Derive the Volume of a Sphere using Calculus

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Homework Help Overview

The discussion revolves around deriving the volume of a sphere using calculus, specifically through the method of integrating a circle at an angle θ (Theta). Participants are exploring the correct approach to set up the integration and are sharing their attempts and reasoning.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants are discussing the integration of a circle and the appropriate thickness of the disks used in the calculation. There are questions about the use of arc length versus vertical thickness in the integration process.

Discussion Status

Some participants have provided guidance on correcting the thickness used in the integration, suggesting that the vertical thickness should be expressed in terms of the angle θ. There is an ongoing exploration of the geometrical implications of these adjustments, with some participants seeking further clarification on the reasoning behind the corrections.

Contextual Notes

Participants have noted issues with access to shared resources and the importance of clearly presenting attempts in the discussion. There is a focus on understanding the relationship between the angle and the vertical distance in the context of the problem.

Shivam
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Homework Statement


Derive the volume of sphere using Calculus. i saw videos on this topic on youtube, but i want to do it by the method of integrating a circle at angle θ (Theta) . i am posting a photo where i explained every thing i did but i couldn't know what i am doing wrong.

Homework Equations


Integrating a circle of radius r at angle θ (Theta)

The Attempt at a Solution


https://drive.google.com/open?id=1Va32w9eCJE_2nH4VIPqG8bFHRzcKuvIF[/B]
IMG_20180823_000643.jpg
 

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Shivam said:
i am posting a photo where i explained every thing i did but i couldn't know what i am doing wrong.
No you did not. You posted a link to your google drive, which other people do not have access to.

Also, you should make the effort of typing out your attempt if you want people to help you.
 
Your error is using ##ds## for the thickness of your disk. It should be ##dy=R\cos\theta d\theta##.
 
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Shivam said:

Homework Statement


Derive the volume of sphere using Calculus. i saw videos on this topic on youtube, but i want to do it by the method of integrating a circle at angle θ (Theta) . i am posting a photo where i explained every thing i did but i couldn't know what i am doing wrong.

Homework Equations


Integrating a circle of radius r at angle θ (Theta)

The Attempt at a Solution


https://drive.google.com/open?id=1Va32w9eCJE_2nH4VIPqG8bFHRzcKuvIF[/B]

I can see your attempted solution. The problem is with ##ds=Rd\theta##. That is arc length along the surface of the sphere. It is not the same as the thickness of your circular section. Can you correct it?
 
Shivam said:

Homework Statement


Derive the volume of sphere using Calculus. i saw videos on this topic on youtube, but i want to do it by the method of integrating a circle at angle θ (Theta) . i am posting a photo where i explained every thing i did but i couldn't know what i am doing wrong.

Homework Equations


Integrating a circle of radius r at angle θ (Theta)

The Attempt at a Solution


https://drive.google.com/open?id=1Va32w9eCJE_2nH4VIPqG8bFHRzcKuvIF[/B]
LCKurtz said:
Your error is using ##ds## for the thickness of your disk. It should be ##dy=R\cos\theta d\theta##.
I got the correct answer by using the correcct thickness you gave me , but i still don't know how did you get that, can you explain please.
 
Shivam said:
I got the correct answer by using the correcct thickness you gave me , but i still don't know how did you get that, can you explain please.

The thickness is the vertical thickness of the slice. The arc length you have is not vertical, it's tangent to the sphere. So it makes a varying angle with the vertical as you move up the sphere. Use trig to turn that into a vertical distance.
 
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LCKurtz said:
Your error is using ##ds## for the thickness of your disk. It should be ##dy=R\cos\theta d\theta##.

Can you show me how did you get that, i thought all day but i can't get it.
 
Shivam said:
Can you show me how did you get that, i thought all day but i can't get it.
Well, you have ##y=R \sin(\theta)##, so ##\frac{dy}{d\theta}=R \cos(\theta)##, so rearranging the differentials gives ##dy=R \cos(\theta)d\theta##. Also, try to understand this result from a geometrical point of view (i.e. how does a small increase in the angle influece the increase in y?)
 
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