how come ribosomes made up of 50S and 30S be equal to 70S?? pls answer in simple words.. as i am poor in understanding.. :( and wat is S here... ?

The 70 S ribosome are found is prokaryotes and 50 S & 30 S are its two subunits where S refers to sedimentation coefficient i.e. how fast the ribosome move in a centrifuge. It should be noted that 50 S and 30 S are not added to make a ribosome. It is named as 70 S ribosome because it moves slowly in a centrifuge, 

Whereas ribosome in Eukaryotes is 80 S as it moves faster then the prokaryotic ribosome in a centrifuge. Hence the name of the ribosome is not based on the subunits but it is based on the speed with which they move in a centrifuge.

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it is not as you are understanding it! 50S and 30s do not combine to form 70S ribosmoe! its like 70S ribosome has two subunits of 50S and 30S. it is not the sum of the other two!

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