fixes free test

Author: Brad Carman

test/Mechanical/translational.jl

@@ -10,7 +10,7 @@ D = Differential(t)
 @testset "Free" begin
     function System(; name)
         systems = @named begin
-            acc = TV.Acceleration()
+            acc = TV.Acceleration(false)
             a = Constant(; k = -10)
             mass = TV.Mass(; m = 100)
             free = TV.Free()