A force pushes or pulls something in a particular direction. Isaac Newton was the first person to make a study of forces. He realized that when an object produces a force on something else, there is an equal and opposite reaction. For example, if you wear roller skates and push against a wall, exerting a force, you will be pushed away with just as much force as you are applying.
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The loudness of sounds is measured in units called decibels (dB).Ordinary speech has a sound intensity of about 20 decibels, while a noisy room full of people produces about 70 decibels. The quietest sound you can hear is about 20 decibels. Loud music at a rock concert reaches a level of about 125 decibels, while sounds of 140 decibels or more actually cause pain in the ear. This level of sound can produce permanent hearing damage if it continues, which is why people operating noisy machinery have to wear ear protectors.
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Sound waves travel very well through water, so they are used to detect submarines or even shoals of fish. A sonar device under a ship sends out sound waves that travel down through the water. The sound waves are reflected back from any solid object they reach, such as a submarine, shoals of fish or the seabed. The echoes are received by the ship and can be used to ‘draw’ an image of the object, and its location, on a screen.
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Thunder is produced when lightning heats up the surrounding air very rapidly, causing it to expand faster than the speed of sound. This produces a ‘sonic boom’ like the one caused by a supersonic aircraft. If you count the seconds between seeing a flash of lightning and hearing thunder and divide this number by 3, it will tell you how many kilometres away from the lightning you are. Sound travels at about 1220 km/h, while light from the lighting strike travels so fast that you see it almost instantaneously.
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At sea level, sound travels though the air at around 340 m pet second, and slightly slower at high altitudes. The denser the substance, the faster sound will travel through it. Sound travels at 1,500 m per second through water, for example. When travelling at such very high speeds, an aircraft begins to build up a huge wave of compressed air in front of it. This was known as the sound barrier because it was an obstacle to high-speed flight. When the aircraft exceeds the speed of sound, it leaves the built-up waves of pressure behind, and these break away, forming a ‘sonic boom’. The speed of sound in air is called Mach 1, and the speed of supersonic aircraft is measured in Mach numbers. Concorde is the only commercial aircraft to have broken the sound barrier.
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You will have noticed that as a car travels quickly towards you, the sound of its engine gets higher, and then becomes deeper after the car has passed by. This is called the Doppler Effect. What happens is that as the car approaches, the frequency of the sound of its engine increases as the wavelength of the sound decreases. Each successive sound wave is a little shorter as the car comes towards you. Then as the car moves away, the process is reversed; the frequency decreases while the wavelength of the sound becomes longer.
You will not notice the Doppler Effect if a supersonic aircraft flies past, because you cannot hear the sound of the plane until it is actually flying away from you.
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Like light, sound can be reflected from certain surfaces. Hard surfaces such as rock or the side of a building reflect sound well; as the sound bounces back you hear an echo. The delay in hearing the echo is due to the comparatively slow speed of sound. Soft materials absorb sound and will not produce an echo. This is why recording studios are lined with felt material, which prevents any unwanted noise from interfering with the recording.
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Compared to other animals, the human ear is not very sensitive. We can hear sounds with a frequency of up to 20 kilohertz (kHz) — a normal speaking voice is about 1 kHz. Bats, dogs and some insects can hear sounds has are pitched much higher, which we cannot hear at all. Children can usually hear a bat squeaking, although this sound can sometimes be as high as 120 kHz, but in adults the ability to hear these high-pitched sounds is usually lost. ‘Supersonic’ dog whistles are used to call dogs, and their sound is so high-pitched that humans cannot hear it at all.
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Sound is a form of vibration passing through the air or some other material. Sound travels in the form of waves but, unlike electromagnetic waves, sound waves cannot pass through a vacuum. The frequency of a sound wave controls its pitch. Long wavelengths produce deep sounds, while short wavelengths produce higher sounds. The loudness of a sound depends on the height of the sound waves, or their amplitude. The higher the amplitude, the louder the sound.
Sound vibrations are received in your ears. They are conveyed to a mechanism inside your ears that first amplifies them, and then converts the vibrations into signals that your brain interprets as sound.
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Lasers are devices that produce a narrow beam of very strong light. Lasers amplify light by causing photons to be bounced back and forth in a substance (solid, liquid or gas), which adds extra energy. The result is that intense light is emitted in a very narrow beam.
Lasers are used to cut metal and other solid material, and even to burn out skin blemishes such as tattoos in a painless way. In CD players lasers scan a beam of laser light across the CD’s silvery surface, reading the tiny changes in light reflected back. They are also used in office printers and scanners. In engineering, the intense narrow beam of laser light is used to measure and align roads and tunnels.
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Although light travels only in straight lines, it can be made to bend around curves and angles using optical fibres. These are bundles of very thin strands of exceptionally clear glass. The fibres are treated so that their outer surface reflects light. When light is shown in one end of the bundle it passes along the fibres, reflecting from the sides as they curve and eventually emerging at the far end.
Optical fibres carry electronic signals to computers, and they are increasingly used in telephone lines. Optical fibres are very useful in medicine to diagnose disease. A flexible fibreoptic probe can be inserted into the body to view the internal workings of an organ.
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When light strikes a transparent surface at an angle, its speed reduces and the rays of light bend slightly. The ‘white’ light we see is actually a mixture of colours: red, orange, yellow, green, blue, indigo and violet; each of these colours is bent by a different amount. So when a beam of white light passes through a glass prism, the light coming out the other side consists of bands of these colours. You can often see this banded effect around the bevelled edges of a mirror.
This process of refraction, or bending, of light also takes place in raindrops when sunlight strikes them, which is why you see the colours of the spectrum in a rainbow.
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Light is given off by atoms that have gained extra energy. This excess energy is released in the form of light or heat. Such atoms become excited by absorbing energy from other source. Atoms can also become excited when heated. The atoms of the heated material move around rapidly and collide with one another, becoming excited and emitting their extra energy as light. The colour of the light produced depends on how much energy is released. A few substances store extra energy and then release it gradually. These substances, such as the glowing paints used on watch faces, are phosphorescent. Other substances only produce light when exposed to other forms of energy; for example fluorescent substances may glow brightly when exposed to ultraviolet light.
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Light travels faster than anything else known to man — it travels through a vacuum at almost 300,000 km per second. This means that light from the Sun takes eight minutes to reach the Earth. It slows down very slightly when it passes through any kind of material, because it collides with atoms. According to Einstein’s special theory of relativity, there is nothing else that can travel as fast as light.
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Light is a form of electromagnetic radiation. It travels as waves that pass freely through space, even in the absence of air. Like other forms of electromagnetic radiation, light waves have a wavelength, and light is the visible part of these waves.
The actual wavelength of the electromagnetic radiation determines the colour of the light. Long wavelengths produce red colours, and very long wavelengths produce infrared light, which we cannot see. Shorter wavelengths produce blue light, or ultraviolet radiation. Strangely, light can also be considered as a form of particle, called a photon, which travels in a straight line. Both descriptions — wave and particle — are correct, and both mean that light is a form of energy.
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