Physics Lecture by Walter Lewin

Introduction

• Lecture duration: ~40 minutes
• Q&A session: ~15-20 minutes
• Book signing after the lecture
• Topic: Pendulums, gravitational acceleration, and energy conservation
  • Example: Pendulum with mass (m) and length (L)

Pendulum Period Calculation

• Period (T) = 2π * sqrt(L / g)
  • L: Length of the pendulum
  • π: Mathematical constant (Pi)
  • g: Gravitational acceleration (~9.80 m/s² in Boston)
• Explanation of units: meters per second per second
  • Speed increases by 9.8 m/s every second when an object is in free fall

Non-intuitive Points About Pendulums

• Period is independent of amplitude (within moderate values)
• Period is independent of the mass of the bob
• Demonstrations with actual calculations and pendulum setup
  • Length (L) of 5.21 meters and mass of 15.5 kilograms
  • Uncertainty in measurement of length (~5 cm, 1% of 521 cm)
  • Impact of length measurement uncertainty: ~0.5% uncertainty in time

Predicting and Measuring Pendulum Period

• Predicted period: 4.58 ± 0.02 seconds
  • Due to uncertainties in length measurement (0.5% of 4.58 seconds)
• Demonstration of reaction time affecting measurement
  • Reaction time assumed to be 0.2 seconds for accurate measurements
  • Conducts measurements at different amplitudes (5 and 10 degrees)
  • Demonstrates measurement accuracy by timing 10 oscillations

Conservation of Energy Examination

• Example: Tennis ball dropping and bouncing
  • Potential energy (mgh) at height h
  • Converts to kinetic energy (1/2 * mv²) when falling
  • Energy loss explanation: due to air drag and heat upon impact
• Pendulum experiment: demonstrating energy conservation
  • Swinging demonstration to prove the maximum height
  • Explanation using potential and kinetic energy conversions

Rayleigh and Mie Scattering

• Rayleigh scattering: Small particles (<1/10 micron) scatter blue light more than red
  • Explanation of P ∝ 1/λ⁴ (λ: wavelength)
  • Demonstration using cigarette smoke (blue light due to Rayleigh scattering)
• Mie scattering: Larger particles (>0.5 micron) scatter all wavelengths equally
  • Demonstration by exhaling smoke held in lungs (white light due to Mie scattering)

Explanation of Sky and Sunset Colors

• Sky's blue color due to Rayleigh scattering of small particles in the atmosphere
• Sunset's red color due to longer travel path scattering away shorter wavelengths
  • Example: More pollution or volcanic eruptions enhance red sunsets

Practical Demonstration

• Creating blue sky and red sunset in a lecture hall using sodium thiosulfate and sulfuric acid
  • Shows blue light (Rayleigh scattering) and red light (Mie scattering) transitions

Final Experiment and Conclusion

• Holds a pendulum to his chin and releases it (demonstrating conservation of energy)
  • Ensures it won't swing higher than initial release point
• Concludes with personal notes and prepares for Q&A

Q&A Session Highlights

1. Explanation of the green flash phenomenon
2. Personal anecdotes and teaching experiences
3. Advice for aspiring physicists: Love for the subject is crucial
4. Preparation process for lectures

Book Signing Instructions

• Write the name clearly on a piece of paper to streamline the process