# Moment of inertia of a half disk about an axis

#### vbrasic

1. Homework Statement
Consider a half disk (of uniform density) with the flat end lying on the x-axis, symmetric about the y-axis (i.e. being cut into two quarters by the y-axis). Calculate the moments of inertia about each of the axes.

2. Homework Equations
$$I_{rr}=\sum_{i}m_ir_i^2$$

3. The Attempt at a Solution
I just need some making sure that I'm setting up the problem correctly.

The distance to the x-axis from any point on the disk is $y$. Or, alternatively, $r\sin{\phi}.$ So we find that, $$I_{xx}=\int_{0}^{R}\int_{0}^{\pi}(r^2\sin^2{\phi})\rho\,rdrd\phi,$$ where $\rho$ is the mass density per unit area. However, my instructor has in his notes that, $$I_{xx}=\int_{0}^{R}\int_{0}^{\pi}(r^2\cos^2{\phi})\rho\,rdrd\phi.$$ I'm not sure why that would be, as the distance from any point to the axis of rotation (x in the case of $I_{xx}$ is $y=r\sin{\rho}.$

Related Introductory Physics Homework Help News on Phys.org

#### tnich

Homework Helper
1. Homework Statement
Consider a half disk (of uniform density) with the flat end lying on the x-axis, symmetric about the y-axis (i.e. being cut into two quarters by the y-axis). Calculate the moments of inertia about each of the axes.

2. Homework Equations
$$I_{rr}=\sum_{i}m_ir_i^2$$

3. The Attempt at a Solution
I just need some making sure that I'm setting up the problem correctly.

The distance to the x-axis from any point on the disk is $y$. Or, alternatively, $r\sin{\phi}.$ So we find that, $$I_{xx}=\int_{0}^{R}\int_{0}^{\pi}(r^2\sin^2{\phi})\rho\,rdrd\phi,$$ where $\rho$ is the mass density per unit area. However, my instructor has in his notes that, $$I_{xx}=\int_{0}^{R}\int_{0}^{\pi}(r^2\cos^2{\phi})\rho\,rdrd\phi.$$ I'm not sure why that would be, as the distance from any point to the axis of rotation (x in the case of $I_{xx}$ is $y=r\sin{\rho}.$
I think you have it right. Maybe the instructor wrote it down wrong.

#### kuruman

Homework Helper
Gold Member
Have you and your instructor defined angle $\phi$ the same way or is one $90^o - \phi$ of the other?
... in the case of $I_{xx}$ is $y=r \sin \rho$.
You mean $y=r \sin \phi$.

#### vbrasic

Have you and your instructor defined angle $\phi$ the same way or is one $90^o - \phi$ of the other?

You mean $y=r \sin \phi$.
Yes -- and not that I know of.

#### kuruman

Homework Helper
Gold Member
OK, then you need to ask yourself, "does it make any difference?" Is the area under the $\cos^2(\phi)$ curve from $0$ to $\pi$ different from the area under the $\sin^2(\phi)$ curve? Draw the curves and see for yourself.

#### vbrasic

OK, then you need to ask yourself, "does it make any difference?" Is the area under the $\cos^2(\phi)$ curve from $0$ to $\pi$ different from the area under the $\sin^2(\phi)$ curve? Draw the curves and see for yourself.
Yeah, it wouldn't make a difference. The integral evaluates to the same result either way.

#### kuruman

Homework Helper
Gold Member
Yeah, it wouldn't make a difference. The integral evaluates to the same result either way.
Yes, and if you want to satisfy your curiosity, you may have to ask your instructor about the definition of $\phi$, otherwise leave it alone.

"Moment of inertia of a half disk about an axis"

### 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