diff --git a/sources/reactphysics3d/physics/Kilogram.cpp b/sources/reactphysics3d/physics/Kilogram.cpp
new file mode 100644
index 00000000..807a6374
--- /dev/null
+++ b/sources/reactphysics3d/physics/Kilogram.cpp
@@ -0,0 +1,46 @@
+/****************************************************************************
+ * Copyright (C) 2009      Daniel Chappuis                                  *
+ ****************************************************************************
+ * This file is part of ReactPhysics3D.                                     *
+ *                                                                          *
+ * ReactPhysics3D is free software: you can redistribute it and/or modify   *
+ * it under the terms of the GNU General Public License as published by     *
+ * the Free Software Foundation, either version 3 of the License, or        *
+ * (at your option) any later version.                                      *
+ *                                                                          *
+ * ReactPhysics3D is distributed in the hope that it will be useful,        *
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of           *
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the             *
+ * GNU General Public License for more details.                             *
+ *                                                                          *
+ * You should have received a copy of the GNU General Public License        *
+ * along with ReactPhysics3D. If not, see <http://www.gnu.org/licenses/>.   *
+ ***************************************************************************/
+
+ // Libraries
+ #include "Kilogram.h"
+
+ // We want to use the ReactPhysics3D namespace
+ using namespace reactphysics3d;
+
+ // Constructor
+ Kilogram::Kilogram(double value) throw(std::invalid_argument) {
+    // Check if the value is positive
+    if (value >= 0) {
+        this->value = value;
+    }
+    else {
+        // We throw an exception
+        throw std::invalid_argument("Exception in Kilogram : Wrong argument, a mass value has to be positive");
+    }
+ }
+
+ // Copy-constructor
+ Kilogram::Kilogram(const Kilogram& mass) {
+    this->value = mass.value;
+ }
+
+ // Destructor
+ Kilogram::~Kilogram() {
+
+ }
diff --git a/sources/reactphysics3d/physics/Kilogram.h b/sources/reactphysics3d/physics/Kilogram.h
new file mode 100644
index 00000000..33f53447
--- /dev/null
+++ b/sources/reactphysics3d/physics/Kilogram.h
@@ -0,0 +1,70 @@
+/****************************************************************************
+ * Copyright (C) 2009      Daniel Chappuis                                  *
+ ****************************************************************************
+ * This file is part of ReactPhysics3D.                                     *
+ *                                                                          *
+ * ReactPhysics3D is free software: you can redistribute it and/or modify   *
+ * it under the terms of the GNU General Public License as published by     *
+ * the Free Software Foundation, either version 3 of the License, or        *
+ * (at your option) any later version.                                      *
+ *                                                                          *
+ * ReactPhysics3D is distributed in the hope that it will be useful,        *
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of           *
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the             *
+ * GNU General Public License for more details.                             *
+ *                                                                          *
+ * You should have received a copy of the GNU General Public License        *
+ * along with ReactPhysics3D. If not, see <http://www.gnu.org/licenses/>.   *
+ ***************************************************************************/
+
+ #ifndef KILOGRAM_H
+ #define KILOGRAM_H
+
+ // Libraries
+ #include <stdexcept>
+
+ // Namespace ReactPhysics3D
+ namespace reactphysics3d {
+
+/*  -------------------------------------------------------------------
+    Class Kilogram :
+        The Kilogram class represents a mass unity. The value inside is a
+        mass in kilogram.
+    -------------------------------------------------------------------
+*/
+class Kilogram {
+    private :
+        double value;                           // Mass value in kilogram
+
+    public :
+        Kilogram(double value) throw(std::invalid_argument);        // Constructor
+        Kilogram(const Kilogram& mass);                             // Copy-constructor
+        virtual ~Kilogram();                                        // Destructor
+
+        double getValue() const;                                    // Return the mass value in kilogram
+        void setValue(double value) throw(std::invalid_argument);   // Set the mass value
+};
+
+// --- Inlines functions --- //
+
+// Return the value in kilogram
+inline double Kilogram::getValue() const {
+    return value;
+}
+
+// Set the value in kilogram
+inline void Kilogram::setValue(double value) throw(std::invalid_argument) {
+
+    // Check if the time value is positive
+    if (value >= 0) {
+        this->value = value;
+    }
+    else {
+        // We throw an exception
+        throw std::invalid_argument("Exception in Time: Wrong argument, a time value has to be positive");
+    }
+}
+
+}
+
+ #endif
diff --git a/sources/reactphysics3d/physics/Time.cpp b/sources/reactphysics3d/physics/Time.cpp
new file mode 100644
index 00000000..3ebe04a4
--- /dev/null
+++ b/sources/reactphysics3d/physics/Time.cpp
@@ -0,0 +1,46 @@
+/****************************************************************************
+ * Copyright (C) 2009      Daniel Chappuis                                  *
+ ****************************************************************************
+ * This file is part of ReactPhysics3D.                                     *
+ *                                                                          *
+ * ReactPhysics3D is free software: you can redistribute it and/or modify   *
+ * it under the terms of the GNU General Public License as published by     *
+ * the Free Software Foundation, either version 3 of the License, or        *
+ * (at your option) any later version.                                      *
+ *                                                                          *
+ * ReactPhysics3D is distributed in the hope that it will be useful,        *
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of           *
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the             *
+ * GNU General Public License for more details.                             *
+ *                                                                          *
+ * You should have received a copy of the GNU General Public License        *
+ * along with ReactPhysics3D. If not, see <http://www.gnu.org/licenses/>.   *
+ ***************************************************************************/
+
+ // Libraries
+ #include "Time.h"
+
+ // We want to use the ReactPhysics3D namespace
+ using namespace reactphysics3d;
+
+ // Constructor
+ Time::Time(double value) throw(std::invalid_argument) {
+    // Check if the value is positive
+    if (value >= 0.0) {
+        this->value = value;
+    }
+    else {
+        // We throw an exception
+        throw std::invalid_argument("Exception in Time : Wrong argument, a time value has to be positive");
+    }
+ }
+
+ // Copy-constructor
+ Time::Time(const Time& time) {
+    this->value = time.value;
+ }
+
+ // Destructor
+ Time::~Time() {
+
+ }
diff --git a/sources/reactphysics3d/physics/Time.h b/sources/reactphysics3d/physics/Time.h
new file mode 100644
index 00000000..c8dc2210
--- /dev/null
+++ b/sources/reactphysics3d/physics/Time.h
@@ -0,0 +1,100 @@
+/****************************************************************************
+ * Copyright (C) 2009      Daniel Chappuis                                  *
+ ****************************************************************************
+ * This file is part of ReactPhysics3D.                                     *
+ *                                                                          *
+ * ReactPhysics3D is free software: you can redistribute it and/or modify   *
+ * it under the terms of the GNU General Public License as published by     *
+ * the Free Software Foundation, either version 3 of the License, or        *
+ * (at your option) any later version.                                      *
+ *                                                                          *
+ * ReactPhysics3D is distributed in the hope that it will be useful,        *
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of           *
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the             *
+ * GNU General Public License for more details.                             *
+ *                                                                          *
+ * You should have received a copy of the GNU General Public License        *
+ * along with ReactPhysics3D. If not, see <http://www.gnu.org/licenses/>.   *
+ ***************************************************************************/
+
+ #ifndef TIME_H
+ #define TIME_H
+
+ // Libraries
+ #include <stdexcept>
+  #include <iostream>
+
+ // Namespace ReactPhysics3D
+ namespace reactphysics3d {
+
+/*  -------------------------------------------------------------------
+    Class Time :
+        The Time class represents a time unity. The value inside is the
+        time value in seconds.
+    -------------------------------------------------------------------
+*/
+class Time {
+    private :
+        double value;                   // Time in seconds
+
+    public :
+        Time(double value) throw(std::invalid_argument);                // Constructor
+        Time(const Time& time);                                         // Copy-constructor
+        virtual ~Time();                                                // Destructor
+
+        double getValue() const;                                        // Return the time value in seconds
+        void setValue(double value) throw(std::invalid_argument) ;      // Set the time value
+
+        // Overloaded operators
+        Time operator+(const Time& time2) const;                            // Overloaded operator for addition with Time
+        Time operator-(const Time& time2) const;                            // Overloaded operator for substraction with Time
+        Time operator*(double number) const throw(std::invalid_argument);   // Overloaded operator for multiplication with a number
+};
+
+// --- Inlines functions --- //
+
+// Return the value in seconds
+inline double Time::getValue() const {
+    return value;
+}
+
+// Set the value in seconds
+inline void Time::setValue(double value) throw(std::invalid_argument) {
+
+    // Check if the time value is positive
+    if (value >= 0.0) {
+        this->value = value;
+    }
+    else {
+        std::cout << "ERROR TIME : " << value << std::endl;
+
+        // We throw an exception
+        throw std::invalid_argument("Exception in test Time: Wrong argument, a time value has to be positive");
+    }
+}
+
+// Overloaded operator for addition with Time
+inline Time Time::operator+(const Time& time2) const {
+    return Time(value + time2.value);
+}
+
+// Overloaded operator for substraction with Time
+inline Time Time::operator-(const Time& time2) const {
+    return Time(value - time2.value);
+}
+
+// Overloaded operator for multiplication with a number
+inline Time Time::operator*(double number) const throw(std::invalid_argument) {
+    // Check if the number is positive
+    if (number > 0.0) {
+        return Time(value*number);
+    }
+    else {
+        // We throw an exception
+        throw std::invalid_argument("Exception in Time::operator* : The argument number has to be positive");
+    }
+}
+
+}
+
+ #endif
diff --git a/sources/reactphysics3d/physics/physics.h b/sources/reactphysics3d/physics/physics.h
new file mode 100644
index 00000000..18c7050a
--- /dev/null
+++ b/sources/reactphysics3d/physics/physics.h
@@ -0,0 +1,27 @@
+/****************************************************************************
+ * Copyright (C) 2009      Daniel Chappuis                                  *
+ ****************************************************************************
+ * This file is part of ReactPhysics3D.                                     *
+ *                                                                          *
+ * ReactPhysics3D is free software: you can redistribute it and/or modify   *
+ * it under the terms of the GNU General Public License as published by     *
+ * the Free Software Foundation, either version 3 of the License, or        *
+ * (at your option) any later version.                                      *
+ *                                                                          *
+ * ReactPhysics3D is distributed in the hope that it will be useful,        *
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of           *
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the             *
+ * GNU General Public License for more details.                             *
+ *                                                                          *
+ * You should have received a copy of the GNU General Public License        *
+ * along with ReactPhysics3D. If not, see <http://www.gnu.org/licenses/>.   *
+ ***************************************************************************/
+
+#ifndef PHYSICS_H
+#define PHYSICS_H
+
+// Libraries
+#include "Time.h"
+#include "Kilogram.h"
+
+#endif