2.4 Waves

2.4 Waves (8.2.4)

Explore this Phenomenon

When you drop a rock in water, waves form on the surface of the water.

1. What observations can you describe about the waves created by the rock?
2. What questions can you ask that would help explain why this happens?
3. How would you explain what is happening?

Waves
Waves are everywhere, but what exactly is a wave?  Waves are a disturbance moving though space.   There are two types of waves, mechanical waves that require a medium, and electromagnetic waves that can travel though a vacuum, so they don't need a medium. Medium is a substance which energy can move through.  Waves have characteristics, properties, and behaviors that can be used to make observations and predictions about what waves can and cannot do. Identifying wave models allows us to understand wave characteristics, properties, and behavior.  

Waves are regular patterns of motion. They transfer energy as they move through various materials like wood, water and even air. The material they transfer energy through is called the medium. Waves do not carry or transfer matter but they do transfer energy from one place to another. When a rock hits the water, the kinetic energy from the rock is carried through the water. The ripples provide evidence that energy is being transferred from one place in the water to another, but the actual water molecules do not move the same distance the wave moves.

How does the flag blowing in the breeze resemble a wave?

Wave Model
Wave amplitude is the maximum distance the particles of the medium move from their resting positions when a wave passes through. The resting position of a particle is where the particle would be if the wave had not traveled through the material. The crest is the highest point of a wave and the trough is the lowest point. Wavelength is the distance from one point on a wave to the same point on the next wave. In the picture it shows a model that represents both the amplitude and the wavelength of a wave.

There are two types of wave movement: Longitudinal Waves and Transverse Waves.  Longitudinal, are also called compression waves.  Sound travels in a longitudinal waves.  These sound pressure waves are generated by the compression and expansion of molecules.  Sound pushes the air molecules forward.  The air molecules become compresses in front of the sound, and less dense behind the sound. 

A transverse wave is a wave in which particles of the medium vibrate perpendicular to the direction that the wave travels.  

In models these Longitudinal and transverse waves look different, but mathematically they are the same.

Energy and Amplitude
A wave that is transferring more energy will have a larger amplitude than a wave that is transferring less energy. A sound wave with higher amplitude is transferring more energy and will sound louder than a sound wave with lower amplitude. 

Frequency- how many cycles per second.   Velocity- how fast the wave is moving to the right

Some interesting facts about sound waves:

-At sea level and room temperature, sound travels at 700 mph

-The higher the temperature the faster sound travels- because the air is less dense.  

-Sound travels more slowly through humid air because it is more dense, and it has to travel through two "media". 

- On a cold day sound travels further but slower.  It bounces off the inversion layer.  However,  when it is snowing,  air near the surface is warmer than the air above.  Sound curves upward.  Also the snow absorbs the sound.  So, snowy days are more quiet. 

Think back to the original question about the ripples in the picture. The ripples were relatively low-amplitude waves with  comparatively little energy because they were caused by a smaller rock. How would the amplitude of the ripples be different if a bigger rock were thrown into the pond? The waves would be higher amplitude waves and would have a greater amount of energy than the waves caused by the smaller pebble.

Below are three different sketches of waves. What do you notice that is similar and different about each of the waves in the sketches?

Describe the similarities and differences between the waves using the words amplitude and energy.

• Which wave has more energy?
• Which wave has less?
• How do you know?

A student wanted to use a rope as a model to investigate how the amplitude of a wave is related to the energy in the wave. The student tied a rope to a tree and then moved the rope up and down at two different heights.

The student found out that the higher you move the rope up and down, the more energy it requires, therefore higher amplitude requires more energy. Watch this video to sum up Wave energy.

https://www.youtube.com/watch?v=abVdZGBRZ7c

To investigate how amplitude is related to the energy in a wave, visit the following digital wave simulator. http://go.uen.org/b12

Putting It Together

1. Explain how your understanding of the waves has changed.
2. Think of another phenomenon that applies to the concept of waves.
3. Explain what is going on with the ripples based on what you have learned in this section.