Weight

Weight is one of the trickiest things to get right in an animation, and can be broken down into many factors that must be combined to imply a character's weight.

Weight is a very complicated subject due to the fact that it is affected by so many factors. It's something I've spent considerable time analyzing, especially in the process of writing this post, since it's a difficult idea to put into words. We all can see what proper weight is, even if we can't explain it. But if done successfully, it adds so much believably to your work.

Opposing Forces

In essence, weight is shown through opposing forces. If you've taken a physics class, you've likely learned about this. Netwton talks about it in his third law:

For every action, there is an equal and opposite reaction.

One example of this is when you are standing, your feet are pushing into the ground with the same force that the ground pushes back. To show these forces in action, we imply them using many of the basics of animation: line of action, balance, timing, overlap, and squash and stretch most importantly.

Line of Action

The posture of a character alone can instantly indicate how heavy a character or any corresponding object is. You can easily tell which of the characters below is holding something heavy or not based on how they're holding it. The girl with the heavy papers is holding them close to her body and leaning back, whereas the two men are holding their food far away from their bodies and are not struggling to lift it with just one hand.

The angle of the hips and shoulders can also indicate where a character's weight is. I often think about this in terms of contrapposto. When a character's weight is shifted to one side, the hips will go up on that side, and the shoulders are usually at the opposite angle to this. Having the hips shift angle each time the weight changes places is an important part of implying weight in your animations.

Balance

It's important to look at your animation from all sides, even though you will only see it from one camera view. Part of the reason for this is to make sure that the character is in balance. To be in balance, the character's center of gravity (the pelvis in humans) must be over the area covered by each of their contact points. For example, if you draw a line between the heels and toes of each foot, that area is the area over which your character must stand to be in balance.

It's very important to keep this in mind during weight shifts as well. A character's weight must be over or almost over one foot, before you can raise the other. This can get trickier when a character is holding another object, since this changes the position of the center of gravity and can create situations where the character must counterbalance to stay upright.

*However, it's important to keep in mind that all of this is a basic rule and can be broken in animations like run cycles, since the body isn't always in balance when in motion.

Timing/Spacing

The timing of your actions tells the viewer how much effort the character needs to start moving, change directions, or stop. Since it takes more effort to move heavy things, heavy characters will move slower and more of the body will be involved in the motion. You can exaggerate this idea in your animation to emphasize force.

The videos below show the differences between a heavy character and a light character:

Overlap

Overlap is also very noticeable in heavy characters because they will have more drag on their arms, heads, etc. This is because each part of a body weighs differently, and they won't all move at once for that reason. And in heavy characters, this delay in movement is more exaggerated, since each part needs longer to start/stop moving.

Squash and Stretch

Squash and stretch is a very useful tool for showing weight, since it indicates how much effort it takes to make a movement. For example, you must anticipate (squash) a lot more in order to jump far/high. A heavier character will also have a much larger compression on impacts, since it takes longer to slow their weight.

To really understand these concepts it takes a lot of practice and studying reference. If you're a beginning animator, I recommend practicing a lot with walk cycles, since these are great for teaching the concept of weight and weight transfer.