January 2011 “We come into this world head first and go out feet first; in between it is all a matter of balance.“ Paul Boese Physics of Balance & Weight Shift This tutorial helps you create realistic poses for your characters by showing you the basic principles of balance. You’ll also see how weigh shift affects a pose and what makes an action pose dynamic. © 2011 Alejandro L. Garcia Creative Commons Attribution- Noncommercial-Share Alike 3.0 United States License Character Design ► Establishing balanced poses for your characters is an important stage in the design process. You don’t want a standing character to look like it’s about to tip over (unless it’s supposed to). So how do you know that a character’s pose is in balance? Consider these various poses; which ones are in balance and which ones are not. How can you tell the difference? Is the character’s weight correctly refl ected in the pose? Understanding the principles of balance is equally important in visual development and for animation since stationary characters and objects in a scene need to be in balance. Balance is not the same as symmetry and symmetry is not the same as balance. The principles of balance also apply to moving characters, which need to be in or out of balance in a way that’s consis- tent with how they’re moving. 2 Animation Physics — Balance & Weight Shift Center of Gravity ► The average position of an object’s weight distribution is called the center of gravity (CG). For simple, solid objects, such as a baseball or a brick, the X X center of gravity is located at the geometric center. If an object does not have a uniform weight distribution then the center of gravity will be closer to where most of the weight is located. For example, the center of gravity for a hammer is located close to where the head connects to the handle. X X Hammer CG X Handle CG Head CG The center of gravity can be located at an empty point in X space, such as the center of a hollow ball. The center of gravity can even be completely outside of an object, such as for a donut or a curved banana. X X Note: Center of gravity is the same as the center of mass when the strength of gravity is constant, as it is on Earth. 3 Animation Physics — Balance & Weight Shift Locating the CG by Suspension ► One way to locate an object’s center of gravity is by means of suspension. An object always hangs such that the CG is directly below the point of suspension. The center of gravity of the hammer is X located inside the handle close to the X hammer’s head. Suspension from a single point is not enough to locate the center of gravity, it only tells you that the CG is somewhere X below the point of suspension. But hanging the object from a second point lets you to fi nd the CG by triangulation. These photos show a doll, in various action poses, suspended from a variety of points. Notice how the position of the CG depends on the orientation of the arms and legs. In this U-shaped pose the CG is located out- side the body, at about the height of the pelvis. With the arms raised the CG shifts higher in the body. The doll has a hollow head so its CG is located a bit lower in the body than for a human. 4 Animation Physics — Balance & Weight Shift Locating the CG by Support ► Another way to locate an object’s center of gravity is by means of support. An object will balance on a pivot point when the CG is exactly above or below that pivot point. Question: If the pivot is below the CG then the balance is unstable; any What’s the easiest way to balance a donut on your fi nger? slight rotation causes the object to tip and fall off the pivot. :rewsnA hguorht regnfi ruoy tuP If the pivot is above the CG then the balance is stable and eht ta detacol ,GC eht taht os eloh eht a slight rotation makes the object just swing back and forth. .regnfi ruoy woleb si ,tunod eht fo retnec XXX X Stable Unstable The toy bird above balances stabily on the tip of its beak. The wing tips are weighted so that the center of gravity is located just below the bird’s beak. The center of gravity is proportionally closer to where most of the weight is located, as in the hammer shown here. 2 inch 6 inch X X Hammer CG X 10 ounce Handle 30 ounce Head 10 pounds Locating the center of gravity by pivoting on 40 pounds a support is based on the same principle as a balance scale. The scale will balance when the leverage is the same on each side. 4 feet 1 foot 5 Animation Physics — Balance & Weight Shift Human Center of Gravity ► Standing upright, an adult human’s center of gravity is located roughly at the center of their torso at about the height of the belly button (at about 55% of the total height). The exact location of a character’s center of gravity will shift depending on the pose. For example, this character’s CG rises a few inches when she raises her arms. X X CG The center of gravity can even be at a point outside the CG body, such as when bent over in an inverted-U pose. X CG The line of gravity is an imaginary vertical line that extends upward and downward from an object’s center of gravity. X When a person is standing fairly straight, the line of gravity CG can be considered to be a plumb line that passes through the pit of the neck. Note: Line of gravity is not the same as the line of action, which indicates the dynamic, visual curvature of a pose (see the Physics of Paths of Action Tutorial). 6 Animation Physics — Balance & Weight Shift Falling & Tipping ► One way to understand balance is from the fact that if nothing prevents a stationary object from lowering its center of gravity then it will move in order to do so. The simplest example is when you drop a ball, it falls. Stepping off of a high tree branch, Tarzan swings downward on a vine. The motion may continue past the lowest point, due to follow-through (inertia), but if he keeps swinging back and forth he’ll eventually come to rest with the CG as low as possible (right under the point of suspension). This simple principle, that an object will generally move so as to lower its center of gravity, helps you predict balance. X X X We know that this brick, sitting on a ramp, is in balance since a rotation to either side would raise the center of gravity. A solid wheel goes downhill since rolling lowers its center of gravity. CG Hole CG X On the other hand, this wheel with a hole that’s X off-center does not roll downhill since that rotation actually lifts the center of gravity. Question: In these photos you see one of my favorite demonstrations in which a hammer is attached to a hinged board. The photo on the left looks normal but how is it that in the right photo we see the hinged board in the raised position (it even supports a small weight placed on top of the board)?! :rewsnA eht raen detacol si GC ehT ta si GC eht ,tfel eht nO .remmah eht fo daeh eht nO .nwod draob eht htiw noitisop tsewol sti eht htiw noitisop tsewol sti ta si GC eht ,thgir eht fi detfil steg GC eht esac siht ni ;pu draob .drawnwod selgna draob 7 Animation Physics — Balance & Weight Shift Base of Support ► An object is in balance if its center of gravity is above its base of support. For the two cylinders below, the left cylinder’s CG is above the base of support so the upward support force from the base is aligned with the downward force of gravity. Balanced Unbalanced CG CG X X The base of support for these tilted cylinders is the area where they are in contact with the fl oor. For the cylinder on the right the CG is not above the base of support so these two forces cannot align and instead create a torque that rotates the object, tipping it over. X CG Balanced Unbalanced CG CG X Base X This wine bottle holder is stable be- cause the CG is right over the base of support. Pour a small amount of water into an emp- tyy soda can and yyou can stand it on its The line of gravity helps you determine balance; if it passes bbeevveelleedd eeddggee.. through the base of support then the object is in balance. If the line of gravity touches the ground at a point outside the base of support then the object will tip over. CG X 8 Animation Physics — Balance & Weight Shift Human Base of Support ► Standing upright, your base of support is the area under your feet (or shoes) including the area between your feet. Roughly speaking, this area is traced from toe to toe and from heel to heel. Base of By moving your feet you can an increase or decrease the Support area of your base of support. The larger the base, the easier it is to keep center of gravity above it and stay in balance. Relaxed Feet Together When an object has multiple bases of contact with the ground (e.g., two feet), the base of support is the entire area that surrounds all the points of contact. Base of Support Support The upward support forces from the two legs of the bench can balance the downward force of gravity (weight). Support Gravity 9 Animation Physics — Balance & Weight Shift Balanced Character Poses ► To determine if a pose is in balance, first estimate the character’s center of gravity. If the line of gravity (the vertical line extending down from the CG) passes through the base of support then the character is in balance. X X The lumberjack character is top-heavy; his CG is roughly in the center of his chest (and a bit forward since his large arm is extended forward). The base of support is traced as the area from toe to toe and from heel to heel, and around the outer edge of each foot. The pose on the left appears to be balance while the pose on the right seems unbalanced. Although the old man on the right is leaning forward, the pose is in balance because the base of support is expanded due to the third base of contact, the cane. As this character leans forward to pick up her sunglasses, in order to maintain her center of gravity above her feet she instinctively moves her lower torso backwards. To understand this motion, try the following demonstration: Stand with your heels touching a wall and try to bend for- ward as if you were picking up an object from the floor. You probably won’t be able to bend more than half-way down before you tip over. 10 Animation Physics — Balance & Weight Shift