Question

Short note on when was non cooperation movement launched and why

Answer 1

Hind swaraj written by Gandhi expressed that British could rule over India because of Indian cooperation, if they had not cooperated, it would have been collapsed. 
many within the congress were reluctant to boycott the council elections for November 1920
finally the congress session at Nagpur December 1920, non cooperation movement was adopted. 
it began in January 1921.