(MOW) Waves Lesson

Waves

We have seen them before, crashing against the beach, carrying a surfer-dude as he hangs ten or roll by bobbing a buoy. We are already somewhat familiar with waves, but now we will look a bit closer and study the ins and outs of an ocean wave. What is a wave? An ocean wave is a rhythmic movement that carries energy throughout sea water. It is important to understand that water is not carried in the wave – it is only energy. The crest is the highest point of a wave and the trough is the lowest point. Amplitude is the vertical distance between a crest and a trough. Wavelength is the horizontal distance between crest and equilibrium or trough and equilibrium. The frequency is how often the crest or trough of a wave passes a certain point in a given period of time. There is a relationship between the wavelength and the frequency of a wave which is presented in the formula below. With this formula, one can determine the speed or velocity of a wave.

Speed = frequency X wavelengthv = fλ 

Let's try working out a few wave problems!

First, we need to discuss units. Frequency is usually expressed in the units of waves/s. Wavelength is usually expressed in meters and speed is usually expressed in m/s.

Now, a practice problem!

An ocean wave has a wavelength of 10 meters and a frequency of 0.31 waves/second. What is the speed of the wave?

First, we need to write down what we know:

λ = 10 meters

f = 0.31 waves/second

Speed = _____

 

Now, let's look at our formula: speed = fλ . Let's put in our numbers.

Speed = (0.31 waves/second)(10 meters)

Speed = 3.1 m/s

 

Let's try another one!

In a stormy sea, 3 waves pass a fixed point every second and the waves are 13 meters apart. What is the speed of the wave?

First, we will write what we know, just like last time. The statement 3 waves pass a fixed point every second is telling us the frequency. So we will write it as 3 waves/second. Let's write the rest of what we know:

λ = 13 meters

f = 3 waves/second

Speed = ______

 

Now, we will use our wave speed formula again to calculate the speed:

Speed = fλ

Speed = (3 waves/second)(13 meters)

Speed = 39 m/s

 

Now, let's try a problem where we are not solving for speed.

A buoy bobs up and down in the ocean. The waves have a wavelength of 2.5 m, and they pass the buoy at a speed of 4.0 m/s. What is the frequency of the waves?

First, we will write what we know. This does not change, regardless of what the problem is asking for.

Speed = 4.0 m/s

λ = 2.5 m

f = ______

 

Now, we will use the wave speed formula to calculate the speed, but we will need to rearrange it. There is a trick to these formulas, which keeps you from having to do a lot of algebra to rearrange the formula. The trick is to put the formula into a triangle, like this:

For this triangle, you put the two parts that are together in the formula on the bottom and speed on the top. Then, you can just fill in your numbers:

Now, we can do what is indicated in the triangle. In this case, we have

4.0 m/s               = 4 waves/second

2.5 meters

 

What causes waves?

Waves are caused by winds or a disturbance in the water such as an earthquake. The size of a wave depends on three factors:  the distance along open water – which is known as the fetch, the strength of the blowing wind and the length of time the wind blows. The biggest waves on the planet are where the wind blows consistently – typically south of the Indian Ocean. Wind waves are caused as the wind blows across a body of water and the resulting friction causes the water to move. The wind transmits energy to water to create a wave. As the wind continues to blow, the wave increases in height.

Earthquakes can cause large ocean waves by transmitting the energy from the moving earth to the water. The resulting wave is called a tsunami. A tsunami in deep water may have a very small wavelength, but when the wave reaches the shore, the friction from the ocean floor causes the tsunami to grow suddenly and crash on the beach at heights reaching 20 meters. 

How do waves affect the shore? Waves usually approach the shore at an angle and thereby shape a beach by eroding the shore at one end and building it up at the other end. The breaker of a wave is the growing swell that approaches the shore, surf zone, and begins to break. Every time a wave breaks on the shore of a beach it carries sand to and from the beach causing different patterns and shapes unique to each shore. Longshore drift is caused when sand is carried by the water along the beach to create a sandbar.

What are the different types of waves? There are several different types of waves. Seiches are standing waves that slosh to and fro in an enclosed or partially enclosed body of water such as deep lakes reservoirs and seas. A swell is a long stable surface wave formed by distant tropical storms and stable wind systems. A short steep wave with a high frequency is refereed to as a chop. A capillary wave is a wave that travels between two fluids and are most commonly observed as ripples. A tsunami is a wave that is created when there is a major disturbance in the open ocean. Tides >are the waves that are generated by the gravitational pull of the sun and the moon. The chart below outlines the major characteristic of each wave type.

Wave	Typical Wavelength	Disturbing Force
Capillary	Up to 1.73 cm	Usually wind
Chop	60 – 150 m	Wind over the ocean
Seiche	Large variable; a function of ocean basin size	Change in atmospheric pressure, or storm surge
Tsunami	200km	Faulting of seafloor, volcanic eruption, landslide
Tide	Half of Earth's circumference	Gravitational attraction between the Earth, Sun and Moon; rotation of the Earth

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