Dynamics provides realistic motion of virtual objects by simulating the behaviour of virtual objects and their responses to external force and torque based on the laws of physics. It has been successfully applied in various fields such as engineering, medicine, training, and high-quality 3D games, but few VR therapy systems currently support physics. This paper presents opportunities for applying physics simulation techniques in post-stroke VR therapy and discusses their potential therapeutic benefits to upper limb motor rehabilitation. A framework for physically-based VR rehabilitation systems is described which consists of functional tasks, such as grasping and catching, and game scenarios designed to encourage patients' physical activity. Activity takes place in hihly motivating, physics-enriched virtual environments where collision response, gravity, restitution, and kinematics are simulated and factors such as gravity and restitution can be scaled to adapt to individual patient's abilities.