# Homework Help: Center of mass of half square without a half circle

1. Jun 23, 2012

### SergioQE

1. The problem statement, all variables and given/known data
Find the position of the center of mass for a thin sheet and homogeneous, with sides R and 2R ,from which has been subtracted a half circle of radius R.

[Xcm=(2/3)*R*(4-pi)]

2. Relevant equations

Rcm=(1/M)*∫rdm

3. The attempt at a solution

By simmetry we know Ycm=0.
For de calculus of Xcm we rewrite the equation: Xcm=(1/Area)*∫xdA

dA=dx*y; $y=2\,R-2\,{\left( {R}^{2}-{x}^{2}\right) }^{0.5}$
For determinate the dA we take a
Xcm=(1/A)*$\int\2\,x\,R-2\,x\,{\left( {R}^{2}-{x}^{2}\right) }^{0.5}$/extract_itex] -R≤x≤R \[Xcm=\frac{{R}^{3}}{3\,A}$

$A=\frac{4\,{R}^{2}-\pi\,{R}^{2}}{2}$

$Xcm=\frac{6\,R}{4-\pi}$

I´ve been attempting in a lot of ways to solve the problem but i didn't get the result. This is an exercise from my exams in Physics I of 1º grade chemical engineering .

PS: I don't know how to make the formulas visible in LaTex

Last edited: Jun 23, 2012
2. Jun 23, 2012

### azizlwl

The CM of rectangle R×2R=CM half circle + CM shaded area.

3. Jun 23, 2012

### takudo_1912

You should use the guyndanh formula for sheet

4. Jun 23, 2012

### Infinitum

I wonder what that is....:uhh:

5. Jun 23, 2012

### SergioQE

Thank you, but when I try doesn't match with the correct answer.

Xcmrectangle= R/2
Xcmcircle= (4/(3*pi))*R

Xcm=(R/2)-(4/(3*pi))*R=0.0755*R

6. Jun 23, 2012

### SergioQE

I don't know what is that, but i´m pretty sure that the problem can be solved just with the center of mass definition.

7. Jun 23, 2012

### Infinitum

That formula is the distance from the base(diameter) of the semi circle... And the semicircle is 'inverted', here

8. Jun 23, 2012

### andrien

you should also consider the mass as proportional to area and not just subtract the x's.

9. Jun 23, 2012

### azizlwl

That from top.
Require reference to x-axis

10. Jun 23, 2012

### SergioQE

Okey, so the center of mass for the semicircle should be R-(4/(3*pi))*R?

Asr=Area of semirectangle, Asc=Area of semicircle.

Then: Xcm=[Asr*(R/2)-Asc*( R-(4/(3*pi))*R)]/(Asr-Asc);

Xcm=0.223*R;(I've reviewed my calculations)

11. Jun 27, 2012

### andrien

Xcm=∫(R/2+√(R^2-X^2)/2)(R-√R^2-X^2)dX/∫(R-√R^2-X^2)dX
limits are from 0 to R OR from -R to R that does not matter because of symmetry.It gives that.verify this yourself.

12. Jun 27, 2012

### SergioQE

Okay so the dA= (R-√R^2-X^2)dx, I understand that. But why you take x=(R/2+√(R^2-X^2)/2)?
And I tried to solve your equation with Maxima but i didn't get the result.

13. Jun 28, 2012

### Yukoel

Hello SergioQE,
You can handle your question without appealing to calculus, provided you know :
(1) Center of mass position of semicircle (removed part) and center mass of full rectangle.
(2) The basic expression for center of mass.
Now the semicircle and the truncated mass (shown figure) form such a system whose center of mass lies on the position of full figure.(use a suitable coordinate system).Please note that they have different masses so you need to be careful.As a matter of fact any calculus derivation of your problem employs the same method.
Does this help?

regards
Yukoel

14. Jul 3, 2012

### SergioQE

So why this expression is wrong?
"Asr=Area of semirectangle, Asc=Area of semicircle.

Then: Xcm=[Asr*(R/2)-Asc*( R-(4/(3*pi))*R)]/(Asr-Asc);"

I've considered the different masses, taking lambda= Mass(sr)/Area(sr), and the same for the sc.

And the result still wrong. I've used the coordinate system shown in the figure.

PS: I add the final expression in LaTex for comfort :
$Xcm=[\frac{{R}^{3}-\frac{\pi\,{R}^{2}\,\left( R-\frac{4\,R}{3\,\pi}\right) }{2}}{2\,{R}^{2}-\frac{\pi\,{R}^{2}}{2}}]$

Last edited: Jul 3, 2012
15. Jul 4, 2012

### Yukoel

Hello SergioQE
regards
Yukoel

16. Jul 4, 2012

### andrien

what do you mean by maxima,just evaluate the integral.in the upper integral,after opening the brackets you will have x^2/2 left whose integral is R^3/6(for 0 to R).the bottom integral is R^2-(∏R^2/4).so center of mass =(R/6)/(1-(∏/4))=4R/6*(4-∏)or 2R/3(4-∏) WHICH IS THE ANSWER YOU GOT THAT.it is now up to you to verify it.
edit:4-∏ is in denominator.

Last edited: Jul 4, 2012
17. Jul 4, 2012

### Yukoel

Hello andrien ,
You have used the inverted coordinate system I think(x=0 at the center of the semicircle).Your answer is correct with respect to that.SergioQE's value of x(cm)
and your value add up to R if my calculations are correct .This shows that you have chosen the center as reference for x=0.Correct me if I am wrong.

regards
Yukoel

18. Jul 5, 2012

of course.