Without friction, anything eventually gets up to speed if you provide a force or torque. What mass and moment of inertia determine is how long it takes for that to happen. In practice (while still neglecting friction) you are probably limited by how long you can take to spool up rotate due to heating form stall currents. How quickly you can spool up is probably limited by core saturation, then heating.
So you need to decide how quickly you need your load to accelerate.
Then you either use all the \$F=ma\$ and \$v = at\$ from high school physics or their angular equivalents which are \$ \omega = I\alpha\$ and \$ \omega = \alpha t\$ depending on whether your 7kg mass is being moved linearly by the motor or being spun by the motor.
If the load mass is being translated by the motor then you need to use the lever arm (aka wheel/gear radius) to convert back and forth between linear force and torque.
If the load is being rotated by the motor, then you need the dimensions and geometry of the 7kg mass to calculate its moment of inertia through calculus or a table. Moment inertia being the equivalent of mass in an rotating system.
If the inertia of the load is much larger than the inertia of the motor's rotor, you can neglect it.
If you can't neglect rotor inertia then:
- For rotational movement you just add it to the load moment of inertia.
- For linear movement, you will have to use the lever arm to convert
the load mass to a moment of inertia and add it to the motor's
inertia.
- Alternatively, for linear movement you can convert the rotor inertia to a linear equivalent mass using the lever arm and do all your calculations in terms of linear forces and masses rather than torques and moments of inertia. Then at the end you can convert the linear force into a torque again.
