Physics in Animation

Within this blog post, I will analysis the different types of physics within animation, these consist of reality physics, movie physics and cartoon physics. This gives context on what type of animation you are trying to make.

MOVIE Physics

https://www.youtube.com/watch?v=3kNqc5Hrh0M&t=141s

Within this clip (please skip to T: 2:20s), from The Hobbit: The Battle of the Five Armies, the character ‘Legolas’ pushes the norms of physics and uses the falling stone bridge rocks to jump back onto the stable bridge. The action is defying logic to impress the the audience of the difficult feat, yet successful execution nonetheless.

This type of animation physics is for entertainment. We know when the physics of the narrative world – no matter the genre – is being broken. Sci- fi and fantasy can push reality physics but not fundamentally break it without explanation.

Culture also plays a big part in what is acceptable in the move. For example, the action sequence within the film Couching Tiger, Hidden Dragon.

In this clip, although they are performing a realistic combat scene, the physics of the situation are pushed when they unnaturally walk on the walls and catching your opponent in the air.

These are real people acting, but the logic of the physics may be stretched to the align with the possibilities of that fictional universe. The audience will know and acknowledge that these physics are stretched but still accept them and that they make sense in the movie scenario.

An audience will know when the physics in the scene are wrong, but still accept them in the fictional universe of the movie.

CARTOON Physics

While most people may be disillusioned to believe that in Cartoon physics ‘anything goes’, however, there is still an order of rules within them. There is an underlying inherent logic that dictates the physics rules of that world. The physics logic of that world are generally used as a source of comedy. These rules are mostly established in 2D, as it is hard to replicate these with 3D rigs.

Cartoon Physics Rule Examples:

• Gravity: Turns on/off when it is comedically appropriate to do so.
• Holes: Certain characters can run through paintings on a wall.
• Death: No character ever dies (99% of the time true) and instead exhibits extreme squash and stretch.

Cartoon Examples:

• Roger the Rabbit
• The Simpsons
• Southpark
• Mask

Mask is a good cross over between cartoon physics and live action.

This is where a live action person acted like a carton character.

REAL Physics in animation

Laws of Motion

1. “An object will not change its motion unless a force acts on it.“

Simple put – Objects want to keep doing what they are doing.

2. “The force on an object is equal to its mass times its acceleration.”

Simply put – Movement = more mass requires force.

3. “When two objects interact, they apply forces to each other of equal magnitude and opposite direction.“

Simply put – Every action has an equal and opposite reaction. Action (down ball) vs Reaction (floor pushes up ball).

Important Elements of Physics in Reality

• Weight
• Mass
• Material
• Speed
• Momentum
• Force

Rules of Physics in Animation (from the video above):

Heavy objects are more resistant. They need more force to start moving. Light objects are less resistant and only need a little bit of force to move a lot.

When a heavy object is moved, it will take longer to get it moving- so there is a longer slow in when it begins to move, and a short slow out. Whereas a light object is easier to move and will move quicker/quickly.

When changing the direction of heavy objects, they will take more time and force to do so. The opposite can be said for light objects.

A heavy object will use more/ longer anticipations and overshoots to show the momentum needed to get the object to move. This show the built up force needed to push its mass to the next position.

Catching a light object would be easy and your hand would not be impacted too much. Whereas, with catching a heavy object- your hand must work more against the mass of the object. You have to go along with it to take the momentum out of it = overshoot. To make the heavy object stop, you need to apply a bigger counter force.

Objects would like to stay in the state that they are in unless a unbalanced force is applied.

Objects at rest will stay as it is unless there is a force on it.

Object in motion also stay within that motion unless other forces are applied. For example, a moving object in space will keep its momentum and travel until an opposing force intervenes. On the other hand, on earth, objects are impacted by gravity, wind, friction of the ground and other natural (like the previously mentioned) or unnatural (human interaction) elements.

Heavy and light objects actually fall at approximately the same speed. They will reach the ground at the same time. There is an exception if there is air resistance affecting the objects but generally that is the rule.

Previously mentioned was that heavy objects need more force to speed them up. The reason for this is because the force of gravity on heavy objects is bigger than on lighter objects. That compensates for heavy objects being slow to start (that is why they generally land at the same time).

Therefore, when we see objects fall, we are not actually receiving information about the weight but about the scale and the distance that they are falling.

Formula for calculating a free fall:

Distance:

Time:

Examples of fall distance and times:

If distance = 1.2 meters (3.9 feet) then the time is 0.5 seconds.

If distance = 20 meters (or 65 feet) then the time is 2 seconds.

Impact

On the moment of impact both heavy and light objects release the kinetic energy they gained during their fall. The speed is the same for both of them, but because of its larger mass and larger force, the heavy object gained more energy.

Energy cannot disappear. It always comes from somewhere and goes to somewhere. For example, a fictional super light and super (not realistic) elastic ball will continuously bounce to the same height. Instead of giving energy to the table, it takes all of its energy onto itself to bounce to the same height, again and again. On the other hand, many objects resist to changing their direction like that, because of their mass and non-elastic material, they turn a good portion of their energy from the fall into something else.

For example, a heavy falling book will create mostly deformation and destruction on both, the book and the table. The energy transferred goes to heat, sound, slight motion, deformation and destruction. There will be a slight horizontal motion after the fall. Further, a tennis ball’s fall energy would somewhat go back into the bounce and some energy would go into the table (as well as slight sound energy). This doesn’t mean that all light objects would bounce, for example fabric or paper will loose energy due to deformation.

Energy is transferable between objects that can move. Such as two tennis balls, when one impacts the other, energy will transfer to the second one and make it move.

These Physics Principles Applied to Animation

Light characters are quicker to speed up, slow down, change direction and stop. They can be influenced by big forces and big masses more easily.

A heavy character will be slower to speed up, slow down, change direction or stop. It is hard for lighter masses to push heavy characters around.

Summary

The motion physics world around us is expremely complex yet we tend to ‘think’ we know what is happening.

Movies do ‘lie’ to you and audiences spot ‘bad’ animation/logic.

You can’t animate ‘good’ cartoon physics until you animate (and practice) animating real physics.