Finding Center of Mass of Disc with Hole

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Homework Help Overview

The problem involves finding the center of mass of a uniform circular disc with a hole, defined by specific mathematical inequalities. The equations provided describe the boundaries of the disc and the hole within it.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the area of the figure and suggest sketching the shape to better understand the problem. Questions are raised about the locations of the centers of mass for both the outer circle and the hole, as well as their respective areas.

Discussion Status

There is a mix of interpretations regarding the wording of the question, with some participants expressing confusion over the definitions provided by the equations. Others assert that the equations define a precise shape, leading to a debate about the nature of the center of mass. Some participants have indicated they believe they have solved the problem, while others are still grappling with its complexities.

Contextual Notes

Participants note that the question may be poorly worded, leading to misunderstandings about the boundaries and definitions of the shapes involved. There is also mention of varying difficulty levels in relation to other problems in the assignment.

Aliasa
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Homework Statement



Find the center of mass of a uniform sheet in the form of a circular disc
with a hole bounded by the equations x^2 + y^2 ≤ 1 and (x - 1/2)^2 + y^2 ≥ (1/16).

Homework Equations





The Attempt at a Solution

 
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Can you find the area of the figure?
 
That is all the info given.
 
Aliasa said:
That is all the info given.

Hi Aliasa. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

Using your knowledge of mathematics, sketch the shape. Then post it here.
 
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SteamKing said:
Can you find the area of the figure?

Aliasa said:
That is all the info given.
Yes, but can you find the area? This is a disk with a hole in it. It would be a good idea to at least draw a picture of the "hole bounded by the equations x^2 + y^2 ≤ 1 and (x - 1/2)^2 + y^2 ≥ (1/16)." Those boundaries are circles. Can you graph the circles?
 
Where is the center of mass of the outer circle located?
Where is the "center of mass" of the hole located?
What is the area of the outer circle?
What is the area of the hole?
 
It turns out the question is horribly worded. The sheet is bounded by the former equation, while hole by the latter -_-. Since the equations are inequality, by taking density into account, I feel the center of mass should be a range too. The hole can have a max radius of .5, sheet, 1. Hole can have a min radius of 1/4, when the sheet can have a minimum of 3/4.
 
Aliasa said:
It turns out the question is horribly worded. The sheet is bounded by the former equation, while hole by the latter -_-. Since the equations are inequality, by taking density into account, I feel the center of mass should be a range too. The hole can have a max radius of .5, sheet, 1. Hole can have a min radius of 1/4, when the sheet can have a minimum of 3/4.
You are misunderstanding the question. The metal template is very precisely defined by the two equations; there is no range of possible shapes and sizes. The centre of mass is an exact point. The two equations together define the metal surface, the hole is where there is metal missing.

Have you sketched the shape yet?
 
If that is the case then the question is trivial. I have solved it if those equations represent what you say. But doing that only takes me 5 minutes or less, which I can't understand. All the other questions on the assignment take in excess of 3 hours.
 
  • #10
The answer is (-1/30,0) btw.
 
  • #11
Aliasa said:
If that is the case then the question is trivial. I have solved it if those equations represent what you say. But doing that only takes me 5 minutes or less, which I can't understand. All the other questions on the assignment take in excess of 3 hours.

That's pretty astounding. Can you provide an example of one of these 3-hour problems?
 
  • #12
Aliasa said:
The answer is (-1/30,0) btw.
Yes. That's correct. Nice job.

Chet
 
  • #13
This is one of those questions. Others seem easy now that I have done them. Yet to start on this one. The astounding thing is there's no mention of 'coefficient of restitution' in lecture notes.
 

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