diff --git a/work/week8/particles/starting_point/CMakeLists.txt b/work/week8/particles/starting_point/CMakeLists.txt
index b73cea7..448732a 100644
--- a/work/week8/particles/starting_point/CMakeLists.txt
+++ b/work/week8/particles/starting_point/CMakeLists.txt
@@ -1,28 +1,28 @@
-cmake_minimum_required (VERSION 2.6)
-project (Particles)
-
-#set(CMAKE_CXX_STANDARD 14)
+cmake_minimum_required(VERSION 2.6)
 set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14")
 
+#add_subdirectory(src) # directory you want to add or contains executable files
+
+
+
 add_executable(particles
   main.cc
   vector.cc
   compute_boundary.cc
   compute_verlet_integration.cc 
   particle.cc 
   planet.cc
   compute_gravity.cc 
   csv_reader.cc 
   particles_factory_interface.cc 
   planets_factory.cc 
   compute_contact.cc 
   compute_kinetic_energy.cc 
   csv_writer.cc 
   system.cc 
   compute_energy.cc 
   compute_potential_energy.cc 
   ping_pong_ball.cc 
   system_evolution.cc 
   ping_pong_balls_factory.cc 
   compute_interaction.cc)
-
diff --git a/work/week8/particles/starting_point/compute_gravity.cc b/work/week8/particles/starting_point/compute_gravity.cc
index 0d6c5db..1cf7b2c 100644
--- a/work/week8/particles/starting_point/compute_gravity.cc
+++ b/work/week8/particles/starting_point/compute_gravity.cc
@@ -1,40 +1,46 @@
 #include <cmath>
 
 #include "compute_gravity.hh"
 
 /* -------------------------------------------------------------------------- */
 
 void ComputeGravity::compute(System& system) {
 
     // Get the total number of particles
     int nparticles = system.getNbParticles();
 
-    // Loop over all the particles and gets each one's parameters
+    //Initialize types 
     Vector v_ij;
     Real r2_ij = 0.;
     Real r_ij = 0;
     Vector force;
+    Vector nv_ij;
 
+    // Loop over all the particles and gets each one's parameters
     for (int pi = 0; pi < nparticles; ++pi) {
         Particle & PI = system.getParticle(pi);
+        //loop over all next interacting particles
         for (int pj = pi+1; pj < nparticles; ++pj) {
             Particle & PJ = system.getParticle(pj);
 
             // Gets the vector between the two particles positions
             v_ij = PJ.getPosition() - PI.getPosition();
 
             // Squared distance  (obtained via vector class)
             r2_ij = v_ij.squaredNorm();
 
             // Obtained distance
             r_ij = sqrt(r2_ij);
 
-            // compute the pair force
-            force = v_ij / r_ij * PI.getMass() * PJ.getMass() * G / r2_ij;
+            //Obtained corresponding normed vector
+            nv_ij=v_ij / r_ij;
+
+            // compute the gravity force vector interaction on i with j
+            force = nv_ij* PI.getMass() * PJ.getMass() * G / r2_ij;
 
-            // add up the force for both concerned particles
+            //F_ij=-F_ji Particles have opposite forces
             PJ.getForce() -= force;
             PI.getForce() += force;
           }
     }
 }