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The concept of arcs is another animation principle you need to be aware of when animating. Now, arcs basically means that natural motion follows in arc. Anything that moves through nature typically moves along a curved arc of some sort. Very rarely do things move in straight lines through nature. Usually straight lines are caused by mechanical action rather than natural action. Now, what causes an arc is one of two things.
The first is in-balance forces. So if we get a force coming from one direction and a force coming from another direction, the sum of those two forces will typically pull an object in a curved path. The second reason things move in an arc is because we can have pendulum motion which is that an object is pinned in one direction or another and it has to follow an arc because it's moving much like a pendulum. Let's go back to our standard bouncing ball, which is one example of something that moves in a straight line.
If the ball is falling straight down, it will tend to fall in a straight line. Now, remember that in this instance we only have one force acting upon this object, which is the force of gravity. So that's why it moves in a straight line. But typically in nature we don't have just gravity moving an object. Either the object has a direction that it's following, it has wind. There are all sorts of things that affect the motion of an object and very rarely does something move exactly straight down.
So in this case we have basically a two-dimensional motion. We have an object moving in more than one direction. We have more than one force acting upon it. So I've graphed this out, so we have the falling motion of the ball but we're adding in just a constant linear motion as if the ball was thrown in a direction. So we have two force vectors or two forces acting upon this ball. We have one that's pulling it straight down and one that's pulling it to the right.
So when we play this, you'll see that the ball actually follows in arc and if you want to see this a little bit clearly, I've ghosted it so you can see that this is actually moving in an arc. Now, the only reason that this moves in an arc is because we have multiple forces acting upon the object and the sum total of these forces moves it in an arc. So let's take a look at this in a character animation context. Again, if you have multiple forces acting upon an object, it'll move in an arc.
In this case, we have a character who is moving from one pose to another. So, as she moves from one pose to another, her legs relax, her body drops down because it's under the force of gravity, and then she kind of stands up again and regains her composure. But if you notice we actually have multiple forces acting upon this character. So, if we just look at the hips, we have the force of gravity pulling down, but we also have the force of the legs pushing it to the side, very similar to our bouncing ball.
In fact, I've graphed this out as well. So let's go ahead and follow this red dot. So basically she moves down and to the side and then back up and to the side. And the sum total of this is just a small arc. So again, even in character motion, things move in arcs. So let's go ahead and just play this at speed so you can see it. A very quick motion but it's still an arc. Now, the other reason that things move in arcs is because of pendulum motion. So let's take a look at that.
Now, what pendulum motion is, is that you have a weight that's pinned at the top, very much like the pendulum on a grandfather clock. In fact, let's go ahead just play this so you can very easily see how a pendulum works. So again, I'm going to go ahead and ghost this and when you see it ghosted, you can see that again this is moving in an arc. And it's pretty obvious because it's pinned, so it actually has to move in a circular arc, not even a parabolic or any other sort of arc.
It's basically a circular arc. Now, this works very similarly to the joints of a character. So, when we actually move that arm, you can see that it's actually moving along an arc. In fact I've gone ahead and added another red dot to this so we can actually see how this moves. So the hand is actually tracing an arc through space. So, as you interpolate or in between your animation you want to make sure that the objects that you're animating move along an arc. A lot of times you'll just move something from one place to another and the computer will in-between it and when the computer in-betweens it, it typically in-betweens in a straight line.
So you need to be conscious of arcs and make sure that you include them in your character's motion.
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