The Physical and Mechanical Aspects of Orthodontic Appliances
These laws were first published in Latin,in 1687.The first law may be literally translated thus, Every body continues in its state of reat or of uniform motion in a straight line ,unless it is compelled to change that state by impressed force. This meano that if a body is at rest it will remain so unless some force acts on it,if in motion ,the velocity of motion must continue uniform unless some force acts to increase it or diminish it. Also the direction of motioA mast continue unchanged and therefom rectilinear unless some force causes it to be diverted. This law therefore supplies us with a definition of force; Force is that which produces or tends to produce, motion 0.' change of motion. Newton's second law of motion may be translated as follows:- Newton's second law of motion may be translated as follows:- The change of motion (produced)is proportional to the impressed force producing it,and pursues the direction in which that force is impressed. This law leads to a method of measuring forces. If we change the velocity with which a mass is moving,we also change its momentum. Change in momentum will serve to measure force.lt seems obvious that whatever change in momentum is produced by a force, twice the force will produce twice the change ,etc.i.e. the change is directly proportional to the force. For a given mass,m,change of momentum ,mv,means change of velocity;the change of velocity per unit time is aceeleration,a;the change in momentum per unit time is therefore malf we employ absolute units (poundals or dynes)this can be shown as; Newton's third law of motion states that 'to every action there is an equaI and opposite reaction'.This law recognises the dual aspect of forces It a tooth is pushed by a finger spring ,the spring is also pushed by the tooth,and an eqpal counter force acts towards the spring unti1 the biology of the system intervenes. This dual stress is called pressure. Retracting incisors against posterior segments it is apparent that the reaction of the posterior segments must be equal and opposite to the incisors.In this case the two forces act away trom each other,and tG this dual stress we give the name tension.