# Area of a circle and pi and generally area

1. Dec 17, 2011

So I always wondered why you multiply by pi when you're finding an area of a circle, for a rectangle you multiply by length and width, I guess that makes sense...

How I see multiplying a length and width is if you have a length of 5 cm and a width of 4 cm, I imagine you just stack 4, 5 cm sticks on top of each other and you get the area is this how it works?

Anyway I have trouble memorizing formulas for area of a circles and cylinders and such and I am sure I would be better at recalling them if I knew why they are... what they are.

A = (pi)(r)^2

What's with pi?

2. Dec 17, 2011

### fashizzle

Also interested in this as well, I've always thought that circumference of a full circle arises from 2*pi*r because radius multiplied by central angle gives you the arc length, and by taking the integral of this with respect to r, you get pi*r^2.

and if you integrate a circle of radius R using polar coordinates, you can change the value of theta to give you the area of swathes within the circle, so pi will change on the basis of whether or not you're dealing with a full circle.

Last edited: Dec 17, 2011
3. Dec 17, 2011

### gsal

4. Dec 17, 2011

5. Dec 21, 2011

### ArcanaNoir

Very nice website. I loved the area explanation.

6. Dec 21, 2011

### all-black

7. Dec 21, 2011

### dextercioby

I was famous for saying this here on PF years ago, so I say it again:

Strictly speaking, the circle has no area. If it did, it would be 0.

8. Dec 21, 2011

### Deveno

well, ok. we should be saying "the area enclosed by a circle", or the area of a circular region (or the area of a regular disc). do you want fries with that?

9. Dec 22, 2011

### HallsofIvy

Or "area of the disk".

10. Dec 22, 2011

### Pengwuino

11. Dec 22, 2011

### I like Serena

It's because pie is defined as the ratio of your upper lip and the length of the curve around your mouth and eyes.

12. Dec 22, 2011

### Deveno

i honestly don't know why, i didn't design this universe.

13. Dec 22, 2011

### Redbelly98

Staff Emeritus
I realize you're speaking tongue-in-cheek, but the real question would be "Why is the circumference of a circle proportional to its diameter?" π is simply the proportionality constant for that relation, by definition.

14. Dec 22, 2011

### Pengwuino

Because the units work out.

I love physics.

15. Dec 22, 2011

### DivisionByZro

To each their own I suppose.

16. Dec 23, 2011

### sankalpmittal

Pi is defined as the ratio of the circumference of the circle to the diameter of that circle originally.

Do this experiment : Draw circles with compass of different radius and measure their circumference by using a thread or a string : Enclose circle boundary with thread and then measure that part of thread with a ruler.

You will find that : C1/D1=C2/D=....=Cn/Dn = pi

So C/D = pi
or C=2*pi*R since D=2R

Here are proofs of area of circle : https://www.physicsforums.com/showthread.php?t=529014

gsal , that site is also nice which you gave !

17. Dec 24, 2011

### Deveno

i have a problem with this, which is:

what is "circumference"? a closely related question is: "what is diameter"?

if you answer something like: "diameter is the length of the longest possible line segment across the circle", or perhaps "the length of any line segment from a point on the circle, through the center of a circle, and terminating at a point on the opposite side of the circle", i would still want to know, what is this "length" thing we are talking about. how do we tell when two lengths are the same, and how do we tell which of two unequal lengths is longer?

if you answer, "we measure them, and compare", i again ask, "how is it we measure things"?

what KINDS of objects qualify as "measurements of lengths", and how do we know that this is a "proper" description (logically consistent)?

now, this is kind of unfair, i actually know the answers to these questions. but i would humbly submit, that when a 6-th grader, for example, is given the definition:

"pi is the ratio of a circle's circumference to it's diameter"

there are several "hidden assumptions"

1) circumference can be unambiguously measured
2) diameter can be unambiguously measured
3) circumference and diameter are "comparable" (the same kind of number), and we may form their ratio (suggests a notion of division)
4) this ratio is always the same, regardless of the length of the diameter (pi is constant)

all of these statements are provable, but some of them are subtler than others.

18. Dec 24, 2011

### logmode

You have me thinking now, and I hope it’s OK to ask a question here. I am not sure, but is this true 2 ∏ = circumference?

19. Dec 24, 2011

### BloodyFrozen

$$C=2\pi r ~~ or ~~C=\pi D$$

20. Dec 24, 2011

### logmode

Am I correct, ∏ is 3.14 radians. If so, 360 degrees is a circle, which is the circumference. 360 degrees converted to radians is 360 x ∏/180 = 2∏, Where am I thinking wrong?