docs: improve text and correct typos

docs/source/simulator/mass_point_model.rst

@@ -4,9 +4,8 @@ Mass Point Model
 Theory of mass point model
 --------------------------
 
-A `mass point model` is an extremely simplified model to simulate the movement
-of some mass on a given topography. It assumes that the flow mass is condensed
-to a single point.
+A `mass point model` is an extremely simplified model to simulate mass movement
+on a given topography. It assumes that the flow mass is condensed to a single point.
 
 Let :math:`Z(x,y)` define a topography in a Cartesian
 coordinate system :math:`\{x, y, z\}`. It induces a local non-orthogonal
@@ -38,13 +37,13 @@ given by
     \frac{g}{\xi}\|\boldsymbol{U}\|^2\right)
     :label: dUTydt
 
-where :math:`\mu` and :math:`\xi` are dry-Coulomb and turbulent friction coefficients
-respectively. :math:`\mathbf{K}` is the curvature tensor :cite:p:`Fischer2012`.
-:math:`\mathbf{U}` prepresents the masspoint's velocity. :math:`U_{T_x}` and
-:math:`U_{T_y}` are the velocity components along :math:`T_x` and :math:`T_y` direction
-respectively.
+where :math:`\mu` and :math:`\xi` are dry-Coulomb and turbulent friction coefficient
+respectively (Voellmy friction model is used here). :math:`\mathbf{K}` is the
+curvature tensor :cite:p:`Fischer2012`. :math:`\mathbf{U}` represents the masspoint's
+velocity. :math:`U_{T_x}` and :math:`U_{T_y}` are the velocity components along :math:`T_x`
+and :math:`T_y` direction respectively.
 
-Equations :eq:`dxdt` to :eq:`dUTydt` can be rewritten in vector format as
+Equations :eq:`dxdt` to :eq:`dUTydt` can be rewritten in the vector format as
 
 .. math:: \frac{d \boldsymbol{\alpha}}{d t}=\boldsymbol{f}(t, \boldsymbol{\alpha})
     :label: dalphadt
@@ -67,7 +66,7 @@ Equations :eq:`dxdt` to :eq:`dUTydt` can be rewritten in vector format as
     \end{array}\right]
     :label: vector-f
 
-Equation :eq:`dalphadt` define an initial value problem. Given initial
+Equation :eq:`dalphadt` defines an initial value problem. Given initial
 :math:`\boldsymbol{\alpha}_0`, the system can be solved forward in time using
 numerical schemes such as the runge-kutta method. Class :class:`.MassPointModel`
 utilizes the explicit runge-kutta method ``dopri5`` provided by