Salam Sejahtera

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Thursday, September 18

High and Low Sounds

THE PITCH OF A SOUND means how low or high it is. In a music band,
the bass drum's deep boom is low-pitched, while the triangle's shrill tinkle is high-pitched. Pitch depends on how many times the sound source moves to and fro, or vibrates, each second. This
is the same as the frequency - how many sound waves are produced each second. The frequency of a wave is measured in units called Hertz, Hz. For example, the note of middle C, in the
middle of a piano keyboard, has a frequency of 261 Hz. Frequency is related to wavelength, since higher frequencies have shorter waves. The length of a middle C sound wave is 126 centimetres.
What sounds can we hear?

We hear many sound frequencies, from the shrill notes of bird song to the deep growl of traffic. But, because of the way our ears work, we do not hear all of the sounds around us. Our ears pick up frequencies from about 20 to 20,000 Hz (Hertz, vibrations per second). We hear sounds below 80 Hz as low, deep booms, thuds or rumbles. Frequencies below about 30 Hz may not be heard clearly, but if they are powerful enough, we can feel them as vibrations in the air and ground. Our ears are most sensitive in the range from 400 to 4,000 Hz. (Human speech tends to be around 300-1,000 Hz.) Sounds above about 5,000 Hz are extremely high-pitched squeaks, hisses and screeches. As people get older, their ears become less sensitive to high notes. So a young person can hear a bat's very high-pitched squeaks, while an older person cannot.
Monday, September 15

Telecom network

A mobile phone sends and receives messages by radio waves. The radio waves travel to and from a tranceiver (transmitter/receiver) station which connect the call into the standard telephone network. Countries are divided up into different areas, called cells, and each cell has its own tranceiver station. In an areas where a lot of people live, there are many small cells because there likely to be many people using mobile phones. In sparsely populated areas populated areas, the cells are larger.

Inside a mobile phone

A "mobile" is a low-power radio transmitter-receiver. It has mouthpiece to change sound waves into electrical signals (like a microphone), and an earpiece to change electrical signals into sound waves (like a loudspeaker). The transmitter-receiver only needs to send and pick up waves from the nearest cell tower, which is usually just a few kilometres away. However hulls or tall buildings may block the radio signals. also, in areas where the cell towers are farther apart, the signals may be too weak to travel to and from the phone.
Saturday, September 13

Simple Machines

Every mechanical device, even the most complicatrd giant earthmover, is made from only four different types of simple machines. These are the lever, the inclined plane or ramp, the wheel and axle, and the pulley. The lever ia a stiff beam or bar that pivots at point called fulcrum. If the fulcrum is closer to one end than the other, you can use the lever to lift a heavy weight more easily. The inclined plane (ramp or slope) is a machine to. It is usually easier to slide a heavy weight up a slope than to lift it straight up. Place two inclined planes back-to-back and they form a wedge, as in a knife blade, axe or chisel. A wedge wrapped around a rod, in the corkscrew-like shape called a helix, forms a screw. Screws are used to lift things and fasten them together.

Building the pyramids
The pyramids were built in ancient Egypt about 4,500 years ago, from thousands of blocks of stone-some weighing many toones. These may have been levered into place, or dragged up a slope built beside the pyramid, or rolled up on logs. No-one really knows.

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