IB Physics 11

Vibrations and Waves

Mr. Jean

January 6th, 2014

The plan:

Video Clip of the dayVideo Clip of the day

Wave interferenceWave interference

In symbolic form

T 1foror

T1f

For a wave, if the distance traveled is a For a wave, if the distance traveled is a

wavelengthwavelength ( (), then the time to travel this ), then the time to travel this

distance is the distance is the periodperiod (T). Then (T). Then

Tv

fv oror

5. TRANSVERSE WAVES

Demonstration: Transverse WavesDemonstration: Transverse Waves

Examples: string musical instruments

ripples on water

electromagnetic waves

6. LONGITUDINAL WAVES

Video - Slinky Longitudinal WavesVideo - Slinky Longitudinal Waves

Parameters

Rarefactions are regions of low density.

Compressions (condensations) are regions of high density.

is the distance between successive rarefactions or successive compressions.

7. INTERFERENCE

Video - Superposition of WavesVideo - Superposition of Waves

SlideSlide - Interference - Interference

Constructive interference occurs when waves are in phase, that is when crests are superimposed and troughs are superimposed.

Destructive interference occurs when waves are out of phase, that is when crests are superimposed with troughs.

Interference is a characteristic of all waves.

Demonstrations with Audacity

Standing Waves

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.

Demo - Rope and strobeDemo - Rope and strobe Demo - Mechanical overhead modelDemo - Mechanical overhead model

There is no vibration at a node. There is maximum vibration at an antinode. is twice the distance between successive

nodes or successive antinodes.

Good Surf Days:

There are two main factors that determine There are two main factors that determine how good the surf will be. how good the surf will be. First, there are the conditions that lead to initial First, there are the conditions that lead to initial

wave formation. These determine the quality wave formation. These determine the quality and size of the ground swell that will hit the and size of the ground swell that will hit the surf spot. surf spot.

The second main factor is the local conditions The second main factor is the local conditions at the surf spot. These have to be right to ensure at the surf spot. These have to be right to ensure the swell can create quality waves. Let's look at the swell can create quality waves. Let's look at both of these factors in turn. both of these factors in turn.

Factors which produce waves:

There are three main factorsThere are three main factors Wind Speed - The greater the wind speed the Wind Speed - The greater the wind speed the

larger the wave. larger the wave. Wind Duration -The longer the wind blows the Wind Duration -The longer the wind blows the

larger the wave. larger the wave. Fetch - The greater the area the wind affects the Fetch - The greater the area the wind affects the

larger the wave. larger the wave.

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

Making the waves: How Waves Are MadeHow Waves Are Made

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

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

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

As the wind continues to blow and the waves generated remain under 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 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 wind will get hold of the small waves much more easily than the calm sea surface. sea surface.

The wave size is dependent on the wind speed generating it. A certain 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 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 the largest waves that can be generated for a given wind speed have formed, the seas are "fully formed." formed, the seas are "fully formed."

The longer period waves are faster and move The longer period waves are faster and move farther ahead of the rest of the slower waves. farther ahead of the rest of the slower waves.

As the waves travel farther away (propagate) As the waves travel farther away (propagate) from the wind source, they start to organise from the wind source, they start to organise themselves into swell lines. themselves into swell lines. Swells are essentially "Wave trains"Swells are essentially "Wave trains"

When everything is right:

Nova Scotia

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

hurricane season. hurricane season.

More properties of waves:

Reflection to fixed ends (Rigid): For fixed end reflection think of the For fixed end reflection think of the

medium as being constrained in its medium as being constrained in its motion. motion.

In the picture to the left you see a In the picture to the left you see a string that is securely fixed to the string that is securely fixed to the wall. wall.

The string (the old medium) is free to The string (the old medium) is free to move up and down, but at the move up and down, but at the boundary where it meets the new boundary where it meets the new medium (the wall) it is constrained – medium (the wall) it is constrained – the string can’t really move up and the string can’t really move up and down like it could before. down like it could before.

In fixed end reflection, the wave that In fixed end reflection, the wave that is reflected back is is reflected back is out ofout of phasephase by by 180180. .

Reflection for non-fixed (not rigid… “open”):

In free end reflection, the In free end reflection, the medium is free to move at medium is free to move at the boundary. The the boundary. The reflected wave will be reflected wave will be in in phasephase. In the drawing on . In the drawing on the right, you see an erect the right, you see an erect pulse traveling into the pulse traveling into the boundary being reflected boundary being reflected with no phase change. The with no phase change. The pulse went in erect and pulse went in erect and came out erect. Water came out erect. Water waves reflecting off a solid waves reflecting off a solid wall are a good example of wall are a good example of free end reflection.free end reflection.

Sound Waves: The disturbance which The disturbance which

travels through air is the travels through air is the compression of air compression of air molecules – they are molecules – they are squeezed together and squeezed together and pulled apart. Sound is a pulled apart. Sound is a series of traveling high series of traveling high pressure and low pressure pressure and low pressure fronts. fronts.

Compr ession

Rar ef act ion

W avelengt h

v

Pressure vs. Time

T ime

Pressure

Wave Videos:

Bright Storm: Bright Storm:

EXCELLENT PHYSICS VIDEOSEXCELLENT PHYSICS VIDEOS http://www.youtube.com/playlist?list=PLF7http://www.youtube.com/playlist?list=PLF7

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