Must-know Physics And Mechanics Principles Used In Animation

Animators use basic physics and mechanics principles they can bend for a more stylized look. In this article, we go through a few examples of physics and mechanics principles to better put words on animation techniques to create more engaging stories.

a month ago   •   6 min read

By Gwénaëlle Dupré
Photo by Paris Bilal / Unsplash

Animation is a reflection of the real world: it has to be just realistic enough to be believable, but also engaging and entertaining. Spider-Man’s movements are inspired by real-life parkour and breakdancing to make his web-slinging feel grounded and dynamic. 

To achieve this, animators use basic physics and mechanics principles they can bend for a more stylized look. In this article, we go through a few examples of physics and mechanics principles to better put words on animation techniques to create more engaging stories.

1. Body Mechanics

Bringing your characters to life requires understanding how real bodies move.

Body mechanics are the foundations of animating realistic actions like walking, running, and climbing stairs. For example, idle animations show characters standing and breathing subtly. You can choose any pose, neutral or action-oriented, and design it to loop seamlessly for a continuous effect. Other cycles and loops where the first and last keyframes are identical create the illusion of continuous movement.

Other body parts like hair, clothes, or even inanimate objects might require dynamic simulations using physics principles. For example, to simulate flowing hair or billowing fabric, enhance the believability of secondary movements (a cape fluttering in the wind), or breathe life into props (fall, bounce, collide, etc.).

For speech animation, a phoneme represents a specific mouth shape associated with a particular vowel or consonant sound. By mastering phonemes, you can create realistic lip movements that perfectly match your character's dialogue.

Source: Blender Artists

2. Timing and Spacing

Animation is not merely the art of drawing cool poses: timing and spacing are equally important to create believable movement, whether it's a powerful jump or a slow walk.

Think of timing as the number of frames dedicated to each action. It controls the speed of your animation. Fewer frames create a faster motion, while more frames slow things down. Imagine a bouncing ball: the higher the bounce, the more frames you'll need to show its peak and descent smoothly.

Frame rate (FPS) refers to the number of images displayed per second. It's the foundation on which timing is built.

Spacing refers to the distance an object moves between frames. It works hand-in-hand with timing to create the illusion of weight and speed. Wider spacing between frames suggests faster movement, while smaller spacing creates a slower, more deliberate action.

You can also combine timing and spacing with motion blur to simulate the blurring of objects during rapid movement. Imagine a fast-moving car – our eyes perceive a blur, not a series of crystal clear frames.

3. Weight and Mass

By understanding the difference between weight and mass, and how 3D software translates these concepts into animation tools, you can make informed decisions about how your objects move. While weight and mass are often used interchangeably, they hold distinct meanings.

Mass is the amount of matter an object contains. It's a constant value regardless of gravity. Imagine a massive boulder on Earth versus the moon – it has the same mass, but its weight differs due to varying gravitational forces.

Weight is the force exerted on an object due to gravity. A heavier object experiences a greater gravitational pull, making it require more force to move and stop.

3D animation software doesn't directly store weight or mass data for objects. However, it offers tools to mimic their effects on movement:

  • Animation Curves - By adjusting animation curves, you control how an object accelerates, reaches peak velocity, and decelerates. Slower curves for heavier objects create a sluggish, delayed response, while sharper curves for lighter objects depict quick changes in direction.
  • Force Fields - Some software allows applying forces like gravity or wind to objects. These forces influence the object's movement, mimicking how weight and external forces affect real-world objects.
  • Physics Simulations - Advanced software offers physics simulations that take into account factors like mass, gravity, and friction. These simulations can create highly realistic movements, but require careful setup and can be computationally expensive.

Pay close attention to how objects of different weights behave. While grounding your animation in a sense of physical reality is important, you'll often benefit from slight exaggerations.

4. Inertia, Momentum, and Force

Inertia, momentum, and force form the foundation of how objects move in the real world.

Imagine a bowling ball rolling down a lane. It keeps moving even after it leaves the bowler's hand. This is inertia in action. Inertia is an object's tendency to resist changes in its state of motion. An object at rest stays at rest, and an object in motion stays in motion (at a constant speed and in a straight line) unless acted upon by an unbalanced force. Inertia helps us depict weight and gradual stops. A character running won't come to a halt instantly. They'll skid a bit, their body continuing to move forward even as their feet come to a stop.

Source: Dreamstime.com

Momentum is the mass of an object times its velocity. Simply put, it's a measure of how much "oomph" a moving object carries. A heavier object or one moving faster will have greater momentum. A character throwing a punch transfers momentum from their body to their fist, resulting in a powerful impact. Likewise, a car with high momentum will take longer to stop than a bicycle.

Force is anything that causes an object to accelerate. It's the push or pull that disrupts an object's inertia and changes its state of motion. For example, friction between a shoe and the ground is a force that slows down a running character. A strong force applied to a character will result in a faster or more dramatic change in movement, while a weak force might lead to a subtle sway or wobble.

For every action, there's an equal and opposite reaction. This is Newton's Third Law of Motion. When a character throws a punch, their arm exerts a force forward (action). In reaction, the character's body experiences an opposite force pushing them back slightly. A character swinging a sword should experience a slight recoil as the blade connects with an object.

5. Center of Gravity

The force that keeps our characters grounded (and makes them fall when they lose balance) is gravity. The Center of Gravity (CoG) is that special point where all the object's weight is perfectly balanced. As animators, we constantly consider the CoG to ensure our characters move realistically and maintain proper balance.

The position of the CoG significantly impacts how your animated character interacts with gravity.

When the CoG falls directly within the base of support (think of the area where your character's feet touch the ground), they'll stand firmly balanced. We call this stable equilibrium. This is the typical pose for a character at rest.

If the CoG shifts outside the base of support, your character becomes unbalanced and starts to tip. This can be used for dynamic actions like leaning into a turn or falling over. By understanding how the CoG moves with different poses, you can create natural-looking weight shifts and falls.

6. Squash and Stretch

Squash and stretch involves exaggerating the way objects deform during movement, creating a sense of weight, flexibility, and humor.

When a bouncy ball hits the ground, it squashes downwards momentarily before stretching back up. This squash and stretch is what we amplify in animation. We might squash a character down as they jump, then stretch them out in mid-air, all while maintaining their overall volume. This exaggeration is what breathes life into the movement.

7. Green Screen / Chroma Key

Let's say you animate a character swimming through a coral reef. Instead of building a miniature reef set, a green screen allows you to film your character in front of a green backdrop. During editing, this green background is replaced with the vibrant underwater scene you envisioned.

Image by Wikipedia

But why green? The magic lies in color differentiation. Green typically doesn't clash with the color range used for characters or objects. This allows digital creation software to easily isolate the filmed element (your character) from the green background: you can just load the animation sequence, adjust the chroma key to identify the green background color, and replace the green background with something else.

Conclusion

Understanding physics and mechanics principles is essential for creating believable and engaging animation. They provide the foundation for realistic movement, weight distribution, and interaction with the environment. By mastering these concepts, animators can breathe life into their characters and tell compelling stories.

But animation isn't just about following cold, hard physics principles: it's an art form that thrives on creativity and expression. Animators often employ artistic license by subtly (or not so subtly) exaggerating movements or squashing and stretching objects to enhance character traits, evoke emotions, or inject humor. This artistic twist, grounded in a solid understanding of physics, is what separates a robotic imitation from a truly captivating animated performance.

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