Physical ScienceCh. 10: Waves

• A wave is a rhythmic disturbance which carries energy through space or matter.

• Many waves, but not all, require a medium to move between points, these are called mechanical waves.

A medium is a material through which a wave transfers energy.

Which of the following would be mechanical waves?

• Light• Sound• Ultraviolet (U.V.) waves

Types of Waves

• Based on the way they move, there are 2 different types of waves:

1. Transverse Waves

2. Compressional Waves

Transverse Waves• In a transverse wave, the wave's medium

move perpendicular to the direction of the wave. Ex. ocean waves, light waves

• For example, as the wave moves left to right, the medium moves up and down.

• Say you're waterskiing down at the lake and you wipe out. As you float in the water and a wave goes by, what does your body do as the wave passes?

Parts of a Transverse WaveThere are 4 main parts to a transverse wave:

• Crest- the highest point on a transverse wave

• Trough- the lowest point on a transverse wave

 

• Amplitude- the distance from either the crest or trough to the resting point of the wave

• Wavelength- distance from crest to crest or trough to trough on a wave

Wave Frequency• Frequency is a property of a wave, not a

part of one.

• Frequency is the number of waves which pass a given point in 1 second.

• Hertz (Hz) is the SI unit for frequency.

1 Hz = 1 wave passing a given point in

1 second

• So if you’re sitting on a boat dock, and 1 waves washes up against the dock every second, then the wave frequency is 1 Hz.

• What if 2 waves hit per second?

• What if 1 waves washes past every 2 seconds?

• When you tune in to a radio station, the numbers on the dial represent the frequency that a particular station is sending their signal out at.

FM – MHz (megahertz), 98.9 = 98,900,000 Hz

AM – KHz (kilohertz), 610 = 610,000 Hz

• Which band, AM or FM, usually has better sound quality?

• Why do you think this is so?

• The period of a wave is the amount of time it takes one wavelength to pass a stationary point. The wave below has a period of about 2 s.

Compressional Waves A compressional wave vibrates in the

direction that the wave is travelling. These waves are also known as longitudinal waves.

Examples: slinky, sound waves

• The 2 main parts of a compressional wave are:A. Compression- area of higher density

within the wave

B. Rarefaction- area of lower density

within the wave pg. 296

Sound WavesSound Waves

Sound waves are a type of compressional wave which require a medium to travel.

Sound waves travel at about 750 mi/hr. This speed is effected slightly by the type of medium, temperature of medium, and elevation.

• When an object such as a jet travels at a speed greater than 750 mi/hr it is said to be supersonic.

• What happens when an object travels at supersonic speeds?

• The ThrustSSC (supersonic car) broke the sound barrier on land, going 760 mi/hr on October 15, 1997.

• The photo below shows the shock wave produced when the car broke the sound barrier.

• A shock wave is a rapid change in air pressure caused by a strong disturbance. The shock wave moves outward from the disturbance.

• Large explosions and sonic booms can produce strong shock waves.

• Think of a rock being thrown into a pond and producing outward ripples. A strong disturbance can produce the same type of ripples, only in the air.

• A boat moving though water can produce waves which are more bunched up in front of the boat in the direction that it is moving. So there are more waves per second (frequency) out in front of the boat.

• Let’s say that the boat travels fast enough for the waves being produced to start to overlap each other, so that a stationary point (x) is hit by more than 1 wave at a time.

• This would cause that point to experience much more wave energy all at once.

• This is fairly easy for the boat to accomplish since all it would have to do is move faster than the waves, which travel relatively slowly. However, the same thing can happen in the air if the object (a jet) can travel faster than the waves in the air (sound waves).

• When the tremendous amount of energy required for this to occur produces waves, and these waves start overlapping and striking a point several at a time, then a sonic boom is experienced.

• Just like the boat produces continual waves, so does the jet. So as long as it stays above 750 mi/hr, a continual sonic boom will be produced following along slightly behind the jet.

• If you are on the ground as the jet goes over, you will here only a single boom though, as the shock wave moves past you following the jet.

• How far away can a sonic boom be felt or heard?

• What happens as the jet drops back below 750 mi/hr?

• Wave speed is the rate at which a wave is traveling, and is measured in m/s.

• The formula for wave speed is:

frequency (Hz) x wavelength (m)

• If a sound wave (335 m/s) has a frequency

of 67 Hz, what is its wavelength?

Velocity = Frequency x Wavelength

so

Wavelength = Velocity / Frequency

= 335 m/s / 67 Hz

= 5 m

Reflection and Refraction• Reflection is where a wave strikes an object and

bounces off.• When a wave strikes a flat reflective surface it will

reflect back at the same angle that it struck the surface at. This is called………

The Law of Reflection

• The law of reflection states that the angle of incidence is equal to the angle of reflection on a flat reflective surface.

Examples of the Law of Reflection:

• How is the Law of Reflection used in laser security systems?

• In addition to reflection a wave can experience refraction.

Refraction is the bending of a wave as it enters a different medium, due to a change in speed.

• Because of light refraction, a person bow fishing would not aim directly at the fish. Where should they aim? Hmmmm………..

Diffraction• Diffraction is the bending of a wave, around a

barrier.

Ex.: Light in the storage room partially illuminating the classroom.

• Sometimes 2 or more waves may come in contact with each other and overlap to form a new wave. This is called interference

• As these waves overlap, they can either multiply and enhance each other, or cancel each other out.

• Constructive interference occurs when the crests and troughs coincide and produce a larger amplitude (greater overall combined energy)

• Examples of constructive interference:

– 2 people trampolining at the same time

– 2 or more people singing together (in sync)

• Destructive interference occurs when the crests of one wave coincide with the troughs of another, creating a smaller amplitude.

• Examples: - excessive echoing in a stadium or arena,

which distorts the P.A. system

- 2 or more people singing together (out of sync)

• Singers doing the National Anthem will often wear earplugs because the strong echo coming back through the stadium can interrupt their timing of the song.

• Many locations, such as auditoriums and modern stadiums are specifically designed to produce constructive interference.

• Waves created through and along the crust of the earth by shifting or breaking tectonic plates are called seismic waves.

• These waves are comprised of both transverse and compressional waves, and can create very damaging earthquakes.

• The San Andreas Fault line in California and the New Madrid Fault in SE Missouri are 2 locations where plates come together, and are therefore more likely to slide or break.

• Underwater earthquakes can often produce tsunamis, giant ocean waves.

• When Doug was driving from Kansas City to Denver, he crossed over from the central time zone to the mountain time zone. When he arrived he went to change the time on his cell phone, but found that it had already changed. How did that happen?

• As Carl was driving through the Ozarks, he found that he could not tune into a radio station and maintain a signal, even for stations nearby. Why was this?

• The South American archer fish shoots a stream of water which knocks unsuspecting prey down into the water where they are eaten. Would the fish in the picture need to aim higher or lower than where it perceives the bug to be?

• A .50 caliber sniper rifle fires a projectile at 1,002 m/s. If the rifle is fired at a target and the bullet takes 2 seconds to reach the target, how long after the bullet strikes will it take before the crack of the gun is heard? (speed of sound = 334 m/s)

• Because sound travels at 750 mi/hr, you can calculate that it will travel about 1 mile in 5 seconds.

1 hr. = 3,600 seconds,

so 750/3,600 = .2 mi/second

So in 5 seconds (5 x .2) sound will travel 1 mile.

• An F-22 Raptor passing overhead breaks the sound barrier, producing a sonic boom. If you hear the boom 1 minute after the jet passed by, how high was the jet flying?