Feynman Lectures Simplified 1D: Sound, Waves, Angular Momentum, Symmetry & Vision

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1D: Sound, Waves, Angular
Momentum, Symmetry & Vision

Feynman Simplified 1D covers a quarter of Volume 1, the freshman course, of The Feynman Lectures on Physics. The topics we explore include:
  •     Angular Momentum
  •     Moments of Inertia
  •     Rotations in Three-Dimensions
  •     Sound & Beat Frequencies
  •     Modes & Harmonics
  •     Fourier Series & Transforms
  •     Complex Waves
  •     Symmetry Properties of Natural Laws


Bow Waves

Bow waves are created by a source moving faster than the wave velocity. Consider the example of a jet flying faster than the speed of sound in air. At each point P along the jet’s path, the jet compresses the air in front of it, creating a pressure wave that spreads outward in all directions at the speed of sound, cs.

Figure 46-1 shows the crests of pressure waves emitted at equally spaced times. All such waves will be tangent to a common line, thus creating a bow wave.

Bow Wave

Figure 46-1 Jet (grey) Creating Bow Wave
An Intense Conical Wave With Half-Angle θ

Feynman notes that a bow wave is created whether or not the jet itself makes any sound — sound is created by the mere act of moving through air faster than the speed of sound. The next figure clearly illustrates a bow wave coming from a bullet.
Sock wave from bullet
Bullets are not sources of sound in and of themselves; it is their high-speed passage through air that creates sound waves.

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