It sounds to me like you want the mass data to be a signal, and not a parameter that is defined by a workspace variable.
I think of Simulink as having data which are either signals or parameters:
- Signal - value updates during simulation time
- Parameter - value does not change unless user manually changes it
To get a mass that changed every two seconds, I would define the mass as an input signal derived from one of the Source blocks (Simulink -> Sources library) such as Inport, From Workspace, Signal Builder, etc.