Lecture Notes on Solid Objects and Speed of Motion

Introduction

• Discussion about a solid metal bar (iron, steel) and how applying force on one end leads to movement observed at the other end.
• Key Question: How long does it take for the other end to move upon pushing one end?

Scaling the Concept

• Example: Scaling the bar to 300,000 km (one light second) to understand the speed of motion.
• Consideration of infinite strength to move a very heavy bar.

Theoretical Possibilities

• Possibilities for Delay:
  • Instantaneous: Because it’s a solid object.
  • Speed of Light-Based: Length divided by speed of light.
  • Speed of Sound in Metal: Length divided by speed of sound.
  • Material Properties and Impact: Delay depends on how the bar is struck and the material properties.

Physics Models and Approximations

• Real-world physics models are complex; approximations are necessary.
• Discussion on different levels of physics modeling from quantum mechanics to rigid body approximations.

Experimental Setup

• Using a solid metal bar and a sensor to measure delay when force is applied.
• Hammer initiates force, pressure sensor indicates force receipt.

Experimental Observations

• Delay measured at 180 microseconds for a 91 cm bar.
• Conclusion: Delay corresponds to the speed of sound in the bar, not instantaneous motion.

Physics Explanation

• Solid objects are not perfectly rigid—a compressive wave needs to travel through.
• Atomic structure of steel and its properties:
  • Steel’s body-centered cubic arrangement.
  • Electrons acting like springs connecting atoms.
  • Elastic deformation modeled as atoms connected by springs.

Sound Waves and Material Properties

• The linkage between the speed of sound and material properties.
• Spring model for atoms and the propagation of compressive waves.

Models and Predictions

• Importance of choosing the right physics model to predict outcomes.
• Speed of sound in steel calculated using material density and spring constant (Young’s modulus).

Practical Application

• Conducting the experiment with different bar lengths (1, 2, 3 feet).
• Verification of theory through experimental data.

Conclusion

• Solid objects are not completely rigid; physics models are simplifications.
• Extensional speed of sound differs from longitudinal speed.

Reflection

• Challenges and insights gained from conducting simple physics demonstrations.
• Importance of approximations and how different models apply to various physics scenarios.

The lecture emphasized the non-rigidity of solid objects in physics and the importance of sound waves in transmitting force through materials. The understanding of sound speed in solid objects gives insight into how force transmission in materials works.