Physics in Animation: The Law of Inertia
Get ready to be swept off your feet because today we’re talking inertiaInertia is the resistance of any physical object to any changes like starting or stopping a motion or changing direction... More! Yep, that’s the physical principle that might literally sweep you off your feet during a bus ride and tosses you around in a roller coaster seat. Now, prepare to find out why it is important for your animation! Professor Alejandro Garcia, who teaches the course “Physics of Animation” at San José State, has returned to tell us about this law of nature that constantly influences you, me, or even a bowl of noodle soup whether still or in motion.
In Part 1 of this series I explained how knowing a little physics will help you observe motion in the real world and, if you want to, create animated motion that’s believable. The principles you’ll learn apply to everything from a simple ball bounce to doing a complicated shot, such as a fight scene.
As mentioned in Part 1, here is my list of the “Principles of Animation Physics”:
- Timing, Spacing, and Scale
- Law of InertiaInertia is the resistance of any physical object to any changes like starting or stopping a motion or changing direction... More
- Momentum and Force
- Center of Gravity
- Weight Gain and Loss
- Action-Reaction
The first principle, Timing, SpacingSpacing is the distance an element travels between two frames of an animation. By increasing and decreasing the spacings... More, and Scale, was covered in Part 1; in this posting you’ll learn the Law of InertiaInertia is the resistance of any physical object to any changes like starting or stopping a motion or changing direction... More, also known as Newton’s First Law of Motion.
Principle #2: Law of Inertia
The Law of InertiaInertia is the resistance of any physical object to any changes like starting or stopping a motion or changing direction... More says that a character will move with constant velocity unless acted on by an unbalanced force. First, let’s be clear what’s meant by an “unbalanced force.” The simplest form of the Law of InertiaInertia is the resistance of any physical object to any changes like starting or stopping a motion or changing direction... More is when there are no forces at all (e.g., an asteroid flying in deep space). In that case the object moves with constant velocity (i.e., constant timing and spacingSpacing is the distance an element travels between two frames of an animation. By increasing and decreasing the spacings... More with a path of action that’s a straight line).
Here’s a more down-to-earth example: A rock sitting on a table has two forces on it: gravity pulling downward and the table’s surface pushing upward; these two forces balance each other. A bowling ball rolling on a smooth floor has a similar pair of balanced forces (gravity and the floor) and, again, these two forces balance. Since the forces are balanced the bowling ball rolls with constant velocity. If there’s friction on the bowling ball then that’s an unbalanced force and so the velocity won’t be constant (the ball will slow down).
At first this all seems purely academic until you realize that follow-throughLoose, attached parts tend to continue moving after the leading element came to a stop. When a leading element is slowin... More in animated motion is entirely due to the Law of InertiaInertia is the resistance of any physical object to any changes like starting or stopping a motion or changing direction... More. Let’s take a simple example of a character standing on a moving bus. When the bus suddenly stops the character goes flying forward. Before the bus hit the brakes he was moving with constant velocity and, by the Law of InertiaInertia is the resistance of any physical object to any changes like starting or stopping a motion or changing direction... More, he’ll continue moving until an unbalanced force acts to stop him (such as when his face hits the floor).
A corollary of the Law of InertiaInertia is the resistance of any physical object to any changes like starting or stopping a motion or changing direction... More is that a character at rest will remain at rest until acted on by an unbalanced force. This means that what we call “dragLoose, attached parts tend to start moving with a delay and lag behind because of inertia. This is clearly visible at th... More” in animation is also entirely due to the Law of InertiaInertia is the resistance of any physical object to any changes like starting or stopping a motion or changing direction... More. Suppose our character stands up again after the bus comes to a stop. Now if the bus suddenly accelerates forward then our standing character seems to fall backwards.
To the passengers on the bus it seems as if there’s a force pulling everything backwards but that’s because they’re moving along with the bus. A stationary observer standing outside the bus would realize that the poor chap that’s falling is actually not moving, he has the bus moving out from under him (see illustration below). The character’s motion is actually the same but it looks different depending on whether the camera is inside or outside of the bus. This reminds us that you have to be careful to consider the effect of the camera’s motion, especially when the camera is accelerating.
The Law of InertiaInertia is the resistance of any physical object to any changes like starting or stopping a motion or changing direction... More also explains the so-called “centrifugal force.” If the bus makes a sudden left turn then the character standing on the bus will continue moving in his original direction, causing him to fall towards the right side of the bus. You experience the same apparent force when you’re in a car that takes a left sharp turn. It feels like you’re pulled towards the outside of the turn (the right side of the car) but in reality your body is simply obeying the Law of InertiaInertia is the resistance of any physical object to any changes like starting or stopping a motion or changing direction... More, which causes you to move in a straight line unless acted on by an unbalanced force (such as your seatbelt).
The Law of InertiaInertia is the resistance of any physical object to any changes like starting or stopping a motion or changing direction... More is also very important to keep in mind when animating overlapping actionDifferent elements of an object or body, come to a stop of different times. This usually happens because an attached, lo... More. As characters turn their bodies their hair and clothing dragLoose, attached parts tend to start moving with a delay and lag behind because of inertia. This is clearly visible at th... More behind due to the Law of InertiaInertia is the resistance of any physical object to any changes like starting or stopping a motion or changing direction... More (an object at rest will remain at rest until acted on by an unbalanced force). Once the hair and clothing are moving they’ll continue moving (i.e., follow-throughLoose, attached parts tend to continue moving after the leading element came to a stop. When a leading element is slowin... More) by the Law of InertiaInertia is the resistance of any physical object to any changes like starting or stopping a motion or changing direction... More. And when a character with long hair turns her head, by the Law of InertiaInertia is the resistance of any physical object to any changes like starting or stopping a motion or changing direction... More her hair flies outward, as if pulled away from the head by a centrifugal force.
Finally, I should mention that when animators talk about creating “weight” what they’re arctually referring to is creating “inertiaInertia is the resistance of any physical object to any changes like starting or stopping a motion or changing direction... More.” To understand this you have to learn the next Principle of Animation Physics: Momentum and Force. But for that you’ll have to wait for Part 3 of this series. Look for it and all the other Principles of Animation Physics appearing here on Animator Island. See you then!
[author] [author_image timthumb=’on’]https://www.animatorisland.com/wp-content/uploads/2011/10/AlejandroGarcia.png[/author_image] [author_info]Professor Alejandro Garcia has taught Physics of Animation at San Jose State since 2009. In 2011 he spent a one-year professional leave as physicist-in-residence at Dreamworks Animation SKG.
Find out more about Animation Physics on his website Animationphysics.org or his YouTube Channel[/author_info] [/author]
This is amazing information just like last time! I have been waiting for Part 2 since you posted the other one last year. It was a long wait. Worth the wait.
Also there are a lot of bolded sentences haha. You should pick maybe only a few instead of making so many bolded. It was distracting but I copied it into Word and made it all non-bold and that was easier on the old peepers. 😉
At a presentation I recently went to, Kip Thorne, Christopher Nolan, and Dr Brian Cox were discussing the physics of the film Interstellar and they said they ran water simulations and got waves that looked like it does. Computers!
Good post.
After looking at a handful of articles on physics in general,
I really appreciate your way of teaching. It is about the how
it is applied to animation and that is the best! Thanks,
Can’t wait for the next one!
Great post !! this is really helping me to understand some aspects of animation, that I was really confused about.
Thank you 🙂