To set the Mechanics Environment defaults select Edit / Mechanics Environment.
System - Choices for the system units are:
- SI (m/kg/sec)
- CGS (cm/kg/sec)
- FPS (ft/lbs/sec)
- IPS (in/lbs/sec)
- Custom - set the Gravity Value and the Gravitational Units check box.
To select the System units,click the row and the arrow icon 
to display the drop down menu of choices.
To change any of the following entries, select the value and retype it.
Gravity Variable - g is a reserved word for the gravitational constant (unless you replace it with another symbol). Do not use this symbol for any other variable.
Gravity Value - set to the gravitational constant (9.8, or a value you can change if you are working in Custom System units)
Time Variable - 
(tau) is a reserved word for time (unless you replace it with another symbol). Do not use this symbol for any other variable. By default is set to 0. You can change this value in the Variables toolbox.
Differential Equation Prefix - choose a prefix for the equations of motion for the Differential Equations function provided in the Mechanics Output menu.
Angle To Horizontal - the default is pi/2, viewing the model from the side with gravity acting straight down. A horizontal model, (angle 0 to horizontal), would view the model from the top and have no gravitational force.
Gravitational Units - when working with standard system units, this selection is grayed and the check box is set by the system: checked when working with weight units (FPS and IPS), cleared when working in mass units (SI and CGS). You can change the entry when working in Custom units.