lecture 8 Flashcards
(24 cards)
Mechanics:
the science that deals with the action of forces on the form and motion of bodies
- Center of mass:
balance point of a system
center of mass with gravity
- Free body in space
- All the points move the same amount in the same
direction - called TRANSLATION or BODILY MOVEMENT
force
Measured in?
Measured in ounces or grams
– A: Point of application = Origin of the arrow
– Line of action: Body of the arrow
– Direction: Body of the arrow and the arrowhead
– Magnitude: length of the arrow
where would point of application be in ortho?
crown
what movement happens here?
rotation with translation, point of application is away from the center of mass
center of resistance of a free body
- One point in a free body on which it can be perfectly
balanced (center of gravity) - Mathematical point at which all resistance to
displacement may be thought of as concentrated
Center of resistance of a tooth
roughly 2/3 from the apex, tooth is not a free body (embedded in bone)
tooth as a constrianed body
- A tooth is a constrained body.
- The center of resistancewill vary with alveolar height
what does decreased alveolar bone support cause in regards to ortho tx?
longer distance from center of mass to the point of application
types of force systems
single point force and force couple
Simple Force
types of simple forces
push, pull, intrusion and extrusion
Force systems in orthodontics
Point of application of the force: almost always at the crown level
* Direction of the force (3 planes of space)
* Amplitude of the force(varies)
Remember that we must also move the roots
simple forces in relation to the center of resistance at the tooth
will cause rotation and tipping, need force systems for better movement
If the line of force goes through the center of
resistance, what occurs?
If the line of force goes through the center of
resistance, pure translation will take place
* translation = all points on a tooth move the
same amount in the same direction
what will occur
pure translation
can we apply a simple force to the center of resistance?
usually not
In orthodontics, a simple force applied to the crown of a tooth will result in?
* crown/apex directions?
In orthodontics, a simple force applied to the crown of a tooth will result in tipping of the crown.
* The crown will tip in the direction of the force, the apex will tip in the opposite direction
how can we get translation in ortho?
- Two forces equal in magnitude and opposite in direction act on an object
- The result is a pure moment, since translatory effect of the forces cancel
- The object will spin around its center of resistance (rotation
moment of force
- Line of action of a force does not pass through the center of resistance
- The force will produce some rotation of the object
- This potential of rotation is called a moment
Moment of a force calc
Mf = F x d
F: magnitude of the force
d: perpendicular distance between the line
of the force and the centre of resistance
moment of force is a measure of?
rotational tendency
Translation
- In orthodontics, translation is often the
movement of choice - You need a combination of a simple force and
a moment of force to obtain translation
