# Using Physical Constants in V9

How is one supposed to work with physical constants in Mathematica V9? When I first open Mathematica, physical constants such as SpeedOfLight are not recognized. If I load the PhysicalConstants package using:

<< PhysicalConstants


I get the following error message:

General::obspkg: "!(\"PhysicalConstants\") is now obsolete. The legacy version being loaded may conflict with current Mathematica functionality. "

However, at this point SpeedOfLight is properly defined!

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In v. 9, speed of light is a quantity. Try

Quantity["SpeedOfLight"]


The output will look like the following:

1 c

However, its FullForm is actually

Quantity[1, "SpeedOfLight"]


You will find addition information about how to use physical constants as quantities in the Compatibility Tutorial and in the Units Overview.

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