“A force
which passes through the centre of resistance will produce pure
translation of a free body”.
Give a short
description of
(a)
The result if the force vector does not pass through
the centre of resistance.
(b)
How the degree of translation (rotation) is effected by
the perpendicular distance from the force vector to the centre of resistance.
(a): For an object in free space, the center of resistance is the same as the center of mass.If the object is partially restrained, as is the case for a fence post extending into the earth or a tooth root embedded in bone, the center of resistance will be determined by the nature of the external constraints.The center of resistance for a tooth is at the approximate midpoint of the embedded portion of the root (i.e., about halfway between the root apex and the crest of the alveolar bone.
ReplyDelete(b):THE CENTER OF RESISTANCE (CR) FOR ANY TOOTH IS AT THE APPROXIMATE MIDPOINT OF THE EMBEDDED PORTION OF THE ROOT. IF A SINGLE FORCE IS APPLIED TO THE CROWN OF A TOOTH, THE TOOTH WILL NOT ONLY TRANSLATE BUT ALSO ROTATE AROUND CR (I.E., THE CENTER OF ROTATION AND CENTER OF RESISTANCE ARE IDENTICAL) BECAUSE A MOMENT IS CREATED BY APPLYING A FORCE AT A DISTANCE FROM CR. THE PERPENDICULAR DISTANCE FROM THE POINT OF FORCE APPLICATION TO THE CENTER OF RESISTANCE IS THE MOMENT ARM. PRESSURE IN THE PERIODONTAL LIGAMENT WILL BE GREATEST AT THE ALVEOLAR CREST AND OPPOSITE THE ROOT APEX.
When a force not acting through the center of resistance produces a moment that tend to rotate a tooth.
ReplyDeleteOrthodontic forces can be treated mathematically as vectors. When more than one force is applied to a tooth, the forces can be combined to determine a single overall resultant. Forces can also be divided into components in order to determine effects parallel and perpendicular to the occlusal plane, Frankfort horizontal, or the long axis of the tooth. Forces produce either translation (bodily movement), rotation, or a combination of translation and rotation, depending upon the relationship of the line of action of the force to the center of resistance of the tooth. The tendency to rotate is due to the moment of the force, which is equal to force magnitude multiplied by the perpendicular distance of the line of action to the center of resistance. The only force system that can produce pure rotation (a moment with no net force) is a couple, which is two equal and opposite, noncolinear but parallel forces. The movement of a tooth (or a set of teeth) can be described through the use of a center of rotation. The ratio between the net moment and net force on a tooth ( ratio) with reference to the center of resistance determines the center of rotation. Since most forces are applied at the bracket, it is necessary to compute equivalent force systems at the center of resistance in order to predict tooth movement. A graph of the ratio plotted against the center of rotation illustrates the precision required for controlled tooth movement.
ReplyDeleteThe magnitude of the moment is represented by the product of the force magnitude,and the perpendicular distance between the line of the force and the center of resistance. The longer the moment arm,the greater the rotation effect.
ReplyDeleteyes... i agree
Deleteyes... i agree
Deleteyes... i agree
Deletea) Moments are produced as aresult of forces acting away from the Center of Resistance or Center of Mass.
ReplyDeleteb) A force applied away from the center of a body will cause the body to move in same previous direction, but rotation will also occur as a result of the moment created by the line of force acting at a perpendicular distance to the center of the body.
a) once a force is not applied in the centre. unequal forces will be exerted on the tooth, therefore the tooth will eith tip of torque
ReplyDeleteb) The more perpendicular the distance from the force vector to the centre of resistance, the more equal the force on the body and the more the distance between the perpendicular vectors the stronger the translation.
a) The centre of resistance of a tooth is constant. in a single-rooted tooth it lies between one third and one half of the root, apical to the alveolar crest while in multi-rooted tooth the centre of resistance lies between the roots, 1-2mm apical to the furcation. To agree with Mr Mahoto once force is not applied in the centre the tooth will tip or torque.
ReplyDeletea) If the force vector does not pass through the centre of resistance, the object (tooth) may tip or rotate depending on the force applied.
ReplyDeleteb) The degree of rotation or translation is determined by the location of the centre of rotation. If the centre of rotation is near the centre of resistance, the body will predominantly rotate. However, if the centre of rotation approaches infinity the body will predominantly translate
a) If the applied force is not on the resistance center, rotation will occur as more force is applied on a certain point.
ReplyDeleteb) ???
A) Moment of farce is when the line of action of an applied force does not pass through the center of resistance; the force will produce some rotation. The potential for rotation is measured as a moment.
ReplyDeleteB) the degree of the multiplied by the perpendicular distance of the line of action of the force to the center of resistance.
M=F×d
Where,
M is the moment,
F, the magnitude of the force and
D, the perpendicular distance of the point of application from center of resistance.
The moment of force can be altered by changing the degree of the force or by changing the perendiclar distance of the line of action of the force to center of resistance. hence, if the degree of the moment needs to be decreased, the force can be applied closer to the center of resistance of the tooth
You have to understand what translation is;
ReplyDelete- a force applied in linewith the centre of resistance
* the tooth is translated with no rotation relative to the force
- the point of attached is irrelevant
* the line of action of force determines the effect on the tooth