squash! Add free body test case

Author: AbdulrhmnGhanem

test/Mechanical/planar_mechanics.jl

@@ -22,6 +22,7 @@ tspan = (0.0, 3.0)
 
     free_falling_displacement = -0.5 * 9.807 * tspan[2]^2  # 0.5 * g * t^2
     @test sol[body.ry][end] ≈ free_falling_displacement
+    @test sol[body.rx][end] == 0  # no horizontal displacement
     # plot(sol, idxs = [body.rx, body.ry])
 end