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Law of inertia, part one. So you might think it's enough to see how things move and that’s
done in a number of tutorials that talk about slowing in an out or path of action. But it's
also very important and useful in animation to understand why things move for the way
that they do. Now the short answer to explain why things move as they do is to understand
how forces affect motion. And that's what we will start doing with this tutorial. So
Isaac Newton established the three basic laws to explain the relationship between motion
and forces. There is a lot of inertia that we will discuss in the this tutorial, the
law of acceleration and the action reaction principle that we will seeing some other tutorials,
and all of these principles that Newton established were, in a sense, rediscovered by Disney and
some of the other early animators when they were studying motion and trying to make their
animations more believable, they observed these principles directly. So let's see how
some of that works with the law of inertia. So we start with a situation where there are
no forces. So if we have something like an asteroids drifting through space, so that
it's away from any gravitational force and such, so the motion is very simple, the asteroid
will float with a constant uniform motion, so it's going to move in a straight line with
constant speed. Now if there is a force acting on the asteroid, for example like a gravitational
pull, then in that case, the motion is no longer constant uniform motion, instead we
will have the possibly slowing out or slowing in and also the path of action could be change
from a straight line to a curve. Now it's rare that we have a situation where there
are absolutely no forces acting up. However it's a very similar situation when forces
are balanced. So we have a pair of forces, let’s say we have a flour sack sitting on
the floor, and the force of gravity is pulling down on the flour sack. However the floor
is exerting an upward force and those two are balanced, and the flower sack just sits
there. Similar situation with a force of tension, the flour sack is hanging from a rope. Now
we shouldn't imagine from these first two examples that when forces are balanced that
means that there's no motion, because we had a similar situation, when we have a bowling
ball rolling along floor, if we don't have much friction then the floor exerts an upward
force on the bowling ball balancing the downward pull of gravity. So in this case again, the
those two forces are in balance. So understanding balance of forces is important because the
law of inertia, this is also known as Newton’s first law, the law of inertia says that an
object moves with constant uniform motion until acted on by an unbalanced force. So
this tells us the bowling ball rolling along the floor with the upward force of the floor
balancing the downward pull of gravity, so then there's no unbalanced force then the
bowling ball should move with constant speed. Of course in reality, there's a small unbalanced
force, namely friction, and so the ball doesn't roll forever. With the constant uniform motion,
it will slow a little bit due to friction, but aside from that asterisk, we have constant
uniform motion. Here is an example of where we might see the law of inertia in practice.
Let's say you're standing on a bus, and are distracted, and the bus is moving along but
then it comes to a sudden stop. Well, you're moving and you continue moving until there
is an unbalanced force that brings you to a stop. So because you continue moving, you
arer seemingly thrown forward, but in reality, what is happening is you are continuing to
move, according to the law of inertia. Let's watch a little scene from this rather cartoony
live-action film ‘Shoot ‘em Up’. Now I don't think I have to tell you that you
should not try it out yourself. Now if we go and look at that stunt, where the hero
in this movie is thrown out of his vehicle into the van. Well, in reality, the stunt
was done somewhat slower than would be implied by the speed of vehicles. But so here, we
see on these frames, the stuntman is actually moving about 10 miles an hour, rather than
look 30 or 40 miles an hour as the car seems to be traveling. Now it's sort of make sense
as they slow this down, not just for the safety a book of the stunt person, but also so that
the action is slow enough that the audience can see it, and you can see what's going on.
And then the stuntman flies into the van at even slower speed, which again is probably
intentional by the directors, just to make the scene even more outrageous and and cartoony.
But in spite of it being distorted, we’re still seeing the law of inertia in practice,
in the scene where it would be character being thrown out of the vehicle. Now as I said the
Disney and the other early animators and saw this principle in action, when they were studying
motion, and here's a quote from ‘The Illusion of Life’: when the character entering a
scene reached the spot for his next action, he often came to a sudden and complete stop.
This was stiff and did not look natural. Walt was concerned: “things don't come to a stop
at once, guys first there's one part and then another.”’ So following these ideas, the
Disney animators came up with the concept of follow-through and overlapping action.
So here we see in a couple frames an example of follow through and overlapping action as
King Julian is dancing and comes to a sudden stop, but the whole body doesn't freezes,
his tail continues moving for a few frames. So this follow through and overlapping action
are just the result of the law of inertia. So the tail continues moving until forces
bring it to a stop. And the we see this in many, many instances in animation. If you
just watch carefully, you'll see situations where there's follow-through, and if you understand
law of inertia, hopefully that follow-through and those overlapping actions will now make
more sense. So in the summary, law of inertia says that an object moves with constant uniform
motion until acted on by an unbalanced force. A force is balanced by an equal force acting
in opposition, for example in the floor pushing you up versus gravity pulling downward. So
the law of inertia is often applicable because it's often the case where forces like gravity
happened be balanced. So we see the law of inertia in practice, very often and that is
encapsulated in the principles of follow-through and overlapping action. An example being the
secondary motion that occurs when a character comes to a sudden stop, things like the hair,
clothing, floppy ears, things like that don't suddenly freeze and stop, but have secondary
motion in the form of follow-through. Well this is the first introduction to forces and
the law of inertia, we will continue looking at more of these in the next tutorials, and
also talk about a closely related topic, which is drag in animation. So see you then.