Physics Model Position, Velocity, Acceleration, Force
A distinguishing characteristic of motion in the real world
is that it is continuous. Objects don't jump
instantaneously from point A to point B, nor do objects at rest
start going 100 mph all of a sudden. Most animation attempts to give the
impression of continuous motion, and using the ideas
in physics is a good way to achieve that.
Acceleration Velocity was introduced in the first two sections as
the change in position over time.Acceleration
has the same relationship to velocity as velocity has to position. It
is the change in velocity over time.
What this means in terms of the variables in a
program is that velocity is added to the object's position at
each animation step, and also that acceleration is added to the
velocity at each step.
Stepping on the gas - source
movie - To restart, right-click and select "restart"
onbeginsprite(me)
sp = sprite(me.spritenum)
x = sp.loch --initialized
to object's position xVelo = 0--zero,
object starts at rest
xAccel = .1--object
will accelerate to the right end
onexitframe()
xVelo = xVelo + xAccel --increment
velocity
x = x + xVelo --increment
position
sp.loch = x --set
sprite position end
You'll see the object flash by the screen again because
Director "wraps" the location properties somewhere around 32000.
•What if the acceleration is -.2? What if the acceleration
is -.2 and velocity is initialized to 10? Try it!
The two-axes demo showed how
diagonal velocity was separated into vertical and horizontal components.
This concept applies to acceleration as well. If you want an object to
accelerate in a diagonal direction, express it as vertical and
horizontal components. Try adding a vertical velocity and acceleration
to the example above and see what happens.
With position, velocity, and acceleration
you have the basic model for continuous motion. Its general
form in programming:
xVelo = xVelo + xAccel
yVelo = yVelo + yAccel
x = x + xVelo
y = y + yVelo
(where x and y represent position)
Forces In the real world, acceleration of
an object is caused by a force acting on the object.
Examples of forces are gravity and friction.
In animation, acceleration can be created without a force,
as it was above by just setting the x-axis acceleration to .2. However,
understanding various forces will give you more ideas to
work with when trying to get a particular motion you want.
