Question about springs and rotational/translational energy

AI Thread Summary
The discussion revolves around calculating the translational speed of a thin uniform rod's center of mass at a specific position and determining the spring constant based on its compression. Participants emphasize the importance of correctly applying energy conservation principles, including gravitational, elastic, kinetic, and rotational energies at different positions of the rod. There is confusion regarding the role of the center of mass in energy calculations, particularly in how to factor it into the equations. The need for credible effort in problem-solving is highlighted, along with adherence to homework guidelines for effective assistance. The conversation underscores the necessity of clearly presenting one's approach to identify errors in calculations.
neel400
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Homework Statement
Hi i need help with this problem ASAP
Relevant Equations
1/2kx^2, mgh, 1/2mv^2, 1/2Iw^2
View attachment 353342
A thin uniform rod has mass M=0.510 kg and length L=0.470 m. It has a pivot at one end and is at rest on a compressed spring as shown in (A). The sequence below shows that the rod is released from an angle θ1=59 degrees, and moves through its horizontal position at (B) and up to (C) where it stops with θ2=111 degrees, and then falls back down. Assume friction at the pivot is negligible.

a) Calculate the (translational) speed of the center of mass (CM) at (B) in m/s.
b) The spring in (A) has a length of 0.1240.124 m and at (B) a length of 0.1540.154 m. Calculate the spring constant k in N/m.

Please help with setting up an equation that can solve for a and b, I'm very lost with what to do.

I originally set it up as A: negative gravitational and elastic energy, B: Kinetic and rotational energy and C: positive gravitational (and tried with and without a rotational energy here), but i got the wrong answer. I'm not sure if it's an error in the way i understood which energies are present and which point, or if there is something wrong in my calculations due to the center of mass (im not very certain how to factor that in -- i used it for the mgh calculations, but not sure if i need to use it elsewhere)
 
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neel400 said:
I originally set it up as A: negative gravitational and elastic energy, B: Kinetic and rotational energy and C: positive gravitational (and tried with and without a rotational energy here), but i got the wrong answer.
The elastic energy will still be relevant at B and C, unless the object is released from the spring.

It stops at C so there is no kinetic energy left (rotational kinetic energy is kinetic energy, too).

We can't tell what went wrong if you don't show your approach.
 
Welcome, @neel400 ! :smile:

Is the described set up similar to this one?

php8qomvh.png
 
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Likes SammyS and kuruman
Lnewqban said:
Welcome, @neel400 ! :smile:

Is the described set up similar to this one?

View attachment 353351
That's it. I remember seeing it when I replied to the OP. It then disappeared in a "Black Hole" probably by a mentor's action.
 
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