diff --git a/applications/newton/invariants/DragForce.nt b/applications/newton/invariants/DragForce.nt
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+#
+#	Authored 2018, Youchao Wang.
+#
+#	All rights reserved.
+#
+#	Redistribution and use in source and binary forms, with or without
+#	modification, are permitted provided that the following conditions
+#	are met:
+#
+#	*	Redistributions of source code must retain the above
+#		copyright notice, this list of conditions and the following
+#		disclaimer.
+#
+#	*	Redistributions in binary form must reproduce the above
+#		copyright notice, this list of conditions and the following
+#		disclaimer in the documentation and/or other materials
+#		provided with the distribution.
+#
+#	*	Neither the name of the author nor the names of its
+#		contributors may be used to endorse or promote products
+#		derived from this software without specific prior written
+#		permission.
+#
+#	THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+#	"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+#	LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+#	FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+#	COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+#	INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+#	BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+#	LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+#	CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+#	LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+#	ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+#	POSSIBILITY OF SUCH DAMAGE.
+#
+#
+#	Description: Physical invariant for a hydrodynamic drag force example
+#
+#	Necessary variables
+#
+#		L : length of cable
+#
+#		D : diameter of cable
+#
+#		V : velocity of the boat
+#
+#		rho : water density
+#
+#		mu : viscosity
+#
+#		F : drag force
+#
+
+include "NewtonBaseSignals.nt"
+
+viscosity : signal =
+{
+	name		= "Poise" English;
+	symbol		= mu;
+	derivation	= mass / (distance * time);
+}
+
+DragForce: invariant(L: distance, D: distance, V: speed, Dense: density, Visc: viscosity, F: force) =
+{
+	
+}