Add first test for PlanarMechanics: a simple pendulum

Author: AbdulrhmnGhanem

test/Mechanical/planar_mechanics.jl

@@ -5,9 +5,27 @@ using ModelingToolkitStandardLibrary.Mechanical.PlanarMechanics
 D = Differential(t)
 
 @testset "Pendulum" begin
+    @named ceiling = Fixed()
+    @named rod = FixedTranslation(rx = 1.0, ry = 0.0)
     @named body = Body(m = 1, j = 0.1)
     @named revolute = Revolute(phi = 0.0, ω = 0.0)
-    @named rod = FixedTranslation(rx = 1.0, ry = 0.0)
-    @named ground = Fixed()
-    @test true
+
+    connections = [
+        connect(ceiling.frame, revolute.frame_a),
+        connect(revolute.frame_b, rod.frame_a),
+        connect(rod.frame_b, body.frame),
+    ]
+
+    @named model = ODESystem(connections,
+        t,
+        [],
+        [],
+        systems = [body, revolute, rod, ceiling])
+    sys = structural_simplify(model)
+    unset_vars = setdiff(states(sys), keys(ModelingToolkit.defaults(sys)))
+    prob = ODEProblem(sys, unset_vars .=> 0.0, (0.0, 60), []; jac = true)
+    sol = solve(prob, Rodas5P())
+
+    # phi and omega for the pendulum body
+    @test length(states(sys)) == 2
 end