I was searching for physics (as I haven’t got enough in college :D) but in animator’s perspective, and I found this site called Animation Physics, it has some great tutorials about animation physics, I picked some common examples to share it here with you, it’s from the balance & weight shift tutorial, download the complete pdf file here.
Enjoy :)
In this first example the object feels light as a beach ball. The lack of weight is indicated by how the character is posed holding the ball in her arms, away from her body. If the ball was very heavy then the center of gravity of her plus the ball would be located in front of her toes. In that case, no matter how strong she is, she’d fall forward (imagine her as a statue holding a heavy, solid ball).
This lift conveys that the water bottle is heavy. The character does not extend her arms away from her body and she also uses her legs to help lift. Regardless of her strength, she needs to keep the heavy bottle close to her body so as to keep the total center of gravity (her plus the bottle) located over her base of support (her feet). She also leans backwards for the same reason.
The hiker’s pose on the left suggests that his pack is light. The pose on the right tells you that the pack is heavy because he has to lean forward to keep the total center of gravity over his base of support (his feet).
Your base of support changes as you rise from a seated position and this affects the poses. When rising slowly the center of gravity needs to remain above this base for the character to maintain balance at all times. Notice that rising from a chair with arms is easier because the hands may be used to expand the base of support.
If the character stands up quickly then some of the poses may be out of balance. If so then the motion slows into the standing pose, much like a rising ball slows into its apex.
Balance may be achieved with support forces pushing or pulling in the horizontal direction when those forces exert a balancing counter-torque. The character’s pose is obviously out of balance. The upward and downward forces are not aligned so they produce a torque that causes a rotation. The character tips backward and falls on her back.
The same pose is in balance when the character leans against a wall since the rotation is prevented by horizontal support forces. There are two horizontal forces that create the balancing counter-torque:
* The wall pushes the character towards the right.
* The frictional force of the ground pushes towards the left.
It’s important to think about these forces because they affect the character’s pose (e.g., the angle of the lower leg as it braces against the frictional force pushing on the foot).
The character’s pose is the same if the force on her back were due to someone trying to push her forward.
