhed wrote:If there is friction, which I think we've established there IS in this problem, and all objects have the same circumfrence...
I believe it will be the sphere, which has the smallest surface area in contact with the slope.
you're right but your explanation is completely off.

Damn. I was sure that was the reason. Meh, the ol' brain isn't in physics mode atm. :icon26:

Guest · Post by **Guest** » Thu Oct 27, 2005 12:56 am

Anyone else wanna try to explain this?

Post by **[xeno]Julios** » Thu Oct 27, 2005 1:00 am

ToxicBug wrote:
werldhed wrote:If there is friction, which I think we've established there IS in this problem, and all objects have the same circumfrence...
I believe it will be the sphere, which has the smallest surface area in contact with the slope.
you're right but your explanation is completely off.

Here's a question, if you have a perfectly spherical object resting on a perfectly flat surface, how much of the sphere is in contact with the surface?

btw i like werldhed's explanation - assuming they're all the same material and mass, then the sphere has least amount of surface area in contact with slope (although more pressure on this point since less area but same force).

Guest · Post by **Guest** » Thu Oct 27, 2005 1:02 am

[xeno]Julios wrote:
ToxicBug wrote:
werldhed wrote:If there is friction, which I think we've established there IS in this problem, and all objects have the same circumfrence...
I believe it will be the sphere, which has the smallest surface area in contact with the slope.
you're right but your explanation is completely off.
Here's a question, if you have a perfectly spherical object resting on a perfectly flat surface, how much of the sphere is in contact with the surface?

btw i like werldhed's explanation - assuming they're all the same material and mass, then the sphere has least amount of surface area in contact with slope (although more pressure on this point since less area but same force).

No, just stop thinking about friction!

Post by **mrd** » Thu Oct 27, 2005 1:02 am

What happened to the original question? lol.

I've read that when things are moving the speed of light and (big maybe here) close to the speed of light, whatever they are "riding" on plays no role in their overall speed relative to the viewer.

Example. Shine a flashlight from some arbitrary point on the ground. The viewer will see the light go at 300,000km/s. But, if you tape a flashlight to a train, airplane, spacecraft, whatever, and then measure the speed of the light, it's still going 300,000km/s. I think I read this in a book called the dancing Wu-Li masters. Now, this either reinforces the fact that you can't exceed the speed of light, OR it says that the speed of light can't be "boosted", per se, by other objects. Maybe it supports both. Anyway, my guess is that since these 2 things, the train and the guy, are both moving pretty damn close to the speed of light, I'm just gunna say they are both going (3/4)c relative to the lamp. The train doesn't boost the guy's speed at all. If the dude is running the opposite direction as the train then it gets kinda tricky. By my definition he should still go (3/4)c relative to the lamp, but maybe he actually does stop? Hard to say.. interesting question.

Not sure how to answer your question TB. I would've thought the sphere for the same reason given above.

Post by **werldhed** » Thu Oct 27, 2005 1:05 am

(@ julios)
Heh..I was pondering that same thing. Perhaps it would be reduced all the way to one sphere atom/molecule touching one slope atom/molecule.
I guess in the theoretically ideal world, no part of the sphere would touch. It would simply come down to repelling atomic forces between the tiny point of the ball and slope. :icon26:

:semantics:

Post by **[xeno]Julios** » Thu Oct 27, 2005 1:08 am

ToxicBug wrote:
No, just stop thinking about friction!

if friction isn't a factor (i.e. surface, air, heat, sound), then the only factor I can think of that would affect velocity is gravity.

unless the mass distribution of the sphere somehow plays a role...

Post by **tnf** » Thu Oct 27, 2005 1:09 am

exactly - no friction, then gravity is all we care about.
so i am guessing we are all missing something in his little questino here.

Post by **werldhed** » Thu Oct 27, 2005 1:12 am

Well, what else sets the sphere apart from the rest? It's three dimensional, so perhaps that plays a role in how straight it rolls?
That doesn't seem correct at all.

Post by **mrd** » Thu Oct 27, 2005 1:15 am

Coins and rings are 3D. I'm guessing it has to do with what you mentioned up there. It only has 1 atom in contact with the surface it's rolling on, but the ring has probably several hundreds maybe thousands of atoms in contact with the surface, and the coin probably has around the same number. Obviously it depends on the thickness of the coin. If there is no friction at all of any kind, then all 3 are gunna roll perfectly straight...

Post by **[xeno]Julios** » Thu Oct 27, 2005 1:15 am

werldhed wrote:(@ julios)
Heh..I was pondering that same thing. Perhaps it would be reduced all the way to one sphere atom/molecule touching one slope atom/molecule.
I guess in the theoretically ideal world, no part of the sphere would touch. It would simply come down to repelling atomic forces between the tiny point of the ball and slope. :icon26:

:semantics:

well if it was a mathematical abstract, so not even atomic, i guess the answer would be something like 1/infinity.

Post by **mrd** » Thu Oct 27, 2005 1:18 am

I know you said don't think about friction, but if the sphere has the smallest contact area, then less force is needed to overcome the stopping power of the friction, right? Gravity will break the friction barrier on the sphere first, so the sphere will begin accelerating a little bit before the other two.

Post by **werldhed** » Thu Oct 27, 2005 1:18 am

I just meant 3-D in terms of basic shape. As in, it can roll properly on any point. A ring or coin on the other hand can only roll in the plane of the circumference. I'm thinking of when you roll a coin and it abruptly changes direction.
But like I said, that's really grasping at straws and probably isn't the real explanation.