Properties of a Wave

Waves have properties… Just like different substances have different properties –Example: Water has a freezing point, a boiling point, etc

Wave Speed The speed of a wave depends on the properties of the medium in which it travels. THIS IS A PROPERTY OF THE MEDIUM NOT THE WAVE (LIKE BOILING POINT, DENSITY, ETC) The speed of a wave does not depend on the amplitude or frequency.

Examples Solids, liquids and gases are different mediums. The same wave will travel through each medium differently. WHY? Solids have molecules that are tightly packed. This is why sound is distorted in water – the sound waves travel through a different medium than air.

What does the speed of a wave depend on? The mass of the particles of the medium (the greater the mass/inertia of the particles, the SLOWER the wave moves) –Density of material However, the material can also absorb sound instead of “passing it on”

What does the speed of a wave depend on? The forces between the particles of the medium (the greater the force between the particles, the FASTER the wave moves). This is why sound moves faster in solids than liquids. The mass of the particles of the medium (the greater the mass/inertia of the particles, the SLOWER the wave moves) However, the material can also absorb sound instead of “passing it on”

Summary Waves have properties that depend on the material the wave is traveling in Speed is affected by the MEDIUM not the amplitude or frequency –Same material = same speed Higher mass/density/inertia = slower speed –This is why sound waves are distorted in water Some materials also ABSORB energy

Amplitude and Energy The amplitude tells us how much energy is transferred This is why loud sounds are shown with larger waves Although a wave with a larger amplitude transfers more energy, it moves with the same speed as a smaller amplitude wave through a given medium. As long as the material is the same, the speed is the same.

Frequency vs Amplitude Frequency is how many cycles of a wave occur in 1 second Amplitude is the height of the wave Amp and Frequency are not the same!!!

Summary of the Amplitude, Frequency, and Speed Energy transferred depends on the AMPLITUDE (larger amp = more energy) Frequency and speed are not the same Frequency of a wave does not change if the medium changes but speed and amplitude will **(will look at more soon!) Wavelength, frequency, amplitude do not affect speed of waves – only the medium does!

Quick Quiz 1. A teacher attaches a slinky to the wall and begins introducing pulses with different amplitudes. Which of the two pulses (A or B) below will travel from the hand to the wall in the least amount of time? Justify your answer.

Answer Both arrive at the same time! Same material!

2. Which of the two pulses (C or D) will travel faster from the hand to the wall? Justify your answer.

Answer Neither! Both will have the same speed. Since they go the same distance they will arrive at the same time!

3. Two waves are traveling through the same container of nitrogen gas. Wave A has a wavelength of 1.5 m. Wave B has a wavelength of 4.5 m. The speed of wave B must be ________ the speed of wave A. a. one-ninth b. one-third c. the same as d. three times larger than

Answer C.

4. The speed of a wave depends upon (i.e., is causally affected by)... a.the properties of the medium through which the wave travels b.the wavelength of the wave. c. the frequency of the wave. d. both the wavelength and the frequency of the wave.

Answer A

Natural Frequency A pendulum or spring (wave) has a natural frequency – the speed at which it vibrates freely. Everything has a natural frequency – bridges, buildings, etc The frequency depends on the length and tension of the medium. Shorter springs and pendulums have higher natural frequencies than longer ones. Example: Guitar Strings – we shorten the strings to change the note (ie the frequency that the string vibrates naturally) To tune a guitar we tighten the strings or loosen them to get them to vibrate at the correct frequency (note).

Resonance Phenomena that occurs when energy is added to a vibrating system at the same frequency as its natural frequency. Resonance makes the amplitude of the vibrations become larger. The waves interact with each other and “add together”. Example: When you push someone on a swing or use their legs to push at the same frequency (at the natural frequency) they will go higher! This is because energy is added at the natural frequency.

Summary Natural Frequency – the speed at which something vibrates at naturally. It depends on properties of the medium (length, tension). Ex: Guitar strings have a natural frequency that corresponds to a note. Resonance - Phenomena that occurs when energy is added to a vibrating system at the same frequency as its natural frequency. Amplitude (and energy) increases. Ex: Using your legs on a swingset to go higher.

Resonance Examples Bay of Fundy Tides When a choir sings Shattering of glass ( ) Allows us to pick up TV/radio signals (electrical resonance) MRIs – Magnetic Resonance Imaging (magnetic field allows doctors to look inside you!) Your car going at a certain speed starts to vibrate! (poor wheel alignment) Tacoma Narrows Bridge (video from before!) – wind was IN PHASE with natural frequency of bridge