Class 1 - Week 6 - Pendulums in Motion

Since the lecture this week has already explained what the successive
breaking of joints is, let's hop straight into how to make a pendulum
look nice. The first thing we'll start with is getting a good reference
of a pendulum. How do we do that you may ask? Well we'll build one! Even
if you don't have a perfect replica of the pendulum in your assignment,
you'd be surprised just how easy it is to make one from odds and ends in
your house. If we take a close look at our pendulum, we'll see that it's
made of 3 solid bars, and a weighted ball on the end. So using string
won't be the best option for this kind of pendulum. We need something
more solid for our reference. One of the best and easiest is to simply
use paper clips and hook them together. This makes an excellent and
quick way of analyzing how a pendulum works in real life. But feel free
to experiment with your references. For example, Beau and I linked up
mousetraps together for our reference, and it worked pretty well!

Now that we have our own pendulum we can reference, let's use that
knowledge to figure out how our pendulum will work when it's attached to
a moving platform. I've split this section into 4 parts: the platform,
moving from a stopped position, moving at full speed, and stopping.

A Pendulum in Motion

Lets begin. The first thing we'll do is take a look at the platform
holding our pendulum. Since this is the main moving force of the shot,
or root, of our pendulum we'll animate it first before doing any
breaking of joints on the pendulum. We want to get the platform moving
in a simple line, or in nice arcs and clean motions, depending on your
mentor's preference. Once we have that done it will be much easier to
get the pendulum working nicely. A nice little tip for getting the
platform to look good is to hide the pendulum by turning off the
visibility of it's layer. That way you don't get distracted by it.

So with the platform set up, the next thing we'll do is begin moving the
pendulum from a stopped position. A lot of people when they first do
this will have the tip of the pendulum swing out in the opposite
direction as the platform begins to move. The problem with this is that
it's physically impossible for a lifeless pendulum to swing out like that
on its own. In the image below is an example of what it should look like:



What's happening is that the tip of the pendulum stays in its original
position until the very last moment. This is because an object at rest
tends to stay at rest, so as the platform moves, the first joint follows
while the others stay still, then the second follows after that,
followed by the third, and lastly the end will begin to move.

Next, we move onto the pendulum in full motion. This is another area
that a lot of people have a bit of trouble with when they begin. The
biggest issue is that they have a tendency to push the pendulum way too
far like in the image below. Remember that the pendulum has weight, and
judging by the size of the end ball, a fairly decent weight on the tip
as well. This combined with gravity and drag means that the pendulum
ball should rarely go over the platform (unless of course the platform is
dropping down fast or has a sudden boost of speed). Depending on the weight
of the pendulum and the speed of the platform, the pendulum can go from a slight
bend to nearly straight out to the side as you can see in the image below.



And lastly, let's talk about getting the pendulum to stop. I'll be
discussing a natural stop here, which means a pendulum swinging on its
own until it stops moving. No hitting walls or getting caught by
something in the scene. So to get your pendulum to slow down and stop,
the beginning animator might think " I'll just have it swing less and
less each time, and make each swing longer than the last. That should
give me a good slow down and stop." The truth is that it's only half
right. The pendulum will swing with less and less energy on each pass,
but the timing actually stays the same throughout. This is why pendulums
work so well in clocks and metronomes. Their timing stays so constant
because the only forces to slow it down are air friction, friction from
the joint that supports it, gravity, and momentum loss. No colliding
with objects that would cause it to lose energy quickly. So with each
swing, the pendulum loses a bit of momentum, making each following swing
shorter, which means that even though the pendulum swings have the same
take the same amount of time, it doesn't have to travel as far and thus
the spacing gets closer and closer until the pendum comes to a gentle stop.

And that's about all I have for this week. Thank you for taking the time
to read my post. I hope it's been helpful, and as always, if you have any
comments, questions or suggestions, you're more than welcome to send
either Beau or me a message on AM.