Lecture Notes: Walter Lewin's Physics Class

Introduction

• Lecturer: Walter Lewin
• Focus of physics: Exploring very small (subatomic particles) to very large (the universe).
• Measurements span 45 orders of magnitude (1 with 45 zeros).

Units of Measurement

• Fundamental Units:

  • Length: Meter (m)
  • Time: Second (s)
  • Mass: Kilogram (kg)

• Derived Units:

  • Length Units: centimeters (cm), millimeters (mm), kilometers (km), inches, feet, miles
  • Time Units: milliseconds (ms), microseconds, days, weeks, hours, centuries, months
  • Mass Units: milligrams (mg), pounds, metric tons

• Discussion on the convenience of metric system vs. imperial system (inches, feet).

• Preference for decimal measurements in the course.

The Powers of Ten Movie

• Introduction of a movie titled "The Powers of Ten" covering 40 orders of magnitude.
• Created by Kees Boeke in the early '50s.

Fundamental Quantities in Physics

• Symbols:
  • Length: [L]
  • Time: [T]
  • Mass: [M]
• Derived Quantities:
  • Speed: [L]/[T] (length per time)
  • Volume: [L]^3
  • Density: [M]/[L]^3
  • Acceleration: [L]/[T]^2 (meters per second squared)

Importance of Measurement Uncertainty

• Emphasis on knowing the uncertainty in measurements.

• Example measuring lengths to highlight uncertainty:

  • Vertical Measurement: 149.9 cm ± 1 mm
  • Horizontal Measurement: 150.0 cm ± 0.1 cm

• If difference in lengths were significant (e.g., one foot), it would be easily noticeable.

Experiment: Measuring Height Difference

• Experiment aimed at testing grandmother's claim that a person is taller lying down than standing up.
• Selected student (Zach) for measurements:
  • Standing Height: 183.2 cm ± 0.1 cm
  • Lying Height: 185.7 cm ± 0.1 cm
  • Difference: 2.5 cm ± 0.2 cm, indicating a small height difference when lying down.

Scaling Argument: Size and Mass of Mammals

• Example of Galileo Galilei’s reasoning on the size limits of mammals.
• Analysis of femur length and thickness relative to size and mass:
  • Length of femur proportional to size
  • Mass proportional to size^3.
  • Pressure on the femur (mass/area) must not exceed a certain level to prevent breaking.

Testing the Scaling Argument

• Collection of femurs from various animals (raccoon, horse, mouse, elephant).
• Measurements of femur length and thickness.
• Results indicate that the ratio of thickness to length (d/l) does not follow predicted scaling laws.
• Conclusion suggests there are biological limits to animal size related to bone structure.

Dimensional Analysis: Falling Apple Experiment

• Discussed the falling time of an apple based on height, mass, and gravitational acceleration.
• Set up two heights: 3m and 1.5m for the falling apple experiment.
• Confirming the time taken to fall is proportional to the square root of the height.
• Measurement results: 781 ms (3m) and 551 ms (1.5m) confirmed the theory.

Conclusion

• Emphasized the importance of dimensional analysis and the limitations thereof.
• Raised awareness about the relationship between mass and falling time.
• Closing remarks on the fundamental understanding of physics and its implications.

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• Next Class: Friday.