Wave Motion

Longitudinal Waves

Different kinds of sounds

Frequency of vibration and pitch:

Large objects, like one of the strings of a double bass, vibrate slowly when plucked. Its frequency of vibration is low and only a small number of sound waves are produced each second. The note produced by the string will have a low pitch.
It is possible for you to see a picture that represents these waves using a piece of apparatus called a cathode ray oscilloscope (CRO).
Small objects, like a violin string, vibrate more quickly when plucked. Its frequency of vibration is higher so more sound waves are produced each second. The note produced by this string will have a higher pitch.
The frequency of an object or wave is the number of complete vibrations it performs each second. It is measured in hertz (Hz) where 1 Hz is one vibration per second.

Musical instruments:

Like all musical instruments, the double bass and the violin can produce notes with a wide range of frequencies and pitch. The changes in frequency produced by the vibrating strings are achieved by altering.
• The length of the string by changing the positions of the fingers on the frets board. The longer the vibrating string, the lower the pitch of the note produced.
• The thickness of the string. The thicker the string, the lower the pitch of the note produced.
• The tension of the string. The greater the tension, the higher the pitch of the note produced by the string.

Changes in the frequencies of the notes produced by wind instruments are achieved by altering the lengths of vibrating air columns.

SOUND

We live in a world filled with a great variety of sounds. We communicate with each other using sounds. Sometimes we like to relax to the playing of music. At other times, we become irritated because of excessive noise. Like light, sounds travel in the form of waves.

Producing sounds:

All sounds are produced by objects that are vibrating. These sounds travel outwards from the source as sound waves.
For example:
The skin on a drum vibrates to produce the sound of the drum beat.
When the clanger hits the inside of a bell, the vibrations are heard as a ringing sound.
The buzzing sound you hear from a bee is created by the vibration of its wings.
In a loudspeaker, electrical energy is changed into vibrations which we hear as sounds or music.

Creating and hearing sound waves:

Sound waves travel from vibrating objects to our ears by means of sound waves. Figure 1 show how a sound wave is created in air.
As the object vibrates to the right, it pushes the air particles close together, creating a compression. As the object vibrates to the left, a region of more spread out particles is created. This is called a rarefaction. When the object has vibrated several times, a series of compressions and rarefactions have been created that are moving away from the object. This is a sound wave. A model of a moving sound wave can be created by pushing and pulling one end of a slinky.

Sound waves must have something to travel through:

Sound waves can travel through solids, liquids and gases. But they cannot travel through space where there are no particles, i.e. they cannot travels through a vacuum.
When there is air in this bell jar we are able to hear the bell ring when the switch is closed. If the air is pumped out of the jar and the switch closed, we can see the bell ringing, but we cannot hear it. This experiment proves that light waves can travel through a vacuum but sound waves cannot.

Speed of sound:

Sound waves travel much more slowly than light waves. This is why sometimes we see an event before we hear the sound, e.g. thunder and lightning or the flash of an exploding rocket followed seconds later by a loud bang.
Sound waves travel at different speeds in different materials. In air the speed of sound is approximately 340m/s, in water is approximately 1500 m/s and in steels it is 6000 m/s. sounds travel most quickly through solids and least quickly through gases.

ELECTROMAGNETIC SPECTRUM - NASA

E-Activity 10

Copy and complete the following sentences.
1. The case of a microwave oven is made of _____ this ____microwaves.
2. Food contains ____molecules. These _____microwaves and become hot.
3. The food is put into containers made of _____, _____ or _____. These materials allow microwaves to pass through them very easily. We say that they _____ the microwaves.
4. We use a _____ dish to collect enough microwaves for a strong signal. This _____ the microwaves on to an aerial.
5. The aerial transfer’s energy from the microwaves as an ______signal.
6. X-rays can pass easily through skin and flesh but not through _____ or _____.
7. Photographic _____ absorbs any X-rays that fall on it. These parts of the film then go _____ when the film is developed.
8. Gamma radiation can kill living _____.
9. It is used to kill harmful _____ or _____ cells inside people’s bodies.
10. Radiation from the sun, or from a sun bed, can give pale skins a _____. But it can also damage skin cells and cause skin _____. These things happen because of _____ radiation.
11. Some substances _____ ultraviolet radiation and use the energy to produce _____. We say that these substances are _____.
12. Toasters and grills cook food using _____ radiation. Foods become hot when they _____ this radiation.
13. Metal things _____ microwaves, even if they are full of small holes. Some microwaves can pass easily through the Earth’s _____. These microwaves are used to carry information to and from _____.
14. Infrared rays are used to control a _____ set or a _____ player, and to send telephone messages along _____ fibers.
15. In microwave ovens, the microwaves are strongly absorbed by _____molecules in food. The energy from the microwaves makes the food_____.

E-Activity 9

1. What substance will radio waves pass through easily?
2. What happens when radio waves are absorbed by an aerial?
3. Why can’t you send a radio message to or from a submarine?
4. Why is it useful to be able to reflect long wavelength radio waves?
5. What is the wavelength of these radio waves?
6. What wavelengths of microwaves are used for satellite television? And why are these wavelengths used?
7. How is your instruction carried to the television set or video player?
8. Why must you point the remote control at the television set or video player?
9. Doctors can use X-rays to see whether your lungs are healthy. How do they know if there is diseased tissue in your lungs?
10. Why can X-rays and gamma rays cause cancer?

Gamma Rays

Gamma rays are electromagnetic waves emitted by radioactive nuclei. They are also released during nuclear reactions.
Wavelengths of gamma rays range from about 10-10 m to less than 10-14 m.
They are very penetrating and cause serious damage when absorbed by living tissues.

X-Rays

X-rays are electromagnetic waves with wavelengths ranging from about 10-8 m (10 nm) down to 10-13 m (10-4 nm).
X-rays are used as a diagnostic tool in medicine and dentistry.
If you have experienced traveling by air, you would have noticed that your luggage are inspected by the security officer present using x-rays.

Ultra-Violet Radiation

Ultra-violet radiation is the radiation beyond the violet end of the visible spectrum.(wavelengths range from 10-8 to 10-7 m)
The main source of ultra-violet radiation is sunlight and it is this radiation which gives rise to suntans.
Ultra-violet radiation from the sun also stimulates our body to produce vitamin D, which we need for healthy bones.
However overexposure to ultra-violet radiation can cause skin-cancer and damage to the retina.
Ultra violet radiation also kills bacteria and viruses. Thus it is used to sterilize hospital operating rooms and surgical instruments.
Low intensity ultra-violet lamps are sometimes placed above grocery meat counters to reduce spoilage.

Infra-Red Radiation

The name infra-red means beyond red. There are waves just beyond the red end of the visible spectrum. In fact, all objects with temperatures above 0 K emit infra-red radiation.
When objects absorb infra-red radiation, they become hotter. This property of infra-red radiation is used to provide heat treatment for various illnesses.
We are not able to see in the dark because our eyes are sensitive only to the visible part of the electromagnetic spectrum. However, an infra-red camera, with special photographic films which are sensitive to infra-red radiation, can be used to take pictures in the dark without using a flashlight.
Infra-red radiation can also be used in intruder alarms. An intruder would unknowingly block the rays and set off an alarm.
One popular use of infra-red radiation today are the remote control devices for many electrical appliance via infra-red radiation which is produced by light emitting diodes (LEDs) inside the unit.

Microwaves

Microwaves are very similar to UHF radio waves. They have wavelengths of a few centimeters.
Microwaves are produced by special electronic devices such as the klystron tube.
Microwaves are used to transmit television signals and are also used in telecommunications.
Due to its short wavelength, relative to radio waves, the microwave signal can travel in a straight line without losing much of its energy.
Radar systems use microwaves to find the direction and distance of objects which reflect the microwaves back to a large receiving aerial mounted near the transmitter.
Nowadays, another common use of microwaves is in microwave ovens.

Radio Waves

Radio waves have the longest wavelengths of the waves in the electromagnetic spectrum. The wavelengths range from several hundred meters (long wave, LW) to a few centimeters (ultra high frequency, UHF).
They are used in radio and television communication to transmit sound and picture information over long distances.

ELECTROMAGNETIC SPECTRUM

Electromagnetic waves

All electromagnetic waves have certain fundamental properties in common. They basically differ from each other in their wavelengths and in the effects produced.
Properties of Electromagnetic Waves
An electromagnetic wave is produced by the simultaneous vibration of electric and magnetic fields.
Features common to all the electromagnetic waves include the following:
They transfer energy from one place to another.
They are transverse waves.
They can travel through a vacuum.
They travel through a vacuum at 300 000 000 meters per second (3 x 108 m/s). This speed is commonly known as the speed of light.
They all show wave properties like reflection and refraction.
They obey the wave equation  = ƒ 
Application of Electromagnetic Waves
Electromagnetic waves have wavelengths ranging from several kilometers (in the case of radio waves) to less than a picometer(10-12 m) (in the case of gamma rays).
The shorter the wavelength, the higher the frequency.
As the frequency gets higher, the energy also increases. This causes different electromagnetic waves to have different properties and applications.

E-Activity 8

1. What is the velocity of a wave of frequency 600 Hz and wavelength 0.5m?
2. What is the frequency of a wave of wavelength 0.9m and velocity 300 ms-1 ?
3. What is the wavelength of a wave of velocity 300 ms-1 and frequency 1000 Hz?
4. A source of frequency 500 Hz emits waves of wavelength 0.2 m. how long does it take the waves to travel 400m?
5. A wave of frequency 500 Hz travels between two points P and Q with a velocity of 300 ms-1 . How many wavelengths are there in PQ if the length of PQ is 600 m?
6. To understand the wave motion of a transverse wave or a longitudinal wave, we have to examine how the wave particles vibrate as the wave propagates through the medium.
7. Surf the Internet to locate information on transverse and longitudinal waves. Choose the web sites that show animation of the waves. Write down the similarities and the differences of the two types of waves and give examples of each.

Waves

There are different kinds of waves. All waves have one thing in common: they transfer energy from one place to another.
• A wave is a phenomenon in which energy is transferred through vibrations. The wave carries energy away from the wave source. You can see the effect of rope waves if you fix one end of a rope by tying it round a rod and move the other end up and down. A series of crests and troughs can be seen to pass along the rope. Each section of the rope is set into an up-and-down motion by the previous section as the wave passes along the rope.
• The rope is the medium through which the wave propagates. Note that the particles in the rope itself do not move forward with the wave.
• A similar effect is obtained with water waves. A small cork on the water surface will bob up and down (or vibrate) as the wave passes, but will not travel forward with the wave. In this case, water is the medium through which the energy is transmitted.
Types of Waves
1. Transverse waves
2. Longitudinal waves
Transverse waves• Water waves and rope waves are examples of transverse waves. Transverse waves are waves which travel in a direction perpendicular to the direction of the vibrations. If you move the end of a slinky coil from side to side, transverse waves are set up. Light waves and other electromagnetic waves are also transverse waves.
At home you produce transverse waves when you shake the dust from a blanket or rug. Watch closely the motion of your hand, flipping the edge of your blanket, and the waves that are produced.
Longitudinal waves
• Another type of wave is the longitudinal wave. Longitudinal waves travel in a direction parallel to the direction of vibrations.
• If a slinky coil is pushed and pulled at one end, longitudinal waves are set up. Watch the way the compression travels along the coil. The compressed coils themselves do not travel. They just vibrate forward and back. Sound waves are examples of longitudinal waves.
Wave Terms• Some of the terms and quantities used to describe transverse wave motion are as follows:
1. The high points are called crests or peaks while the low points are called troughs.
2. The amplitude is the maximum displacement from the rest position. It is the height of a crest or depth of a trough measured from the normal undisturbed position.
3. The wavelength, l, is the distance between two successive crests or two successive troughs. It is also equal to the distance between any two identical points on successive waves, for example points A and B, and points C and D in figure
4. The frequency, ƒ, is the number of crests (or troughs) that pass a point per second. This is equivalent to the number of complete waves generated per second. Frequency is measured in hertz (Hz). A frequency of 1 hertz means that one wave cycle or one oscillation is completed per second.
5. The period, T, is the time taken to generate one complete wave. It is also the time taken for the crests, or any given point on the wave, to move a distance of one wavelength. T = 1 / ƒ
6. The speed, n, of the wave is the distance moved by a wave in one second. Since the wave crest travels a distance of one wavelength in one period, the wave speed, n = l/T or n = ƒ l
7. In longitudinal waves, the part where the particles of matter are closest together is called the compression. The part where the particles are spread apart is the rarefaction.

Three Laws of Motion

E-Activity 7

1. Copy and complete the table using the results of all six experiments shown in the diagrams.
2. What do you notice about the first and last columns in the table?
3. Look at the examples in the picture.
Use the formula to work out the missing items.
The first one is done for you.

Force, mass and acceleration

Applying a force
Students investigate the relationship between force, mass and acceleration in the laboratory.
• How do the students apply a constant force to accelerate the trucks?
• When they apply the same force to different masses, which accelerates faster-the larger mass or the smaller mass?
• If they keep the mass constant but increase the force, how does the acceleration change?
• Give one reason why it’s difficult to do the experiment accurately.
Experiments in space
A spaceship is a better place to measure the accelerations produced by different forces.
• Why is a spaceship a good place to do experiments on force and acceleration?
How force affects acceleration
The diagrams show some experiments in a spaceship.
The same 1 kg mass is accelerated using different forces.
• How much acceleration does a 1 N force give tgo the 1 kg mass?
• How much acceleration does a 2 N force give?
Copy and complete the sentence.
• When the force on an object is doubled, its acceleration is _______________
How mass affects acceleration
A 1 N force is then used to accelerate different masses.
• How much acceleration do you get when the mass is twice as big?
• How much acceleration do you get when the mass is three times as big?
Copy and complete the sentence.
• If the force stays the same, an object with twice as much mass is given half as much __________
A force of1 N acting on a mass of 1 kg produces an acceleration of 1 m/s2.

Friction V-Speed and Safety

Friction and the motor car

In order that a car can change its speed or direction, there must be friction between its tyres and the road surface. A new tyre with lots of tread has a rough surface and will provide lots of grip. A worn tyre with a smoother surface will provide less grip and the cart will be much more difficult to control and stop.
Having tyres which are in food condition is important if you need to stop quickly, but there are several other important factors which will affect how quickly a car can stop. These are:
• The reaction time of the driver,
• The efficiency of the breaking system of the car,
• The speed of the car,
• The weather/road conditions,
Figure illustrates how the speed of a car affects the total distance it will travel before it stops. The total stopping distance is equal to the thinking distance + the breaking distance.
Braking force
When a car driver brakes, the braking force between the brakes and the wheels slows the wheels down.
Explain why you must brake harder to stop in the same direction when you are traveling at higher speed.
Tyres must grip
Friction between the tyres and the road makes the tyres grip the road. When you brake, the wheels slow down. The friction with the road must then increase to slow the whole car. The friction force depends on the tyre design and the road surface.
• Why does the car skid if you brake too hard?
• Why must the driver brake carefully when the road is wet?
• Why must the driver brake very carefully when there is ice on the roads?
• Why is it safer to have a rough road surface before a pedestrian crossing?
Why do tyres have tread?
You need good tyres to stop quickly. Tyres can grip the road only if they are touching it. They lose their gripwhen the road is wet. The tread on a tyre is designed to push away the water. In dry conditions, the tread doesn’t help. In dry weather, racing cars use tyres with no tread.
Why do racing drivers stop to change their tyres when it starts raining?
Why do worn tyres increase the chance of a skid?
You can’t stop instantly
If someone steps out in front of a car, it takes time for the driver to react. This is called the reaction time. The distance the car travels during the reaction time is called the thinking distance.
Look at the diagram. Why does it take time to react?
Look at the table.
• What is the thinking distance when traveling at 30 miles per hour?
• What happens to the thinking distance if the speed is doubled?
When the driver presses the brake pedal, it takes time for the brakes to slow the car down. During this time, the car travels a distance called the braking distance.
Look at the table.
• What is the braking distance for a speed of 30 miles per hour?
• What happens to the braking distance if the speed is increased?
• Copy and complete the formula.
• Stopping distance = thinking distance + _________ distance
After drinking alcohol, people may feel perfectly normal but their reactions are actually much slower.
• Why is it a bad idea for people to drive after drinking alcohol?
• What other factors could affect the reaction time?

Friction IV-Speed time graph for a skydiver

Friction III-Terminal velocity

Friction II-Drag

Drag

Whenever an object moves through the air it experiences frictional forces or drag. Which try to prevent its motion. The faster the object moves, the greater the drag. To reduce these forces objects such as cars, trains and aircrafts are shaped so that they cut through the air. They are streamlined to reduce air resistance.
Animals such as dolphins, sharks and penguins have streamlined shaped so that when they move through the water, frictional forces are as small as possible.

Friction I

One of the most common forces which can act upon an object is friction. Whenever an object moves or tries to move, friction is present. Friction is a force which opposes motion.
On some occasions friction can prove very useful. For example, when you walk or run, you push yourself forward by pushing backwards on the ground. Friction between your foot and the floor helps you to do this. If there was no friction, i.e. like on a slippery ice rink, your feet would slip!
Smooth surfaces reduce the friction between objects while rough surfaces increase the frictional forces. Trainers and football boots are designed to prevent your foot from slipping by increasing the frictional forces between you and the ground. In contrast, skates and skis are designed with smooth surfaces which keep friction to a minimum.
Where there is contact between surfaces, friction can be reduced using a lubricant such as oil, friction between two surfaces can cause the surfaces to wear away and become hot.