# iTutor Math Updates

- Details
- Written by Super User
- Hits: 160

Hi, I'm building a vplotter (pen suspended by two strings of varying length) and I am facing a problem that looked quite simple at first
but I'm finding myself struggling with my rusty math skills.
Maybe someone could point me in the right direction.
So here's the problem:An object, to make it easier, a rod, is suspended at it's ends to a ceiling by two strings.I know the coordinates of points A and D as well as the length of the strings a,c and the distance between B and C (b in the drawing).
I need to find the coordinates for B and C when the system is in equilibrium.
This is where I am at:
Point B lies on the circle around A with the radius a. C lies on the circle around D with the radius c. So, with (x-a)

^{2}+(y-b)^{2}=r^{2 }(Circle) I have: (1) (x_{B }- x_{A})^{2 }+ (y_{B}- y_{A})^{2}= a^{2}(2) (x_{C}- x_{D})^{2}+ (y_{C}- y_{D})^{2}= c^{2}Also, |BC| = b (distance between B and C is given). So: (3) b = sqrt( (x_{B}-x_{C})^{2}+ (y_{B}-y_{C})^{2}) => b^{2}= (x_{B}-x_{C})^{2}+ (y_{B}-y_{C})^{2}This is where I'm getting into trouble (or maybe I already am). 4 unknowns, only 3 equations. My system still has a degree of freedom, I suppose the physical equivalent of shifting the rod around on the two circles. I was hoping I'd get another equation by looking at how gravity exerts forces on the rod and those forces split into tangential and perpendicular forces at points B and C. Knowing that the system is in equilibrium, all those forces should add to 0. This is where I am stuck. Any help would be greatly appreciated. Maybe someone can point me in the right direction to move on or point out any mistakes so far. Thanks