Model ElementUnits defines the units for the model being defined.

Class Name



Units allows you to specify four types of units: Mass, Length, Time, and Force. The last is obviously a function of the first three and can be automatically calculated for a consistent set of units. However, many unit systems are not "consistent", so it may be necessary to specify a force unit also.

Attribute Summary

Name Property Modifiable by command? Designable?
force Enum (Force, default="NEWTON") NO NO
length Enum (Length, default="METER")
mass Enum (Mass, default="KILOGRAM")
time Enum (Time, default="SECOND")
ucf Double (default=None)
*Force=["CENTINEWTON",    "DYNE",          "KILOGRAM_FORCE",   "KNEWTON",
        "NANONEWTON",     "NEWTON",        "OUNCE_FORCE",      "POUND_FORCE",

*Length=["ANGSTROM",      "CENTIMETER",    "FOOT",             "KILOMETER",
         "INCH",          "METER",         "MICROINCH",        "MICROMETER",
         "MILE",          "MILLIMETER",    "MILLS",            "NANOMETER","YARD"]

*Mass=["GRAM",            "KILOGRAM",      "KPOUND_MASS",      "MILLIGRAM",
       "MICROGRAM",       "NANOGRAM",      "OUNCE_MASS",       "POUND_MASS",
       "SLINCH",          "SLUG","TONNE",  "US_TON"]

*Time=["DAY",             "HOUR",          "MICROSECOND",      "MILLISECOND",
       "MINUTE",          "NANOSECOND",    "SECOND"]


Units may be defined in three different ways.
#1: Explicit specification of mass, length, time and force units
Units (force=string, length=string, mass=string, time=string)

#2: Selecting a predefined set of unitsUnits (system=string)

#3: With a units consistency factor only 
Units (UCF=double)


Explicit specification of mass, length, time, and force units.
Defines the units for force. Select one from:
  • "Ounce_Force"
  • "Pound_Force"
  • "Kpound_Force"
  • "Dyne"
  • "Newton"
  • "Kilogram_Force"
  • "KNewton"
  • "CentiNewton"
  • "MilliNewton"
  • "MicroNewton"
  • "MegaNewton"
  • "Poundal"
  • "NanoNewton"
The force_unit attribute is mandatory.
Defines the units for mass. Select one from:
  • "Gram"
  • "Milligram"
  • "Kilogram"
  • "Megagram"
  • "Pound_Mass"
  • "Slug"
  • "Ounce_Mass"
  • "Kpound_Mass"
  • "Tonne"
  • "Microgram"
  • "Nanogram"
  • "US_Ton"
  • "Slinch"
The mass_unit attribute is mandatory.
Defines the units for length. Select one from:
  • "Inch"
  • "Foot"
  • "Mile"
  • "Millimeter"
  • "Centimeter"
  • "Meter"
  • "Kilometer"
  • "Nanometer"
  • "Angstrom"
  • "Yard"
  • "MilliInch"
  • "MicroInch"
  • "Mils"
  • "Micrometer"
The mass_unit attribute is mandatory.
Defines the time units. Select one from:
  • "Millisecond"
  • "Second"
  • "Minute"
  • "Hour"
  • "Microsecond"
  • "Nanosecond"
  • "Day"
The time_unit attribute is mandatory.
Selecting a predefined set of units
MotionSolve knows about the following sets of units. You can select from one of the following
  • "CGS"
  • "FPS"
  • "IPS"
  • "MKS"
  • "NONE"
When "NONE" is specified, MotionSolve uses the "MKS" system.
The system attribute is mandatory when you want to use a predefined set of Units.
Specify a units consistency factor
In lieu of specifying the mass, length, and time units, you can alternatively specify a Units Consistency Factor, UCF. This factor should ensure that for the set that's chosen, Newton's second law, F=MA, can still be used.
UCF has a default value of 1.0, so that when neither the units nor the mass multiplication factor are defined, the software assumes that the model has been defined in some (unknown) set of consistent units.


  1. Define the SI unit system.
    si = Units (force="Newton", mass="Kilogram", length="Meter", time="Second")
  2. Set the UCF for example in Comment 1.
    myunits = Units (ucf=1000)
  3. Define the following units system.
    • Mass in Kilograms
    • Length in yards
    • Time in days
    • Force in Newtons
    weirdUnits = Units (force="Newton", mass="Kilogram", length="Yard", time="Day")
  4. Define the model to be in the British units system.
    BritishUnits = Units (system="FPS")


  1. UCF is defined to the quantity that enables Newton's Second law to be valid for an inconsistent set of units. Thus the 2nd law may be written as:
    F = m * a * UCF

    Therefore, UCF = F / (m*a).

    Let's see how this works for the example below.
    • Mass is specified in grams
    • Length is specified in micrometers
    • Time is specified in milliseconds
    • Force is specified in Newtons
    UCF = F ÷  ( m*a )   UCF =  ( 1 N )  ÷  ( 1g *  ( 1 μ meter ÷  ( 1 ms * 1ms ) ) )     UCF =  ( kg m s 2 )  *  ( 1 ms * 1ms ) ( 1g * 1 μ meter )   UCF = 1  kg m s 2  *  ( 1e-3s * 1e-3 s ) ( 1e-3 kg * 1e-6 m ) UCF = 1000

    We see that UCF does not have any units. It is just a factor.

  2. See Properties for an explanation about what properties are, why they are used, and how you can extend these.
  3. For a more detailed explanation about Units, see Parameters: Units.