Help with coupled spring and pendulum system

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Homework Help Overview

The discussion revolves around a coupled system involving a block and a pendulum, where the original poster seeks to derive the equations of motion for the system. The context includes a frictionless surface and small angle approximations, with specific attention to the forces acting on both the block and the pendulum.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • The original poster attempts to derive the equations of motion by analyzing the forces acting on both the block and the pendulum, expressing concerns about the role of the spring force and the tension in the pendulum. Some participants question the number of equations relative to the degrees of freedom in the system and suggest the need to eliminate certain variables.

Discussion Status

The discussion is ongoing, with participants providing guidance on the inclusion of spring forces in the equations of motion. There is a recognition of the limitations in techniques available to the original poster, and multiple interpretations of the forces at play are being explored.

Contextual Notes

The original poster notes restrictions on the use of certain techniques, such as Lagrangian methods, and expresses uncertainty about approximating vertical displacements for the pendulum. There is also a mention of the need to clarify how forces are applied in the system.

J.Sterling47
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Homework Statement


Given the system in the image below, I need to find the equation of motions for the coupled system. The surface where the block moves is frictionless. The red line is position where the block is at equilibrium. At equilbrium x1 and x2 = 0. After finding the equation of motion for the coupled system, I need to find the normal modes which I can do but I'm having trouble finding first part. Also the angle is small angle approximation.
http://imgur.com/JPzK1Jn
http://imgur.com/JPzK1Jn

Homework Equations


F= ma
sinθ = tanθ = θ = [x2 - x1]/L

The Attempt at a Solution


Ok so taking the horizontal and vertical forces of each:

Block:
Fx1 = -kx1 - Tsinθ
Fy1 = N - mg - Tcosθ

Mass on Pendulum:

Fx2 = Tsinθ **************
Fy2 = mg - Tcosθ*****I feel like the spring should add a force here, but I'm not sure how.
If I proceed adding a -kx1 to Fx2 and solving for the equations of motion using the small angle approximation I get

a1 = -k/mx1 + T/Lm(x2 - x1)

and a2 the same which is wrong. Also I cannot get rid of T. I am not so concerned over the normal modes because I can do that after. We did not do any langrange stuff so I am not allowed to use it. Also because of the small angle, I asked if I was allowed to approximate the vertical displacement for the pendulum to be zero, but I cannot so I have to find another way.
 
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You have written four equations of motion, but this system only has two degrees of freedom. You should be able to eliminate some variables after you get your equations of motion correct.

To be sure, the spring force does enter into the equation of horizontal motion for the block. How do springs generate force?
 
OldEngr63 said:
You have written four equations of motion, but this system only has two degrees of freedom. You should be able to eliminate some variables after you get your equations of motion correct.

To be sure, the spring force does enter into the equation of horizontal motion for the block. How do springs generate force?

Yeah I'm stuck here. I can't figure out how to eliminate T and I'm also unsure of how to make Fx2 correct. My problem is that I'm very limited with techniques here, as this is pretty much the beginning of an intro to classical mechanics.

I'm pretty sure adding -kx1 to horizontal motion of the pendulum is incorrect as then the two equations that I care about are identical which is wrong. -kx2 wouldn't make much sense.
 
Does the spring act on the block or on the pendulum?

Where are your FBDs?
 

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