# Rotating reference frame help

#### natalie

1. The problem statement, all variables and given/known data

I have attached the problem as a picture on this post, am really really unsure on how to start!

so far the only thing i can think of doing is using this equation

$$(\frac{d^{2}r}{dt^{2}})_{s}$$ = $$( \frac{d^{2}r}{dt^{2}})_{s'} + 2ω \times (\frac{dr}{dt})_{s'} + \dot{ω} \times r + ω \times [ω \times r]$$

And now just solve for r, but in the s reference frame ?

any help appreciated really stuck.

#### Attachments

• 66.8 KB Views: 216
Related Advanced Physics Homework News on Phys.org

#### voko

Not even part (i)?

#### natalie

Not even part (i)?
No, I dont see how it works?

#### voko

Assume that the rod is pointing momentarily north.

What forces acting on the bead, in the inertial frame, can you think of? What are their directions?

#### vanhees71

Gold Member
Hm, I must admit that I always have trouble using forces to derive the equations of motion. If you have Hamilton's principle at hand, it's way simpler to use the Euler-Lagrange equations and then work out the forces at the very end ;-)).

#### voko

then work out the forces at the very end ;-)).
That's cheeeeeating! :)

#### natalie

Assume that the rod is pointing momentarily north.

What forces acting on the bead, in the inertial frame, can you think of? What are their directions?
so basically, if i understand correctly, the rod is lying in the x' axis. we have a weight force, and a normal force. thats in the intertial reference frame? the weight force cancels out the normal....

Gold Member

#### voko

so basically, if i understand correctly, the rod is lying in the x' axis. we have a weight force, and a normal force. thats in the intertial reference frame? the weight force cancels out the normal....
If the weight cancelled the normal force, the bead would have zero resultant force acting on it. What it is the motion under zero resultant force? Does that seem plausible in the situation at hand?

"Rotating reference frame help"

### 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