Force is the change of momentum in time. But suppose a body is at rest on the floor. The system is in mechanical equilibrium, meaning that the forces acting on it sum up to zero and there is no momentum because there isn't any displacement.
But if there isn't any displacement at all, how can I say that these forces (each one being cancelled) do exist? Don't I need acceleration, and thereby velocity and displacement to observe it's existence? Does something like a microscopic displacement exist?