# I don't get why this troll physics is wrong.

#### Ryumast3r

To put the above posts into "time to teach a 6th grader why pi = 3.14 and not 4" speak:

It doesn't equal 4 because, no matter how many times you cut the squares smaller and smaller, there will always be area of the squares that is not touching the circle.

Here's a little picture I drew in paint to help visualize:

In the above picture, there is a section of it that is red. You'll notice that this area exists on the other pictures as well (but it's blue, I believe). This area, no matter how many times you cut away at the square, still exists to an extent. At some point, it's a very small extent, but the upper-right corner of the square will never touch the circle, there will always exist 2 more upper-right corners for every single upper-right corner that you cut away.

#### Eldar

To put the above posts into "time to teach a 6th grader why pi = 3.14 and not 4" speak:

It doesn't equal 4 because, no matter how many times you cut the squares smaller and smaller, there will always be area of the squares that is not touching the circle.

Here's a little picture I drew in paint to help visualize:

In the above picture, there is a section of it that is red. You'll notice that this area exists on the other pictures as well (but it's blue, I believe). This area, no matter how many times you cut away at the square, still exists to an extent. At some point, it's a very small extent, but the upper-right corner of the square will never touch the circle, there will always exist 2 more upper-right corners for every single upper-right corner that you cut away.
I agree with you completely, but the diameter is specified as being 1 and not 4, the perimeter is 4 :P I'm sure you realised this but just had a quick lapse and mistyped.

#### Unrest

It doesn't equal 4 because, no matter how many times you cut the squares smaller and smaller, there will always be area of the squares that is not touching the circle.
I don't think that counts as a reason. The area keeps getting smaller, and it approaches something, presumably it approaches the area of the circle.

The pi=4 comes from the perimeter, not the area. The perimeter seems to remain 4 no matter how many squares (or rectangles, which is what the original picture shows) you cut away.

But how's this for a simple proof:

pi < 4 (already been http://en.wikipedia.org/wiki/Proof_that_22/7_exceeds_%CF%80" [Broken])
therefore pi doesn't equal 4

Last edited by a moderator:

#### Ryumast3r

I don't think that counts as a reason. The area keeps getting smaller, and it approaches something, presumably it approaches the area of the circle.

The pi=4 comes from the perimeter, not the area. The perimeter seems to remain 4 no matter how many squares (or rectangles, which is what the original picture shows) you cut away.

But how's this for a simple proof:

pi < 4 (already been http://en.wikipedia.org/wiki/Proof_that_22/7_exceeds_%CF%80" [Broken])
therefore pi doesn't equal 4
See below as well, What I meant is that there is area unaccounted for (though it becomes infinitesimally small), which also means that there is a section of the perimeter that does not touch the circle.

Put more simply what I was trying to say: Every time you cut a corner, 2 more appear that do not touch the circle, ergo the perimeters of both objects will never touch at every point.

I agree with you completely, but the diameter is specified as being 1 and not 4, the perimeter is 4 :P I'm sure you realised this but just had a quick lapse and mistyped.
Yeah, that's what I meant. I had a test and... well... you know what happens after tests. :P

Last edited by a moderator:

#### Femme_physics

Gold Member
haha That's a good one, I'll have to remember that next time I want to mess with someone.

I think of it this way. Since "removing" the corners like that doesn't change the perimeter at all, it will fail to converge on the perimeter of a circle. So it's rather unlike, say, increasing the number of sides of a polygon inside the circle. That one converges on the real perimeter. His example does not.

The fact that the perimeter never changes as he removes the corners is pretty much proof that the technique will never work. It needs to converge to a smooth curve (to it's limit) that equals the perimeter of the circle, which this will never do (being jagged).

Not sure if that's 100% clear, someone else can probably put it better.
Great explanation, actually. *thumbs up*

However, I will definitely use it to mess with ppl! :)

Edit: Wait, I'm confused. If this fails to prove that an area is really the sum of infinitely small rectangles, why should I trust taking the integral to give me a correct approximation?

#### D H

Staff Emeritus
Edit: Wait, I'm confused. If this fails to prove that an area is really the sum of infinitely small rectangles, why should I trust taking the integral to give me a correct approximation?
The jagged curve *does* converge to the circle in almost every1 sense of the word "converge". The area *does* converge to that of a circle. The distance between any point on the jagged curve and the circle *does* converge to zero.

Think of the upper (or lower) half of the jagged curve / circle as a function of x. While the jagged semi-curve converges uniformly to the semi-circle, the derivatives of those curves are miles apart. In fact, the jagged semi-curve is nowhere differentiable in the limit N → ∞. Just because two functions converge to one another does not mean that their derivatives, or their lengths, do so.

1Addendum: Well not quite every sense. The jagged curve does not converge smoothly to the circle. The jagged curve is nowhere differentiable in the limit N → ∞ while the circle (upper semicircle) is infinitely differentiable almost everywhere.

Last edited:

#### hillzagold

It's the idea that with every step, the difference between areas of the jagged figure and the circle is constantly shrinking, until the difference is zero, and also that the jagged figure and the circle are the exact same shape. Because the jagged shape always has a perimeter of 4, the circumference of the circle is also 4, and not 3.14.

Without getting into differentials and tangents and other topics more advanced than the thought that went into the trolling, I prefer the simpler answer that the jagged shape never does become a circle. In math it's called an infinitesimal, where 1/$$\infty$$ (or any other mind-bogglingly small number) is not zero. The difference between the jagged shape and the circle keeps shrinking, and you can choose to disagree with me and believe whether or not the difference becomes zero. I'm no math professor, so I won't claim to be more than pretty sure about this.

#### disregardthat

Without getting into differentials and tangents and other topics more advanced than the thought that went into the trolling, I prefer the simpler answer that the jagged shape never does become a circle.
Approximation by polygons will also never become the circle, but still the perimeter of the polygons will approach the circle. The main problem here is that one will not be certain that length is preserved under the limit operation unless the (almost everywhere) derivatives of the sequence curves (such as the jagged curves in our example) has a (almost everywhere) continuous (maybe simply integrable, not sure) limit. If this condition is satisfied, and it is for polygons, we will have length preservation. And this is why we can trust the approximation by polygons, as someone commented on earlier.

#### Unrest

The difference between the jagged shape and the circle keeps shrinking
I think you're mixing up area with perimeter by using the vague word "shape". You can't say "the difference between these two shapes is less than the difference between those other two shapes". Difference implies subtraction, and subtraction isn't defined on "shapes". This confusion is what the original picture plays on. The area of the jagged shape does approach the area of the circle, but the length of the perimeter doesn't even begin to.

Here's an even more extreme example of how our intuition about the relationship between area and perimeter length doesn't work: http://en.wikipedia.org/wiki/Koch_snowflake" [Broken]

Last edited by a moderator:

#### guss

How do you know it is true that the shape never becomes a circle?

#### olivermsun

Well, any iteration of the jaggy shape is going to have lots of sharp corners on it, so it pretty clearly does not satisfy the definition of a circle.

#### DaveC426913

Gold Member
How do you know it is true that the shape never becomes a circle?
The distance from corner to opposite corner will always be x+y, regardless of how short you make x and y. Even teeny tiny jags too small to see will still never make x+y equal to root(x2 +y2).

Try it. Make x=y = .0000000000000000000000000000000000000001
Now make x=y=.0000000000000000000000000000000000000000000000000000000000000000000000000000000000001

Not only will they will never reach it, they never even start towards it.

Another way of looking at it:

If the jags are the size of an atom, and the circle is 10 trillion atoms high, then each jag still contributes 1 atom's-worth of y, times 10 trillion equals the side of a square. Not a circle. This is true no matter how small you choose to go.

Thus, no matter how small you make the jags, they're still jags, and they still add to the perimeter of a square, not a circle.

Last edited:

#### guss

The distance from corner to opposite corner will always be x+y, regardless of how short you make x and y. Even teeny tiny jags too small to see will still never make x+y equal to root(x2 +y2).

Try it. Make x=y = .0000000000000000000000000000000000000001
Now make x=y=.0000000000000000000000000000000000000000000000000000000000000000000000000000000000001

Not only will they will never reach it, they never even start towards it.

Another way of looking at it:

If the jags are the size of an atom, and the circle is 10 trillion atoms high, then each jag still contributes 1 atom's-worth of y, times 10 trillion equals the side of a square. Not a circle. This is true no matter how small you choose to go.

Thus, no matter how small you make the jags, they're still jags, and they still add to the perimeter of a square, not a circle.
Sorry, I'm not following. What corner to what corner? What are you saying x and y are?

#### DaveC426913

Gold Member
Sorry, I'm not following. What corner to what corner? What are you saying x and y are?
Simply put, when the circle is enclosed in a square, the square's perimeter is going to be 2x+2y, where x and y are both diameters of the circle. As you add more jags, the jags get smaller, but the perimeter does not decrease - it is still 2x+2y (A straight vertical line from top of square/circle to bottom of square/circle, no matter how much you subdivide it, will always add up to y).

No matter how small the jags get, the perimeter of the shape remains 2x+2y. Seriously, the jags could be microscopic, and yet the perimeter never wavers from 2x+2y.

#### Attachments

• 7.1 KB Views: 310
Last edited:

#### VS63

As the steps get smaller and smaller, each one can be defined as a right angled triangle, and therefore calculated out using Pythagoris Theorum. At some point the triangles will become so small that the length of the hypotenuse will almost exactly equal the length of the arc of the perimeter of the circle at that point. In fact, being a straight line, the hypotenuse will actually be minutely shorter than the arc.

It can readily be seen that at the 45degree position the corresponding triangle will have a horizontal and vertical size of 1 unit, and a total length of 2 units, whatever size that unit might be. Using Pythagoris, it can also be seen that the Hypotenuse of that triangle will have a length of sqrt(2) or approx 1.4142 units, which is obviously less than 2. In all cases of any triangles defining the perimeter of the circle, the hypotenuse will be less than the sum of the horizontal and vertical sides. Once all the hypotenuse' are added together, their sum should approximately equal PI, although as noted earlier, because the hypotenuse' are all contained within the arc of the circle, they will actually sum to slightly less than PI.

#### Samuelb88

The reason why the circumference of the square doesn't approach $$\pi$$ as we change the shape of the square to that of the circle is that the edges doesn't touch the circle tangentially, as mentioned in earlier posts.

Let us zoom in sufficiently close to a section of the circle such that the section of the circle looks like a straight line. No matter how many times we repeat the process of changing the shape of the square to that of the circle, we will always be able to zoom in sufficiently close such that:

http://imageshack.us/photo/my-images/847/zoom.jpg/

Now if the edges touched the circle tangentially and we zoomed in sufficiently close to a given section, it would look like the section of the circle and the tangential intersection of the approximating curve would be "on top" of each other.

*Credit of the explanation should be given to my classmate, whose name I have forgotten.

** My picture isn't showing up on my screen, so for those who cannot see it as well, the picture is that of a straight inclined line and two lines, a vertical line that intersects the inclined line at 45 degrees and a horizontal line that intersects the inclined line at 45 degrees. It is just a simple picture of the circle and on of the fringes of the square whose perimeter has been changed to fit that of the circle zoomed in sufficiently small.

Last edited:

Wow mathematics knowledge here is terrible. There is a whole field of mathematics devoted to this. It is called the theory of measures. And this problem is not trivial at all.

1) The rectangle curve sequence does converge to the circle (for every epsilon there is a delta...)
2) The length of the rectangle curve is always 4
3) The "Manhattan metric" has little to do with it, as I can produce sequences of curves that have different lengths than the limiting length in that metric too.
4) "Tangentialness" and differentiability have no direct meaning for the result either (I could build a wiggling sine wave approach to the circle and get the same result)

This means that arc length must be defined in a non intuitive way in two dimensions.
The reason for this is that in to dimensions there is always enough space for a two dimensional curve to have infinitely many wiggles of non zero length. This is studied further in fractal dimensions. A common way is the integral over the tangent vectors of the curve using the Lebesgue measure, but the Lebesgue measure is fairly abstract.

But I guess this doesn't really help. The main message is, that you have to know exactly what you are doing when dealing with infinities. If you really want some brain damage look up the Banach Tarski paradox. It has been shown that you can make one sphere into two identical ones by taking it apart and putting it back together.

Why is such a simple problem being made to look so complicated? It has already been pointed out many times here that no matter how many right angled sections are used the total perimeter of the surrounding shape remains constant (see for example Dave's posts above)This result is simple and intuitive and all that is needed to see it is to spend a few minutes with pencil and paper.

#### granpa

well it shows that the length of the hypotenuse doesnt simply follow
from the length of the other 2 sides.

That may be so (I'm not sure what hypotenuse you are referring to) but the problem as originally set does not require any reference to hypotenuses.

#### disregardthat

4) "Tangentialness" and differentiability have no direct meaning for the result either (I could build a wiggling sine wave approach to the circle and get the same result)
Uniform convergence (almost everywhere) of the differentiated curves have everything to do with this. This is the essential property that fails which makes lim(length) =/= length(lim). This has been stated several times in this thread. That the non-differentiability in the corners doesn't matter has also been stted.

Having quickly scanned through the latest posts on this thread it appears to me that an emphasis has been placed on area calculation which is a different problem to that originally posted which implied that pi=4 comes from perimeter calculation.

#### DarthPickley

here is an analagous situation: take the function given by the limit as a --> 0+ of the function f(x) = a*sin(x/a).
Clearly, this converges to the function F(x) = 0.

However, suppose that this is like physics, so you are going to talk about the energy of the wave (no quantum stuff here though! It's not a photon or any duality with a particle)
So, you say that the energy is proportional to frequency and to amplitude, so E = h*f*A, which in this case gives E = h*(1/a)*(a) = h. And suppose that the constant h = 1. then Energy E=1 is constant for all a>0.

However, the energy in the simple function that the function converges to as a approaches 0, which is F(x) = 0, will just be 0. F'(x) is always 0 as well.

So, just because the points in the curve approach 0, other properties of the curve, such as its energy (aka the maximum slope), are not also converged to in the model.

Here is another example, this time more relevant, because it uses perimeter of a circle:
you have the polar curve defined by r = 1 + sin(a*Θ)/a . As a --> ∞, r --> 1, and the curve converges to a circle.
However, it should be easy to see that for any finite, positive integer a, taking the path defined by r = 1 + sin(a*Θ)/a will be longer (and more windy) than taking the path r = 1 (a circle). (also it will be longer by a factor of about 1.2)
But the limit of these perimeters is not equal to the perimeter of the circle.

For something like that to work, I think it has to have the lines that are approximating it to converge towards being the same location AND direction as the curve is.
One way you could do this is you could take the original method of having lines that only go horizontal and vertical, and then just take the convex hull of that curve (basically wrap a string tightly around the rectangularized curve of perimeter 4, and find the length of that string)

#### RationalPi

Take a 1 cm diameter ring and a 3 meter length of fishing line.

Wrap fishing line tightly around the ring such that the ring is completely covered in fishing line.

The circumference of the pipe is equal to 3 meters.

Problem?

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving