diff --git a/HySoP/hysop/physics/compute_forces.py b/HySoP/hysop/physics/compute_forces.py
index 8d421bec1c31f56f309c6c3c8e1822a285a07189..685ba683b56803dc984b114e06de639ea7145e2d 100644
--- a/HySoP/hysop/physics/compute_forces.py
+++ b/HySoP/hysop/physics/compute_forces.py
@@ -204,7 +204,7 @@ class Compute_forces(object):
         self.coef = 2. / (uinf ** 2 * PI * self.obstacle.radius ** 2)
 
 
-    def apply(self, dt, velo, vort, Re):
+    def apply(self, t, dt, velo, vort, Re):
         """
         Computation of the drag according to the "impulsion" formula presented by
         Noca et. al in Journal of Fluids and Structures, 1999
@@ -239,10 +239,10 @@ class Compute_forces(object):
         comm = MPI.COMM_WORLD
         comm.Reduce([localForce, MPI.DOUBLE], [force, MPI.DOUBLE], op=MPI.SUM, root=0)
 
-        f = open('NocaForces.dat', 'w')
-        f.write(force, force / self.coef \n)
-        f.close()
-
+        if (self.topo.rank == 0):
+            f = open('NocaForces.dat', '+w')
+            f.write(t, force, force / self.coef '\n')
+            f.close()
 
     def integrateOnBox(self, force, vort, chi, dvol, dt):
         """