Chapter 15 – Characteristics of Waves

8th Grade Science

Mr. Smolka

Section 1 – What are waves?

• Wave – a disturbance (in the force) that moves energy from place to place.

• Medium - Can be ANYTHING a wave will travel through. Some examples are water and air.

• Some waves can travel through and require a medium in order to travel. Some other types of waves do not require a medium to travel. The type of wave that requires a medium to travel in is called a Mechanical Wave.

What causes waves?

• Waves are created when a source of energy causes a medium to vibrate.

• Example: Your vocal chords cause a disturbance (in the force) in air. This disturbance is a wave. Your ears interpret these waves as sounds.

What does a wave look like?

Transverse Waves

• Waves that move matter perpendicular (90 degrees) from the direction of energy.

• The medium will move, but in most transverse waves, it will go right back to where it started as shown here.

• Notice that the blue circle simply moves up and down and not left to right.

• Think of a fishing bob moving up and down in a pond. It barely moves back and forth at all.

Longitudinal (Compression) Waves

• These types of waves move matter back and forth parallel to the direction of the energy.

• The leading edge (front) of the wave, compresses the medium and the trailing edge (back) expands the medium.

Notice how the particle is compressed and expanded, but it doesn’t really move from it’s original location.

Drums compress the air and thenExpand (or decompress) the air Rapidly. This produces sound.

The picture below shows air molecules Being compressed and decompressedBy the vibrating “skin” of the drum head.

Ocean Waves

• It may appear as if ocean waves move in from a great distance, however ocean waves just move the water a few feet in a circular motion.

• Wind pushes the surface of the water down, creating the initial energy required for the wave.

• The wave moves along the surface and deposits the energy as a breaker on the beach.

The surface wave is the only visible portion of the wave. In actually, the circular motion occurs beneath the surface as well and deteriorates as the depth increases. Eventually, the circular motion beneath the surface is slowed and stopped as the wave approaches the beach.

• Water waves are an example of waves that involve a combination of both longitudinal and transverse motions. As a wave travels through the waver, the particles travel in clockwise circles. The radius of the circles decreases as the depth into the water increases.

Credits

• http://www.kettering.edu/~drussell/demos.html