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696 Chapter 12: Animation
Sliding Friction—
Determines how difficult it is for
an objec t to keep moving over a surface (the higher
this value, the more d ifficult for the object to keep
moving). Once two objects be gin to slide over
one another, static friction disappears and sliding
friction takes over. Generally, sliding friction is
lowerthanstaticfrictionduetosurfacetension
effects. For example, once steel starts sliding over
brass (a value of static friction that mig ht run
from .05 to .2), the sliding friction drops to a
significantly lower value: .01 to .1.
Edit Object Dialog
Utilities p ane l > Utilities rol lout > More button >
Dynamics>Dynamicsrollout>EditObjectbutton>Edit
Object dialog
TheEditObjectdialogisthemaininterfacefor
setting dynamic properties for any object in the
simulation. You assign dynamics propert ies one
object at a time. Select an object in the list under
Object, and then set that object’s parameters. For
example, you can cause the object to be affected by
gravity or wind, and specify collisions with other
objects.
Inter face
Object—Displays the name of the object for
which you’re sett ing the dynamic properties. All
settingsintheEditObjectdialogaffecttheobject
listed here. To change the object you’ re affecting,
open the list, and choose from a list of all objects
assigned to the simulation.
D yn am ic Controls group
Use Initial State—When tur ned on, the simulation
solves for the motion, energy, and momentum
of the object at the simulation’s starting frame
(specified in the Start Time spinner in the Time
and Simulation rollout > Timing g roup.) If Use
Initial State is turned off, the object is assumed
to b e sitting motionless at the starting frame.
The Dynamics utility overwrites keys. Either the
object’s keys are clocked with the simulation, or
it’s not moving (or both). If you don’t solve for
the initial state, the object’s keys are interpolated
at the instant the simulation starts and all
subsequent motion comes from the interaction of
the simulation itself.
This Object is Unyielding—Lets you use
key f rame-animated objects in dy namics
simulations. Objects with this option turned on
areimmovablerelativetootherobjectsthatcollide
with them, but can be animated (keyf ramed).
You can animate objects with this option turned
on by themselves or as part of a keyframed
hierarchy. For collisions, objects with Use Initial
StateturnedoncannotmoveobjectswithThis
Object is Unyielding turned on.
Move Pivot to Centroid—When this button is on
(green) and you exit the Edit Object dialog by
clicking OK, the object’s pivot point is moved to its
center of mass. Having the pivot aligned with the
center of mass speeds up the dynamics ca lculation
and makes manual keyframing of the objects
appear more natural. However, if the pivot point
is already positioned as part of a linked hierarchy,