Derivation of Minkowski norm of the four-momentum

[TheCanadian]

I have attached a derivation of the Minkowski norm of the four-momentum but just don't quite see how the writer arrived at $-m^2 c^2$ from what was given. How exactly does this quantity follow from $-\frac {E^2}{c^2} + p^2$? I feel like it might be very obvious, so any explanation would be great!

 

[Dale]

I would say that it is a definition rather than a derivation. We use the norm of the four momentum often enough that it deserves a name, so we define the name "mass" to refer to it.

 

[Aditya Choudhary]

Guys ...is it so that every physical quantity is related to time?
like ...acceleration is rate of change in the velocity of an object over time...similar is for velocity n power...but what abt other...

cuz simply p=mv ...i don't get it...but ...by equating ...p=mv n F=ma...we can get F=p/t...n can't get this as well...

please sombody helppppp...m a 11th class student n can't go further if i can't get the basics clearly...pleasezz