Physics 11 n Mr. Jean n May 28 th, 2013

The plan: n Review of yesterday u Constructive & Destructive Interference u Fixed Ends u Open ends u Waves when they meet n What makes a good surfing day n Visible spectrum n Doppler Effect n Shock Waves n Return Pendulum Lab n Speed of sound Lab soon!

Constructive & Destructive Interference:

When waves meet:

Standing Waves:

Waves and Wave Length: n The standing wave to the left represents one half of the wavelength of the wave or ½. n This would be a complete wave cycle or 2/2 or 1. n This would be 3/2 or 1 ½ wave.

Fundamental Frequencies:

Interference is a characteristic of all waves.

Standing Waves n n When two sets of waves of equal amplitude and wavelength pass through each other in opposite directions, it is possible to create an interference pattern that looks like a wave that is “standing still.” It is a changing interference pattern. n Demo - Rope and strobe n Demo - Mechanical overhead model

Visible Spectrum:

DOPPLER EFFECT n n Refers to the change in frequency when there is relative motion between an observer of waves and the source of the waves n Doppler Movie - URL URL

When a source of waves and an observer of waves are getting closer together, the observer of the waves “sees” a frequency for the waves that is higher than the emitted frequency. When a source of waves and an observer of waves are getting farther apart, the observer of the waves “sees” a frequency for the waves that is lower than the emitted frequency.

n All waves exhibit the Doppler effect. n n A particularly interesting example is used by astronomers to determine if light emitting objects (such as stars) are getting closer to us or farther away. n n On average most stars are moving farther away, and their light spectra are “red shifted.”

Red Shift: Moving Away from source Actual Visual Spectrum Observed Visual Spectrum

Blue Shift: Moving towards source Actual Visual Spectrum Observed Visual Spectrum

n Radar bounced off a spinning planet can exhibit a Doppler effect and lead to a determination of the spin rate of the planet. n This was used to discover that Venus has a retrograde spin. n Of course police use the Doppler effect to catch speeding motorists.

BOW WAVES n n Waves in front of moving object pile up. n n The familiar bow wave generated by a speedboat knifing through the water is a non-periodic wave produced by the overlapping of many periodic circular waves. It has a constant shape.

Video Demonstration: n Demo 2.1 – Waves & Wake from Phys123 u Next Day

SHOCK WAVES n n Just as circular waves move out from a swimming bug, spherical waves move out from a flying object. If the object flies faster than the waves, the result is a cone-shaped shock wave. n n There are two booms, one from the front of the flying object and one from the back.

- slower than the speed of sound n Subsonic n Supersonic - faster than the speed of sound n n Mach Number = speed of sound speed of object

Good Surf Days: n There are two main factors that determine how good the surf will be. u First, there are the conditions that lead to initial wave formation. These determine the quality and size of the ground swell that will hit the surf spot. u The second main factor is the local conditions at the surf spot. These have to be right to ensure the swell can create quality waves. Let's look at both of these factors in turn.

Factors which produce waves: n There are three main factors u Wind Speed - The greater the wind speed the larger the wave. u Wind Duration -The longer the wind blows the larger the wave. u Fetch - The greater the area the wind affects the larger the wave.

n Closely packed isobars (areas of equal pressure shown on a weather map) mean strong winds. It is these deep low pressures that generate the big swells. The swell will be bigger and last longer...

Making the waves: n How Waves Are Made u Waves are generated by wind. Offshore storms generate winds which blow on the surface of the sea and create ripples, much in the same way as the ripples in your post surf cuppa are made when you blow on it to cool it down.

n Where it all begins: u Offshore storms generate winds. u The ocean would be almost completely flat without wind.

n The stronger and longer the wind blows, the more effect it has on these ripples and the larger they become. Initially the waves will just be small chop, but these will soon increase in size.

n As the wind continues to blow and the waves generated remain under the influence of the wind, the smaller waves will increase in size. The wind will get hold of the small waves much more easily than the calm sea surface. n The wave size is dependent on the wind speed generating it. A certain wind speed will only be able to generate a wave of certain size. Once the largest waves that can be generated for a given wind speed have formed, the seas are "fully formed."

n The longer period waves are faster and move farther ahead of the rest of the slower waves. n As the waves travel farther away (propagate) from the wind source, they start to organise themselves into swell lines. u Swells are essentially "Wave trains"

When everything is right:

Nova Scotia n Nova Scotia is quickly becoming a surf destination for surfers around the world. u Especially late August into October for hurricane season.

Speed of sound lab soon: