Is it a good idea to pursue B.Tech in mechanical after doing B.Sc. in physics?

It is a good idea, but you have done B.Sc in physics, then you have lot of scope. You can go for M.Sc and explore the world with the knowledge of physics. Best of luck.

It is a good idea to pursue B.Tech in mechanical engineering after B.Sc in physics. You are eligible for lateral entry into second year of B.Tech - ME in lot of engineering colleges which have lateral entry provision of admission.