Newton's Third: Part Deux

And Kevin follows up with:

One of my AM students pointed out that San Jose State University is going to be offering a class in Physics for Animators. In a bit of serendipity, the professor of that class commented on my last post, of Hancock violating Newton’s Third Law of Motion, and provided a link to the program’s website.

This is a great idea, and I heartily support it. I’ve taken a look at the most recent tutorial (”The Physics of Timing”), and I have a few suggestions, mostly related to terms animators use. In this case, when we talk about “timing,” we’re generally talking about when key actions in our animation occur, or how long those actions take. The Physics of Timing Tutorial is mostly about the displacement, from moment to moment, of objects in a gravitational field. That’s very different from the issue of timing. So, in animation terms, what is this really tutorial about? Spacing!

This excellent tutorial is about the spacing of a falling (or sliding, or rising) objects. While a physicist may talk about displacement, we talk about spacing instead, because we’re concerned with the movement of things within screenspace, so we’re looking at the relative spacing from one frame to another, not actual distances.

Substituting ‘Spacing’ for ‘Timing’ may seem a subtle point, but there’s already way too much confusion regarding timing and spacing, which I referred to here and here.

Also, the tutorial repeats the hated “animations” language. Ugh, ugh, and double ugh. I know it’s common for students and people who have worked outside the mainstream of traditional animation to talk about “animations,” but it still sounds as wrong as talking about composing “musics.”

Finally, in the last image of the tutorial, this beautiful stroboscopic shot of a bouncing ball is reproduced. [see the original post at the link for the illustration. -- Hulett]

What’s wrong with this picture? The path of action is off. The arc that the ball traces in space isn’t smooth after each bounce. Now, this is a real photograph, so what gives? I’ll reprint the Widimedia explanation:

Note that the ball becomes significantly non-spherical after each bounce, especially after the first. That, along with spin and air-resistance, causes the curve swept out to deviate slightly from the expected perfect parabola. Spin also causes the angle of first bounce to be shallower than expected.

It’s important to note these discrepancies for students, especially when otherwise showing “idealized” examples (for example, air resistance in the rest of the tutorial is explicitly ignored). If a student copied these arcs exactly, I’d correct them. And they’d be confused, because they were perfectly copying a real example. The problem is, they’d be copying something they didn’t fully understand, and so we need to keep things as simple and clear (and exclude spin and air resistance) in these examples.

Those are quibbles, and meant as constructive criticism and clarification. This is a wonderful idea, and I highly recommend it! I look forward to what Professor Garcia et al. come up with.