Waves and Vibrations

Waves

Waves are everywhere. Whether we recognize it or not, we encounter waves on a daily basis. Sound waves, visible light waves, radio waves, microwaves, water waves, etc.

Waves

For many people, the first thought concerning waves conjures up a picture of a wave moving across the surface of an ocean, lake, pond or other body of water.

Waves are created by a disturbance (such as a rock thrown into the water, a duck shaking its tail in the water or a boat moving through the water).

Waves

Another picture of waves involves the movement of a slinky or similar set of coils. If a slinky is stretched out from end to end, a wave can be introduced into the slinky by either vibrating the first coil up and down vertically or back and forth horizontally.

Waves

• Waves are everywhere.• In science, the two most common types of

waves are sound waves and light waves.

Definition

• A wave can be described as a disturbance that travels through a medium from one location to another location.

Definition

• A medium is a substance or material which carries the wave.

In the case of the slinky wave, the slinky is the medium. In the case of a sound wave, air is the medium.

Clarification

You have perhaps heard of the phrase news media. The news media refers to the various institutions (newspaper offices, television stations, radio stations, etc.) within our society which carry the news from one location to another. The news moves through the media. The media doesn't make the news and the media isn't the same as the news. The news media is merely the thing that carries the news from its source to various locations. In a similar manner, a wave medium is the substance which carries a wave (or disturbance) from one location to another.

Waves

When a wave is present in a medium (that is, when there is a disturbance moving through a medium), the individual particles of the medium are only temporarily displaced from their rest position. There is always a force acting upon the particles which restores them to their original position.

Waves

In a slinky wave, each coil of the slinky ultimately returns to its original position. In a water wave, each molecule of the water ultimately returns to its original position. And in a stadium wave, each fan in the bleacher ultimately returns to its original position. It is for this reason, that a wave is said to involve the movement of a disturbance without the movement of matter.

Waves

• Waves transport energy, not matter from one place to another.

Check for Understanding

TRUE or FALSE: In order for John to hear Jill, air molecules must

move from the lips of Jill to the ears of John.

Check for Understanding

• False.A sound wave involves the movement of energy

from one location to another, not the movement of material. The air molecules are the particles of the medium, and they are only temporarily displaced, always returning to their original position.

Check for Understanding

Minute after minute, hour after hour, day after day, ocean waves continue to splash onto the shore. Explain why the beach is not completely submerged and why the middle of the ocean has not yet been depleted of its water supply.

Check for Understanding

Ocean waves do not transport water. An ocean wave could not bring a single drop of water from the middle of the ocean to shore. Ocean waves can only bring energy to the shore; the particles of the medium (water) simply oscillate about their fixed position. As such, water does not pile up on the beach.

Types of Waves

• A transverse wave is a wave in which particles of the medium move in a direction perpendicular to the direction which the wave moves.

Types of Waves

• A longitudinal wave is a wave in which particles of the medium move in a direction parallel to the direction which the wave moves.

Types of Waves

Another way to categorize waves is on the basis of their ability or inability to transmit energy through a vacuum (i.e., empty space). Categorizing waves on this basis leads to two notable categories: electromagnetic waves and mechanical waves.

Types of Waves

• An electromagnetic wave is a wave which is capable of transmitting its energy through a vacuum (i.e., empty space).

Electromagnetic waves which are produced on the sun subsequently travel to Earth through the vacuum of outer space.

Types of Waves

• A mechanical wave is a wave which is not capable of transmitting its energy through a vacuum. Mechanical waves require a medium in order to transport their energy from one location to another.

Types of Waves

A sound wave is an example of a mechanical wave. Sound waves are incapable of traveling through a vacuum. Slinky waves, water waves, stadium waves, and jump rope waves are other examples of mechanical waves; each requires some medium in order to exist. A slinky wave requires the coils of the slinky; a water wave requires water; a stadium wave requires fans in a stadium; and a jump rope wave requires a jump rope.

Transverse WavesThe dashed line drawn through

the center of the diagram represents the equilibrium or rest position of the string. This is the position that the string would assume if there were no disturbance moving through it. Once a disturbance is introduced into the string, the particles of the string begin to vibrate upwards and downwards.

Parts of a Wave

Parts of a Wave

The wavelength is another property of a wave which is portrayed in the diagram.

• The wavelength of a wave is simply the length of one complete wave cycle.

• The wavelength can be measured as the distance from crest to crest or from trough to trough.

Parts of a Wave

Check for Understanding

• The wavelength of the wave in the diagram above is given by letter ______.

• The amplitude of the wave in the diagram above is given by letter _____.

Check for Understanding

The wavelength is A.

The amplitude is D.

Check for Understanding

Indicate the interval which represents one full wavelength.

a. A to C b. B to Dc. A to Gd. C to G

Check for Understanding

Answer: DThe wavelength is the distance from crest to

crest, trough to trough, or from a point on one wave cycle to the corresponding point on the next adjacent wave cycle.

Wave Properties

• The frequency of a wave refers to how often the particles of the medium vibrate when a wave passes through the medium.

Clarification

Frequency is a part of our common, everyday language. For example, it is not uncommon to hear a question like "How frequently do you mow the lawn during the summer months?" Of course the question is an inquiry about how often the lawn is mowed and the answer is usually given in the form of "1 time per week."

Wave Properties

• In mathematical terms, the frequency is the number of complete vibrational cycles of a medium per a given amount of time.

Wave Properties

Given this definition, it is reasonable that the quantity frequency would have units of cycles/second, waves/second, vibrations/second, or something/second.

• Another unit for frequency is the Hertz (abbreviated Hz) where 1 Hz is equivalent to 1 cycle/second.

Clarification

If a coil of slinky makes 2 vibrational cycles in one second, then the frequency is 2 Hz. If a coil of slinky makes 3 vibrational cycles in one second, then the frequency is 3 Hz. And if a coil makes 8 vibrational cycles in 4 seconds, then the frequency is 2 Hz (8 cycles/4 s = 2 cycles/s).

Wave Properties

The quantity frequency is often confused with the quantity period. Period refers to the time which it takes to do something.

When an event occurs repeatedly, then we say that the event is periodic and refer to the time for the event to repeat itself as the period.

Wave Properties

• The period of a wave is the time for a particle on a medium to make one complete vibrational cycle. Period, being a time, is measured in units of time such as seconds, hours, days or years.

Wave Properties

• Frequency refers to how often something happens. Period refers to the time it takes something to happen.

Clarification

Frequency and period are distinctly different, yet related, quantities.

As an example of the distinction and the relatedness of frequency and period, consider a woodpecker that drums upon a tree at a periodic rate. If the woodpecker drums upon a tree 2 times in one second, then the frequency is 2 Hz. Each drum must endure for one-half a second, so the period is 0.5 s. If the woodpecker drums upon a tree 4 times in one second, then the frequency is 4 Hz; each drum must endure for one-fourth a second, so the period is 0.25 s.

Equations

Wave Properties

• The amount of energy carried by a wave is related to the amplitude of the wave.

• A high energy wave is characterized by a high amplitude; a low energy wave is characterized by a low amplitude.

Wave Speed

If one watches an ocean wave moving along the medium (the ocean water), one can observe that the crest of the wave is moving from one location to another over a given interval of time. The crest is observed to cover distance. The speed of an object refers to how fast an object is moving and is usually expressed as the distance traveled per time of travel.

Wave Speed

• In the case of a wave, the speed is the distance traveled by a given point on the wave (such as a crest) in a given interval of time. In equation form,

Check for Understanding

Noah stands 170 meters away from a steep canyon wall. He shouts and hears the echo of his voice one second later. What is the speed of the wave?

Because the sound echoes, the distance is 2x170 or 340 m.

Speed = 340 m/ 1 sSpeed = 340 m/s

Equations

Speed = Wavelength • Frequency• The above equation is known as the wave

equation. It states the mathematical relationship between the speed (v) of a wave and its wavelength (λ) and frequency (f). Using the symbols v,λ , and f, the equation can be rewritten as

v = f • λ

Wave Interference

What happens when two waves meet while they travel through the same medium? What affect will the meeting of the waves have upon the appearance of the medium? Will the two waves bounce off each other upon meeting (much like two billiard balls would) or will the two waves pass through each other?

Wave Interference

• Wave interference is the phenomenon which occurs when two waves meet while traveling along the same medium.

Wave Interference

• Constructive interference is a type of interference which occurs at any location along the medium where the two interfering waves have a displacement in the same direction.

Constructive Interference

Wave Interference

• Destructive interference is a type of interference which occurs at any location along the medium where the two interfering waves have a displacement in the opposite direction.

Destructive Interference

Wave Interference

• Wave interference is temporary. After the waves meet and interfere, they will continue on as if nothing happened.