diff --git a/js/model/GravityForceLabModel.js b/js/model/GravityForceLabModel.js
index 4e486f0d..4164f4e1 100644
--- a/js/model/GravityForceLabModel.js
+++ b/js/model/GravityForceLabModel.js
@@ -24,6 +24,12 @@ define( require => {
   const Vector2 = require( 'DOT/Vector2' );
   const Vector2Property = require( 'DOT/Vector2Property' );
 
+  // constants
+  const MASS_OPTIONS = {
+    leftObjectBoundary: GravityForceLabConstants.PULL_LOCATION_RANGE.min,
+    rightObjectBoundary: GravityForceLabConstants.PULL_LOCATION_RANGE.max
+  };
+
   /**
    * @param {Tandem} tandem
    * @constructor
@@ -55,9 +61,9 @@ define( require => {
     const density = 150; // in kg/m^3
 
     const mass1 = new Mass( massValue1, position1, GravityForceLabConstants.MASS_RANGE, density,
-      this.constantRadiusProperty, baseColor1, tandem.createTandem( 'mass1' ) );
+      this.constantRadiusProperty, baseColor1, tandem.createTandem( 'mass1' ), MASS_OPTIONS );
     const mass2 = new Mass( massValue2, position2, GravityForceLabConstants.MASS_RANGE, density,
-      this.constantRadiusProperty, baseColor2, tandem.createTandem( 'mass2' ) );
+      this.constantRadiusProperty, baseColor2, tandem.createTandem( 'mass2' ), MASS_OPTIONS );
 
     // leverage ISLCModel, in "mass" mode
     ISLCModel.call( this, PhysicalConstants.GRAVITATIONAL_CONSTANT, mass1, mass2,
diff --git a/js/phet-io/gravity-force-lab-phet-io-elements-baseline.js b/js/phet-io/gravity-force-lab-phet-io-elements-baseline.js
index ad244c1a..e53c9aee 100644
--- a/js/phet-io/gravity-force-lab-phet-io-elements-baseline.js
+++ b/js/phet-io/gravity-force-lab-phet-io-elements-baseline.js
@@ -1562,7 +1562,7 @@ window.phet.phetio.phetioElementsBaseline = assert &&
       "phetioTypeName": "PropertyIO<RangeIO>"
     },
     "gravityForceLab.gravityForceLabScreen.model.mass1.positionProperty": {
-      "phetioDocumentation": "The position of the object along the track, in meters.",
+      "phetioDocumentation": "The position of the object along the track, in meters. The origin is in the center between the two objects.",
       "phetioDynamicElement": false,
       "phetioEventType": "MODEL",
       "phetioFeatured": false,
@@ -1614,7 +1614,7 @@ window.phet.phetio.phetioElementsBaseline = assert &&
       "phetioTypeName": "PropertyIO<RangeIO>"
     },
     "gravityForceLab.gravityForceLabScreen.model.mass2.positionProperty": {
-      "phetioDocumentation": "The position of the object along the track, in meters.",
+      "phetioDocumentation": "The position of the object along the track, in meters. The origin is in the center between the two objects.",
       "phetioDynamicElement": false,
       "phetioEventType": "MODEL",
       "phetioFeatured": false,