Friction occurs when materials are in contact with each
other but have different velocities. This demo deals with air
friction, which is a useful one in animation. An object moving
through air experiences friction with the air, which pushes against the
object.
To "model" a force such as air friction, the
first questions to ask are what direction does it act in, and
what determines its strength?
Unlike surface gravity in the last demo, whose direction
is always downward, air friction always pushes in the direction
opposite the object's velocity. And its force increases
as velocity increases.
How can this be expressed in terms of the variables
in the physics model? Since the direction and force of air friction
depend on velocity, the formula will be relying on xVelo
and yVelo. It's form is friction
= -velocity.
--acceleration is sum of forces
xAccel = xGrav + xFric
yAccel = yGrav + yFric
--increment velocity and position
xVelo = xVelo + xAccel
yVelo = yVelo + yAccel
x = x + xVelo
y = y + yVelo
--set sprite position
sp.loch = x
sp.locv = y
end
Air friction - source
movie - To restart, right-click
and select "restart"
Due to air friction, the motion is similar to a feather
or a snowflake. The object cannot exceed its terminal velocity,
which is the velocity at which the force of friction pushing
up equals the force of gravity pulling down. At terminal velocity, what
does the expression yGrav + yFric equal?
Since there is no horizontal motion, all
the "x" statements could be removed without affecting
the program. However, this script shows the general form
of the two-dimensional physics model. And now its easy to add a little
horizontal motion, by either setting the initial horizontal velocity
or gravity's horizontal component (funky gravity, try
it!).
Why multiply velocity by .06 to get friction?
It scales the friction down in relation to velocity,
and was a matter of testing values until the animation looked like I wanted.
The .06 could be considered the viscosity of the air—the
greater you make it, the more it looks like the object is moving through
molasses. The expression for the basic idea behind a force, like friction = -velocity, is usually then scaled so that
its effect is proportionally correct in relation to other
elements of the animation.
The type of behavior that air friction causes is useful
in animation, because it is a general way of limiting the velocity
of an accelerating object, even though you may not be trying
to imitate an object moving through air.
Sliding Friction
Another common type of friction is sliding friction, where one object
slides on a surface. This friction still resists an object's motion,
but it acts differently depending on whether the object is stationary
or moving, called "static friction" and "kinetic
friction". A demo might be made later, but for now I'll just describe
what happens briefly.
As you begin to push on a stationary object, it doesn't
move. Static friction increases to oppose the force of
your push, until you push hard enough and the object begins to move. At
this point the friction between the object and the surface lessens
and is called kinetic friction. Kinetic friction is constant
regardless of velocity.
What is sliding friction good for in animation, other than
animating a sliding object? It doesn't seem to be as useful as air
friction. You might use the idea of the static friction part to
make an object look like it was "stuck"
until it was pushed on hard enough.
