History:
Levers have been used since prehistoric times for cultivation, excavation, and moving large objects. Such implements as hoes, slings, and oars were conceived and constructed to enhance human effort.As early as 5000 B.C.E., a simple balance scale employing a lever was used to weigh gold and other items. A Greek device called a steelyard improved on these simple scales by adding a sliding weight to enhance precision. Around 1500 B.C.E., the shaduf, a forerunner of the crane, made its appearance in Egypt and India as a device for lifting containers of water.
The earliest extant writings regarding levers date from the third century B.C.E. and were provided by Archimedes—behind his famous remark Give me the place to stand, and I shall move the earth stands a correct mathematical principle of levers (quoted by Pappus of Alexandria) and of the various methods possibly used by builders.
http://www.newworldencyclopedia.org/entry/Lever
Chemical and physical properties:
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Exceptionally strong relative to its weight
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A good heat
and electrical insulator;
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of increasing importance
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It is a renewable and biodegradable resource.
Chemical and physical changes:
Good general rule: A chemical change changes the substance so that you can't ever use it the way you did before. A physical change retains the character of the substance.
Burn a candle, and you can never burn that candle again. It has undergone a chemical change. Cutting that same candle into a bunch of small pieces is a physical change, because if you were to melt the wax and mold it into a candle like it was in the first place, you could still burn that candle. The little pieces of wax act just like the big piece of wax.
I know, I know, it's not quite perfect, but it's still a good way to remember the difference.
Same thing goes for wood. Burning wood is a chemical change, because you can't turn it back into a log that you can burn.
Cutting wood into smaller pieces of wood is a physical change. Each individual piece of acts just like the bigger piece. You can pile a bunch of little tiny toothpicks together and they will burn just the same way a log does.
Baking a cake is a chemical change. You can never get the uncooked flour and eggs and stuff out of that cake again.
Heating ice to become water is a physical change. All you have to do is put it in the freezer and it's ice again.
