The physical forces and energy that impinge on the patient are influenced by the laws of physics. One of those laws, the law of inertia, states that:
a) An object at rest will stay at rest unless acted upon by an external force.
b) For every action, there is an equal and opposite reaction.
c) The acceleration of an object is directly proportional to the force applied and inversely proportional to its mass.
d) The velocity of an object remains constant unless a net force acts upon it.