pip من کار نمیکنه

pip : The term 'pip' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of  

the name, or if a path was included, verify that the path is correct and try again.    

At line:1 char:1

+ pip install tk

+ ~~~

   + CategoryInfo          : ObjectNotFound: (pip:String) [], CommandNotFoundException

   + FullyQualifiedErrorId : CommandNotFoundException