Physics Lecture Notes by Walter Lewin

Introduction

• Lecturer: Walter Lewin
• Course Overview: Explore physics from the very small (subatomic) to the very large (universe).
• Magnitude Range: 45 orders of magnitude (1 with 45 zeroes).

Fundamental Units

• Length: Meter
• Time: Second
• Mass: Kilogram

Derived Units

• Common derived units for length:
  • Centimeters, millimeters, kilometers, inches, feet, miles
  • Astronomers use astronomical units and light-years.
• Common derived units for time:
  • Milliseconds, microseconds, days, weeks, hours, months, years, centuries.
• Common derived units for mass:
  • Milligrams, pounds, metric tons.

Preference for Decimal System

• Decimal system is preferred for calculations.
• The imperial system (inches, feet) is seen as cumbersome.

Movie Introduction

• The Powers of Ten: A movie covering 40 orders of magnitude, originally conceived by Kees Boeke in the 1950s.

Fundamental Quantities in Physics

• Symbol Definitions:
  • Length: [L]
  • Time: [T]
  • Mass: [M]

Derived Quantities

• Speed: [L]/[T]
• Volume: [L]^3
• Density: [M]/[L]^3
• Acceleration: [L]/[T]^2 (meters per second squared)

Measurements and Uncertainty

• Importance of measurement uncertainty:
  • Any measurement without uncertainty is meaningless.
• Example of a measurement setup with an aluminum bar demonstrating vertical vs. horizontal length.
  • Uncertainty Example:
    • Vertical measurement: 149.9 cm ± 0.1 cm
    • Horizontal measurement: 150.0 cm ± 0.1 cm

Testing Grandmother's Statement

• Grandmother's claim: A person lying down may appear longer than standing.
• Measurement of a volunteer (Zach) showed a discrepancy of about one inch in height between standing and lying down.

Galileo's Scaling Argument

• Question: Why are mammals not larger?
• Argument: Bone strength limits size.
  • Mass relates to the volume (size) while bone thickness must increase with size.
• Scaling relationship established:
  • Thickness of femur (d) is proportional to the length (l) to the power of 3/2.

Experimental Validation

• Femur measurements from raccoon, horse, opossum, mouse, elephant:
  • Results showed that the ratio of thickness to length did not scale as expected.

Dimensional Analysis

• Question: How does the height affect the time of a falling object?
• Relationship expressed in powers:
  • Time (t) proportional to height (h) and gravitational acceleration (g).
• Final conclusion:
  • Time t is proportional to the square root of h/g.

Experimental Results for Falling Apples

• Experiment setup to measure fall times from 3m and 1.5m.
• Measurements:
  • t1 (3m): 781 ms ± 2 ms
  • t2 (1.5m): 551 ms ± 2 ms
• Confirmed that the time of fall is independent of mass and proportional to the square root of height.

Conclusion

• Importance of understanding measurement uncertainty in physics.
• Acknowledge limitations in dimensional analysis and results.
• Encouragement for students to think critically about physics principles and experiments.

Next Lecture

• Reminder for students to prepare for the next class on Friday.