In summary, the conversation discusses a problem involving a quarter disc with a given areal density and radius. The conversation includes attempts at solving the problem using different methods, with the final solution being found through a mistake in the area element for polar coordinates. The correct solution is 8.55 g.
  • #1
Grace Pseudonym
4
0

Homework Statement



A quarter disc of radius 3 cm lies in the first quadrant. The areal density is (1.2 g/cm3)x + (0.7 g/cm3)y. Determine the mass of this object.

Homework Equations



1_1.png


The Attempt at a Solution



For my bounds:
x: 0 to 3
y: 0 to Sqrt[3 - x^2]

2_1.png


When I took this integral I got 2.07846 + 5.87878 I, which is obviously an imaginary number. Any idea how to get a real number? I'm not sure where I'm going wrong-- I thought this was simple calculus 2.
 
Physics news on Phys.org
  • #2
UPDATE: I have converted to polar coordinates, giving density = 1.2 r Cos[theta]+ 0.7 r Sin[theta]. Then I solved the following integral:

3_1.png


This gave me 8.55 g which was incorrect. Where am I going wrong?
 
  • #3
Your bound for y is wrong, draw a diagram with your bounds (e.g. y as a function of x) and you'll see that's not the bounds of a circle quadrant. Hint: the relation is x^2 + y^2 = 3^2
 
  • Like
Likes Grace Pseudonym
  • #4
Emily Herschbein said:
UPDATE: I have converted to polar coordinates, giving density = 1.2 r Cos[theta]+ 0.7 r Sin[theta]. Then I solved the following integral:

3_1.png


This gave me 8.55 g which was incorrect. Where am I going wrong?
If you check the units of your expression, you'll see that they don't work out. Your mistake is in the area element for polar coordinates.
 
  • Like
Likes Grace Pseudonym
  • #5
UPDATE NO. 2:

I figured it out.

they.png
 
  • #6
Thank you everyone for your help :)
 
  • #7
You stumbled onto the correct answer, but your work isn't correct.
 

Related to Mass Given Density Function and Area

1. What is a mass given density function?

A mass given density function is a mathematical representation that describes the relationship between the mass of a substance and its density. It is typically used to calculate the mass of a substance given its density and volume.

2. How is a mass given density function different from a regular density function?

A regular density function only describes the relationship between density and volume, while a mass given density function also takes into account the mass of the substance. This allows for more accurate calculations of mass given a known density and volume.

3. How is a mass given density function used in the real world?

A mass given density function is commonly used in fields such as physics, chemistry, and engineering to calculate the mass of a substance in various scenarios. It is also used in industries such as manufacturing and construction to determine the amount of material needed for a project.

4. Can a mass given density function be used for all substances?

No, a mass given density function is specific to a particular substance or material. Each substance has its own unique density, so a different mass given density function would need to be used for each substance.

5. How is area related to mass given density function?

Area is not directly related to a mass given density function. However, the concept of density can also be applied to two-dimensional objects, in which case area would be used instead of volume. In this scenario, a mass given density function would describe the relationship between the mass and area of the object.

Similar threads

  • Introductory Physics Homework Help
Replies
9
Views
1K
  • Introductory Physics Homework Help
Replies
8
Views
1K
  • Introductory Physics Homework Help
Replies
8
Views
3K
  • Introductory Physics Homework Help
Replies
11
Views
4K
  • Introductory Physics Homework Help
2
Replies
44
Views
3K
Replies
19
Views
1K
  • Introductory Physics Homework Help
Replies
13
Views
3K
  • Introductory Physics Homework Help
Replies
10
Views
1K
  • Introductory Physics Homework Help
2
Replies
57
Views
4K
  • Introductory Physics Homework Help
Replies
15
Views
5K
Back
Top