--- a/gfx/layers/AxisPhysicsModel.cpp
+++ b/gfx/layers/AxisPhysicsModel.cpp
@@ -42,23 +42,23 @@ AxisPhysicsModel::AxisPhysicsModel(doubl
 }
 
 AxisPhysicsModel::~AxisPhysicsModel()
 {
 
 }
 
 double
-AxisPhysicsModel::GetVelocity()
+AxisPhysicsModel::GetVelocity() const
 {
   return LinearInterpolate(mPrevState.v, mNextState.v, mProgress);
 }
 
 double
-AxisPhysicsModel::GetPosition()
+AxisPhysicsModel::GetPosition() const
 {
   return LinearInterpolate(mPrevState.p, mNextState.p, mProgress);
 }
 
 void
 AxisPhysicsModel::SetVelocity(double aVelocity)
 {
   mNextState.v = aVelocity;

--- a/gfx/layers/AxisPhysicsModel.h
+++ b/gfx/layers/AxisPhysicsModel.h
@@ -32,27 +32,27 @@ public:
    * Advance the physics simulation.
    * |aDelta| is the time since the last sample.
    */
   void Simulate(const TimeDuration& aDeltaTime);
 
   /**
    * Gets the raw velocity of this axis at this moment.
    */
-  double GetVelocity();
+  double GetVelocity() const;
 
   /**
    * Sets the raw velocity of this axis at this moment.
    */
   void SetVelocity(double aVelocity);
 
   /**
    * Gets the raw position of this axis at this moment.
    */
-  double GetPosition();
+  double GetPosition() const;
 
   /**
    * Sets the raw position of this axis at this moment.
    */
   void SetPosition(double aPosition);
 
 protected: