Lecture on Waves and the Speed of Sound

Introduction to Waves

• Oscillations can travel through a medium (like air or water) as waves.
• Waves are mechanical disturbances not carrying matter.
• Gravity waves are an example of advanced physics experiments today.

Historical Context: Measuring the Speed of Sound

• 17th-century state-of-the-art experiments included measuring the speed of sound.
• Isaac Newton attempted to measure the speed of sound using a pendulum and echo in a corridor at Trinity College.
• Pendulum Method: Adjust pendulum length to match the time taken for sound to travel down a corridor and back.

Understanding Sound

• Sound is a type of wave: a disturbance that travels at a definite speed.
• Sound travels fast but not infinitely; this is evident in echoes and the time difference between seeing lightning and hearing thunder.

Waves in Nature

• Waves can be found in water, light, sound, and pressure.
• Waves are common natural phenomena and can be human-made (e.g., crowd waves).
• Historical examples include human shockwaves and nuclear shockwaves.

Mechanical Waves

• Mechanical waves arise when oscillators are linked, and disturbance moves through them.
• Medium Dependency: Wave speed depends on the medium and how tightly linked matter is.
• Example: In crystals, mechanical waves pass atom to atom like masses connected by springs.

Musical Sound Waves

• Musical instruments generate continuous sound waves through air.
• Waves have characteristics such as amplitude (size of disturbance) and frequency (inverse of period).
• Sound Properties: Tone depends on frequency, loudness on amplitude.

Wave Equations and Properties

• Wavelength: Distance between compressions, equals period times wave speed.
• For sound waves, speed remains constant in air regardless of frequency, wavelength, or amplitude.
• Water waves differ in speed, with long waves traveling faster than short ones in deep water.

Types of Waves

• Longitudinal Waves: Oscillations in the direction of wave travel.
• Transverse Waves: Oscillations perpendicular to wave travel.
• Water waves involve circular motion, not purely longitudinal or transverse.

Factors Affecting Wave Speed

• For masses and springs, wave speed depends on stiffness, mass, and equilibrium distance.
• For water waves, gravity and wavelength affect speed.

Sound Wave Dynamics

• Generated by vibrating objects, compressing, and expanding air density.
• Speed depends on air pressure and density.

Newton's Theory of Sound Speed

• Newton calculated sound speed using atmospheric pressure and air density.
• His initial result was 979 feet per second.
• Experimentally measured as 1,142 feet/second by William Durham.

Resolving Discrepancies in Measurements

• Newton's calculated and measured speeds differed by 20%.
• Newton's attempt to reconcile this involved adjusting for molecular space and water vapor.
• Final adjusted speed: 1,143 feet/second.
• Later understood: Air compression heat affects speed, known posthumously.

Closing Remarks

• Newton's adjustments show early scientific method and reasoning.
• For more educational resources, visit www.learner.org.