Vibrations and Waves, Behavior PHYSICS Vibrations and Waves, Behavior

Vibrations and Wave Types Vibration: in a general sense, anything that switches back and forth, to and fro, side to side, in and out, off and on, loud and soft, or up and down is vibrating. A vibration is a wiggle in time. Wave: a wiggle in both space and time is a wave. A wave extends from one place to another. Vibrations and waves: the source of all waves is something that is vibrating. Waves are propagations of vibrations throughout space.

1. VIBRATION OF A PENDULUM What does the period (T) depend upon? Length of the pendulum (l). Acceleration due to gravity (g). Period does not depend upon the bob mass or the amplitude of the swing. Vibration of a pendulum. The to-and-fro vibratory motion is also called oscillatory motion (or oscillation).

2. Mechanical Waves Waves that require a medium. Transverse, Longitudinal, and Combination waves. Energy is transferred from one particle to the next as the wave travels through the medium.

3. Sound Waves (Longitudinal) Molecules in the air vibrate about some average position creating the compressions and rarefactions. We call the frequency of sound the pitch.

3. Electromagnetic Waves Electromagnetic waves are special in the fact that they do not need a medium to propagate through. But what is creating the disturbance? What is emitting this energy? ELECTRONS

Making electromagnetic waves A vibrating electric charge creates an electromagnetic wave that travels outward in all directions from the charge. Only one direction is shown here.

The Electromagnetic Spectrum Electromagnetic waves are described by different names depending on their frequency and wavelength.

Electromagnetic Waves Electromagnetic waves are everywhere. Light is only a small part of them Radios TVs Microwaves Light (Visible/UV/InfraRed) Radiation Lasers CD/DVD players X-Rays

Wave Behavior Refraction Reflection Diffraction Interference Constructive Resonance Standing Waves Destructive Sound Beats Doppler Effect Polarization

Reflection and Refraction http://www.schooltube.com/video/9b9e3e3cf65c21bb4c3d/Bill-Nye-The-Science-Guy-on-Light-Bending-Bouncing-Full-Clip

Law of Reflection The angle of incidence equals the angle of Reflection: Normal Incident ray Reflected ray Angle of incidence Angle of reflection Mirror

Refraction of Ocean Waves Wave fronts are shown in white heading toward the beach. The water gets shallow at the bottom first, which causes the waves to slow down and bend, and the wavelength to decrease. By the time the waves reach shore, they’re nearly parallel to the shoreline. The effect can even be seen on islands, where winds nearly wrap around it and come toward the island from all sides.

Law of Refraction A wave entering a more dense material slows down and bends toward the normal. A entering a less dense material speeds up and bends away from the normal.

Diffraction Pics When waves pass a barrier they curve around it slightly. When they pass through a small opening, they spread out almost as if they had come from a point source. These effects happen for any type of wave: water; sound; light; seismic waves, etc. http://www.teachertube.com/viewVideo.php?video_id=123581

Wave Interference When two wave pass each other their superposition causes reinforcement or cancellation.

Constructive interference Reinforcement when the crest of one wave overlaps the crest of another Their individual effects adds together, resulting in a wave increased in amplitude

Destructive Interference Cancellation when crest of one wave overlaps trough of another reducing their individual effects Water waves show these best Out of phase- the crest of one wave arrives at a point at the same time as a trough of the second wave arrives, effects cancel each other In phase- two waves crests and troughs arrive at a place at the same time, effects reinforce each other

Sound Wave Interference Interference occurs when two sounds of difference frequency are heard superposed. Constructive interference causes louder sound and destructive inference cause fainter sound. This alternating pattern produces a beat. A piano tuners listens for beats to disappear.

Beats f beat = | f1 - f2 | f combo = ( f1 + f2 ) / 2 soft loud We’ve seen how many frequencies can combine to produce a complicated waveform. If two frequencies that are nearly the same combine, a phenomenon called beats occurs. The resulting waveform increases and decreases in amplitude in a periodic way, i.e., the sound gets louder and softer in a regular pattern. Hear Beats When two waves differ slightly in frequency, they are alternately in phase and out of phase. Suppose the two original waves have frequencies f1 and f2. Then their superposition (below) will have their average frequency and will get louder and softer with a frequency of | f1 - f2 |. f beat = | f1 - f2 | f combo = ( f1 + f2 ) / 2 Beats Animation (click on “start simulation”) soft loud

Water Wave Interference Left side is theoretical drawing of an interference pattern. Right side is the actual interference pattern.

Resonance Objects that oscillate or vibrate tend to do so at a particular frequency called the natural frequency. If a periodic force, like an occasional push, matches the period of one of the masses, this is called resonance, and the mass’s amplitude will grow. M (continued) m Resonance Animation

Resonance Question Explain how you could get a 700 lb wrecking ball swing with a large amplitude only by pulling on it with a scrawny piece of dental floss. answer: Give the ball a little tug, as much as you can without breaking the floss. The ball with barely budge. Continue giving it tugs every time the ball is at its closest to you. If you match the natural frequency of the ball, its amplitude will slowly increase to the desired amount. In this way you are adding energy to the ball very slowly. Wrecking Ball

Tacoma Narrows Bridge Even bridges have resonant (natural) frequencies. The Tacoma Narrows bridge in Washington state collapsed due to the complicated effects of wind. One day in 1940 the wind blew at just the right speed. The wind was like Jane pushing Tarzan, and the bridge was like Tarzan. The bridge twisted and shook violently for about an hour. Eventually, the vibrations caused the by wind grew in amplitude until the bridge was destroyed. Click the pic to see the MPEG video clip.

Resonance & Standing Waves Occurs when a wave reflects upon itself and interference causes the pattern Nodes remain stationary Anti nodes-occur half way between nodes

Standing Waves and Harmonics Change the frequency in a standing wave and more nodes/antinodes appear in the event; a Harmonic.

8. DOPPLER EFFECT Refers to the change in frequency when there is relative motion between an observer of waves and the source of the waves Doppler with Sound Doppler with Water

The Doppler effect- Change in frequency due to the source or receiver greater the speed the source, greater the Doppler effect Stationary bug Bug swimming Blue Shift-increase in frequency Red Shift- Decrease in frequency

Doppler Shift Gives Radial Velocity True Velocity Tangential Velocity Radar Radial Velocity

Bow waves V-shaped pattern made by overlapping crest

Shock Waves Produced by supersonic aircraft, three-dimensional cone shaped Sonic boom – sharp crack heard when conical shell of compressed air that sweeps behind a supersonic aircraft reaches listeners on the ground below.

A Sonic Boom Credit: Ensign John Gay, USS Constellation, US Navy Supersonic! The shock wave energy absorbed by water vapor when released causes The shock wave energy absorbed by water vapor when released causes it to condense in response to the drop in pressure. A Sonic Boom Credit: Ensign John Gay, USS Constellation, US Navy

Polarization

Applications of Polarizers Polarizing sunglasses are used to reduce the glare of reflected light The LCD (liquid crystal diode) screen on a laptop computer uses polarized light to make pictures. That’s why with polarizing sunglasses on you can’t read an LCD display!

Review Questions

1. Doubling the mass of a simple pendulum undergoing small oscillations does what to the period of the pendulum? (a) cuts it in half (b) increases it by the square of 2 (c) nothing (d) doubles it (c) nothing

2. What dictates the frequency of a sound wave? (a) wavelength (b) medium (c) source of the sound (d) speed (e) amplitude (c) source of the sound

3. What determines the speed of a wave? (a) the frequency (b) the wavelength (c) the amplitude (d) the period (e) the medium of transmission (e) the medium of transmission

4. A skipper on a boat notices wave crests passing his anchor chain every 5 seconds. If the wave crests are 15 m apart, what is the speed of the water waves in m/s? (a) 5 (d) 10 (b) 15 (e) 3 (c) 75 (e) 3

5. For a medium transmitting a longitudinal wave, the areas of the medium where the density of the medium is temporarily increased are called... (a) rarefactions (b) compressions (c) density holes (b) compressions

6. When you move away from a fixed source of sound, the frequency of the sound you hear... (a) is greater than what the source emits (b) is less than what the source emits (c) is the same as what the source emits (b) is less than what the source emits

7. Sonic booms from a plane are produced... (a) because the plane breaks through the sound barrier. (b) when the plane reaches the speed of sound. (c) by the plane traveling faster than the speed of sound. (d) by the plane traveling slower than the speed of sound. (c) by the plane traveling faster than the speed of sound