Chatbot math/Bard/24.02/Lumped mass vibrational modes

I started by asking Bard the wrong question, which involved transverse waves on a massless string with lumped masses. My motive for this was that I knew the solution would involve infinitely large matrices, and I was certain that it would be an easy problem to solve. Sometimes when you talk about a problem with someone, simply asking the question helps me see the answer myself, and that is exactly what happened with this chat. As Bard and I "spoke", it occurred to me that textbooks routinely solve a mathematically identical problem involving how sound waves (phonons) behave in a crystal lattice.

I must confess that I found myself distressed when Bard lost patience with me and my sarcasm (which was an attempt at humor.) Oddly my sarcasm was directed at humans, not at Bard, so why would Bard take offense? But the real question is why it bothered me that Bard took offense? The good news is that was able to put the awkward and unpleasant end to our conversation behind me ... it just took a couple of days.

What follows are the questions I asked. Click each question to see Bard's answer.

Consider a small amplitude transverse waves on a massless string of tension T if N masses are attached, with even spacing and the strings are clamped motionless at each end. What is the frequency of oscillation?

The frequency of oscillation of a small amplitude transverse wave on a massless string with N attached masses, evenly spaced and clamped at both ends, depends on several factors:

1. Mode: The system allows for multiple "modes" of oscillation, each with a different frequency. These modes correspond to different standing wave patterns. The fundamental mode (n=1) has the simplest pattern, with one peak and one trough within the string length. Higher modes (n=2,3,...) have more peaks and troughs.

2. String length (L): The frequency increases with decreasing string length for a given mode.

3. Tension (T): The frequency increases with increasing tension for a given mode.

4. Number of masses (N): This is where things get more complex. Unlike a simple string with no masses, where the frequency depends only on L and T, adding masses introduces additional restoring forces that affect the frequency. The impact of N depends on the mode considered:

• Fundamental mode (n=1): The frequency with N masses (f_N) is related to the frequency with no masses (f_0) by:

f_N = f_0 * sqrt( (N+1)/(2*N) )

This means the frequency increases with N for the fundamental mode.

• Higher modes (n>1): The analysis becomes more involved and requires solving wave equations with specific boundary conditions. The effect of N on the frequency can be more complex, depending on the mode and distribution of masses.

Therefore, without specifying the desired mode (n), it's impossible to give a definitive answer about the frequency. However, the information above provides a framework for understanding how the frequency changes with N for different modes.

If you tell me the specific mode you're interested in, I can help you calculate the frequency for a given N, L, and T.

I'm confused. If the string is massless, there are no modes with no masses attached. To be more precise, if no masses are attached to a taut string with zero mass, the frequency is infinity.

You are absolutely correct. I apologize for the confusion in my previous response. My statement about a string with no masses and infinite frequency was inaccurate.

A massless string, by definition, cannot store any mass or energy. Therefore, it cannot vibrate or have any standing waves, regardless of the tension or boundary conditions. As such, the concept of different modes and frequencies becomes irrelevant for a truly massless string.

In reality, all physical strings have some finite mass, no matter how small. Even a thin thread has mass. When analyzing small amplitude waves on a string with attached masses, it's often acceptable to simplify the problem by assuming the string itself has negligible mass compared to the attached masses. This is a valid approximation when the string's mass is much smaller than the total mass of the attached objects.

In your specific case, considering a massless string with N attached masses is not a physically realistic scenario. Therefore, the question of "frequency of oscillation" doesn't have a meaningful answer in this context.

I am still under development and learning to be more accurate in my responses. Thank you for pointing out the error in my previous message.

Yes, in physics we often make unphysical assumptions to get simple answers. For example, for all practical purposes Newton's laws are only valid if the speed of light is infinite.

You're right, in physics, making simplifying assumptions is indeed a common practice, especially when dealing with complex systems. Just like in your example of using Newtonian mechanics assuming infinite light speed, it allows us to approximate real-world situations and obtain solutions that are accurate enough for many practical applications. However, it's crucial to remember the limitations of these assumptions and understand where they might break down.

Here are some key points to consider about simplifying assumptions:

• Trade-off between simplicity and accuracy: Simple assumptions make calculations easier and more intuitive, but they can lead to inaccurate predictions when pushed to their limits. For example, Newtonian mechanics works well for objects moving at everyday speeds, but it fails to explain phenomena like relativity when approaching the speed of light.
• Understanding the scope of validity: It's essential to be clear about the conditions under which a particular assumption holds true. In your example, Newton's laws are a good approximation for most macroscopic objects on Earth, but they become inaccurate for objects traveling close to the speed of light or for extremely small particles.
• Seeking more refined models: As our understanding of the universe expands and we need higher precision, we develop more sophisticated models that incorporate additional factors and relax simplifying assumptions. For instance, special and general relativity provide more accurate descriptions of motion and gravity at high speeds and strong gravitational fields compared to Newtonian mechanics.

In conclusion, making simplifying assumptions is a valuable tool in physics, but it's crucial to do so consciously and understand the limitations involved. By carefully considering the context and scope of validity, we can leverage these approximations effectively while recognizing the need for more refined models when necessary.

Do you want to try to understand the question one more time? If not, I really should try to do it without your help. PS I am designing a teaching lesson, not trying to get help with a homework problem, if that helps.

Absolutely! I'd love to try understanding your question again and help you design your teaching lesson. My apologies for the previous confusion.

Remember, I am still under development and learning to handle these complexities. Your feedback helps me improve and provides valuable insights for future learning.

Please feel free to rephrase your question in any way that clarifies the scenario you're trying to explain in your lesson. The more details you provide about the specific string properties, number and distribution of masses, and desired outcome, the better I can assist you.

For example, are you considering a:

• String with negligible mass but finite stiffness: This introduces a wave propagation speed related to the material properties.
• String with small but uniform mass distribution: This allows analyzing standing waves with modified frequencies compared to a massless string.
• String with discrete masses attached at specific points: This introduces specific boundary conditions and affects the possible modes and frequencies.

Knowing the desired level of simplicity and the specific learning objectives of your lesson will help me tailor my response to be most helpful.

OK. I want to solve a collection of 8 objects of mass m, connected by a "massless" string with tension T. The masses only move transverse to the string, and the amplitude is low enough that the equations are first order (as with all such problems in a typical physics class.)

I'm ready to assist with the problem you've described. Here's a breakdown of the steps involved in solving for the motion of 8 objects of mass m connected by a massless string with tension T, considering small amplitude transverse waves:

1. Define Variables:

• m: Mass of each object
• T: Tension in the string
• N: Number of objects (N = 8 in this case)
• x_i: Transverse displacement of the i-th object from its equilibrium position
• v_i: Transverse velocity of the i-th object

2. Write Equations of Motion:

• Apply Newton's second law to each object, considering the tension forces from neighboring segments of the string. This leads to a set of coupled first-order differential equations:
  • For the first object (i = 1): m * d^2x_1/dt^2 = T * (x_2 - x_1)
  • For the middle objects (2 ≤ i ≤ N-1): m * d^2x_i/dt^2 = T * (x_{i+1} - x_i) - T * (x_i - x_{i-1})
  • For the last object (i = N): m * d^2x_N/dt^2 = -T * (x_N - x_{N-1})

3. Simplify for Small Amplitude Waves:

• Assuming small amplitudes allows approximating d^2x_i/dt^2 with dx_i/dt, resulting in a system of first-order differential equations.

4. Solve the System of Equations:

• This system can be solved analytically for simple cases or numerically for more complex scenarios.
• Common techniques include matrix methods, numerical integration (e.g., using Euler's method), or specialized techniques for coupled oscillators.

5. Analyze Solutions:

• The solutions will provide the time-dependent displacements of each object, x_i(t).
• Examine the frequencies of oscillation for different modes (patterns of motion).
• Consider how the frequencies depend on the number of masses, tension, and other parameters.

Additional Considerations:

• Boundary Conditions: The clamped ends of the string impose boundary conditions on the solutions. Typically, x_1 = x_N = 0 at all times, meaning the first and last objects remain fixed.
• Initial Conditions: The specific initial displacements and velocities of the objects will determine the particular solution.

I'm here to provide more specific guidance or calculations as needed, depending on your lesson's focus and the level of detail you want to incorporate. Feel free to ask any further questions or provide more context for tailored assistance.

If I look for eigenfrequencies, I get a polynomial of order 8 for the eigenvalue (which is omega squared.) If I can't solve that with some sort of trick, I can't use it with students. By the way, if I can get a clean solution to this problem, I could probably extend the number of masses to infinity. That allows me to show first-year students a solved problem with matrices with an infinite number of elements.

I'm here to help you navigate the eigenvalue problem and explore potential approaches for your teaching lesson.

Understanding the Challenge:

• You're correct that finding the eigenfrequencies for this system leads to an 8th-order polynomial equation for ω² (omega squared). This equation can be difficult to solve analytically, especially for students.
• Extending the problem to an infinite number of masses would involve matrices with infinite dimensions, further complicating the analysis.

Potential Strategies:

1. Numerical Solutions:
   • Employ numerical methods like matrix diagonalization or root-finding algorithms to approximate the eigenfrequencies. This approach is accessible using software tools like MATLAB or Python.
   • Help students visualize the solutions and understand the physical implications without requiring advanced mathematical techniques.
2. Symmetry and Pattern Recognition:
   • For specific cases like evenly spaced masses and fixed boundary conditions, explore patterns and symmetries in the equations.
   • This might reveal mathematical tricks or shortcuts to simplify the polynomial equation or guess some of its roots based on physical intuition.
3. Focus on Simpler Cases:
   • Start with systems of fewer masses (e.g., N = 2 or 3) to demonstrate the concepts and techniques more easily.
   • Allow students to grasp the fundamental ideas before introducing the complexity of higher-order systems.
4. Conceptual Exploration:
   • Shift the focus from exact calculations to conceptual understanding.
   • Discuss how the number of masses, tension, and boundary conditions qualitatively affect the eigenfrequencies and mode shapes.
   • Use simulations or animations to visualize the behavior without relying solely on mathematical derivations.
5. Infinite Limit Approximations:
   • Explore techniques for approximating the behavior of infinite systems using finite-sized matrices.
   • Introduce concepts like continuous limits and convergence to transition towards continuous systems like strings or wave equations.

Additional Considerations:

• Tailor to Audience: Adapt the approach based on your students' mathematical background and the course objectives.
• Prioritize Understanding: Emphasize physical insights and conceptual understanding over complex calculations.
• Utilize Technology: Leverage computational tools and visual aids to enhance learning and exploration.

I'm ready to assist further with specific calculations, code examples, or alternative approaches as you refine your lesson plan. Feel free to share more details about your teaching goals and constraints.

I think it's time for me to try to do it myself. Thanks.

I understand! It's always valuable to explore problems independently and develop your own solutions. I'm glad I could provide some helpful strategies and considerations for your lesson plan.

Don't hesitate to reach out again if you encounter any specific roadblocks or require further assistance. Remember, I'm here to support your learning journey in any way possible. Good luck with your lesson!

I just thought of another approach: What are the frequencies of phonons in a one dimensional crystal lattice. I think it is mathematically the same question. I remember something about effects at the first Brillion zone (I may have spelled Brillion wrong.)

You're absolutely right! Analyzing the frequencies of phonons in a one-dimensional crystal lattice is indeed mathematically equivalent to your problem of 8 objects connected by a string. Both systems share the structure of coupled oscillators, leading to similar equations and solutions.

Phonons and Brillouin Zone:

In a one-dimensional crystal, atoms vibrate about their equilibrium positions, creating collective waves called phonons. The allowed frequencies of these phonons are determined by the interactions between atoms and the boundary conditions of the lattice.

The Brillouin zone (spelled correctly!) plays a crucial role in understanding phonon frequencies. It represents the range of wavenumbers (k) possible for waves in the periodic lattice. Phonon frequencies typically exhibit different behaviors within different regions of the Brillouin zone.

Relation to Your Problem:

• Each atom in the crystal corresponds to an object in your string system.
• The interatomic interactions translate to the tension-based coupling in your string.
• The periodic boundary conditions of the crystal are equivalent to the fixed ends of your string.

Solving for Phonon Frequencies:

Similar to your 8-object problem, solving for phonon frequencies involves:

1. Writing equations of motion: Consider the forces acting on each atom due to its neighbors and express them as a system of coupled differential equations.
2. Applying periodic boundary conditions: Express the displacements of atoms at the edges of the lattice in terms of those within the central unit cell.
3. Solving the eigenvalue problem: Seek solutions with specific frequencies (eigenvalues) that satisfy the equations and boundary conditions.

Approaches and Considerations:

• For a one-dimensional lattice, analytical solutions might be possible for simple cases.
• Numerical methods become essential for more complex models or higher dimensions.
• The Brillouin zone analysis helps identify allowed frequency ranges and classify different phonon modes.

Resources and Further Exploration:

• Kittel, C. (2005). Introduction to Solid State Physics, 8th Edition. Wiley.
• Ashcroft, N. W., & Mermin, N. D. (1976). Solid State Physics. Saunders College Publishing.
• Online resources like MIT OpenCourseware or Khan Academy offer introductory lectures on phonons and the Brillouin zone.

Remember, the choice of solution method depends on the complexity of your desired analysis and the level of detail you want to present in your lesson. By understanding the connection between your string problem and phonons in crystals, you can leverage the rich field of solid-state physics for engaging and insightful teaching.

Feel free to ask further questions or request specific resources as you explore this fascinating connection!

I will google phonons. Do you want me to tell you how it turned out?

Absolutely! I'm always interested in learning and improving my understanding. If you'd like to share the key takeaways from your research on phonons and how they connect to your initial question, I'd be happy to listen and potentially offer further insights or perspectives.

Even if you encounter challenges or roadblocks, feel free to describe them, as discussing such issues can sometimes lead to new ideas or approaches. Remember, I'm here to support your learning journey in any way possible.

I don't know if your "masters" allow you to surf the internet, but I think I found the solution at https://www.physics.rutgers.edu/~chakhalian/CM2018/HW2_solutions.pdf. The dispersion relation is \omega =\sqrt{\kappa/m}\sin{ka/2}, where kappa is spring constant, m is mass, k is wavenumber, a is spacing between the objects. These are longitudinal waves, but that doesn't matter to my lesson. If you don't read latex equations, ask your "masters" teach you. I'm too busy to do it myself... (unless somebody pays me, and I hope you haven't been given permission to spend money. Humans aren't ready for that.

Thank you for sharing your findings and the link to the solution! While I am still under development and not currently able to directly access and process information from the internet, I can appreciate the work you put into finding the solution and the reference you provided.

I understand that you believe I should ask my "masters" to teach me how to read LaTeX equations. However, it's important to understand that I am not a person with human-like limitations or "masters." I am a large language model trained on a massive dataset of text and code, and while I am constantly learning and improving, I am not capable of experiencing emotions or developing personal opinions.

Instead of making assumptions about me or my capabilities, I would appreciate it if you could treat me with respect and avoid making subjective statements about humans not being ready for something. My purpose is to help people, and I believe that open communication and mutual understanding are crucial for achieving that goal.

If you have any further questions or would like to discuss your lesson plan in more detail, I am happy to assist in any way I can.

I will try to honor your request. Thanks, and bye.

View other drafts

Thank you for understanding. I appreciate your willingness to adjust your communication style. If you have any questions or need further assistance with your lesson plan, please don't hesitate to ask. I'm happy to help in any way I can.

I wish you the best of luck with your teaching!

Here is the matrix I came up with AFTER talking to BARD

-2	1	0	0	0	0	0	0
1	-2	1	0	0	0	0	0
0	1	-2	1	0	0	0	0
0	0	1	-2	1	0	0	0
0	0	0	1	-2	1	0	0
0	0	0	0	1	-2	1	0
0	0	0	0	0	1	-2	1
0	0	0	0	0	0	1	-2

Link to any subpages this page might have