# ME 431 Homework

## Homework 03

Snow.... Snow Snow Snow Snow Homework 03 Snow.

Lyric of the day
Anyway the wind blows doesn’t really matter to me, to me.

Dr. Quinn,

On Problem #7, is there a relationship between the inner and outer radii on the disk? The two cables are attached at different radii which would affect the moment equation and only one radius is given.

Randall Schelcher
You can assume that the inner radius is half the outer radius.

On problem 3, unless told otherwise, I am assuming the object at the end of the link is a point mass. There is no radius specified. Is this correct?
Yep...
Dr. Quinn,

On problem #3 I have a question about taking a derivative of one of my kinematic relationships. I drew a free body diagram with my I and J unit vectors displaying positively to the right and up, respectively. My spring force is in the positive i direction and labeled as k*x and the viscous damper is in the negative i direction and labeled as
-bź and finally the weight of the mass point is in the negative j direction and labeled as -mg. This is a 1 dof system in which the it can be explained in terms of its position (theta) about the fixed point O. So, the compression of the spring, x, is equal to l*sin theta, and the compression of the damper, z, can be expressed as
-l*sin theta. Now my confusion is when i take a derivate of z to get ź. Is it simply -l*cos(theta) or -l*thetadot?

Attached is an image
I believe that it would be -l*cos(theta)*thetadot from the chain rule.
Thanks Brandon! I see that now.. That is why my damping dropped out of my governing equation haha.

Professor,

Will you be posting the hwk 3 solutions today before 2pm?

They are up...
Dr. Quinn,

For problem 3 solutions you labeled the spring force in the j - direction and the mass at the end of the rod in the i direction.. is there a reason for that?
Yes. I made a typo in the solutions. However, the resulting equation of motion is correct.

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