Help with coupled spring and pendulum system

AI Thread Summary
The discussion focuses on deriving the equations of motion for a coupled spring and pendulum system, emphasizing that the surface is frictionless and the small angle approximation applies. The user struggles with incorporating the spring force into the equations, particularly in the horizontal motion of the block and the pendulum. There is confusion about how to eliminate the tension (T) from the equations and correctly account for the forces acting on both components of the system. The need for free body diagrams (FBDs) is highlighted as essential for clarifying the forces involved. Overall, the user seeks guidance on correctly formulating the equations of motion while adhering to the constraints of their coursework.
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?
 
Kindly see the attached pdf. My attempt to solve it, is in it. I'm wondering if my solution is right. My idea is this: At any point of time, the ball may be assumed to be at an incline which is at an angle of θ(kindly see both the pics in the pdf file). The value of θ will continuously change and so will the value of friction. I'm not able to figure out, why my solution is wrong, if it is wrong .
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