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Another important concept related to center of mass is base of support, or every point of contact an object has with the stabilizing surface beneath it. For a person standing on the ground, his feet are his bases of support. If he lifts one foot, then he has one base of support. Conversely, if he gets on his hands and knees, he has four.
The center of mass of a horse is just below it's withers (where its neck meets its back) and behind the girth (the front elbows)
Center of mass is important to:
A stationary horse has a rectangular shaped base of support - it is very stable while standing still. The faster a horse moves, the fewer hooves there are on the ground at a time and so the horse must rely more on momentum to move it forward. A slow moving horse cannot rely on momentum as much and relies more instead on keeping the most hooves on the ground as possible - thus enlarging its base of support.
The center of mass of a horse moves forward slightly as it increases speed since it puts more weight onto its forehand.
Collected vs extended gaits:
A horse moving in a collected gait it puts more of its weight onto his hind and "collects" itself up, raising its back. It results in the slight shift of center of mass back as compared to an extended, or entirely forward, gait. (Photo courtesy of Riding Art, "A Horse's Balance")
Horses generally move on the forehand so their center of mass is not located in the exact center of their bodies, or the center of the rectangle of bases of support. Instead, it is shifted forward slightly. A rider seated behind the horses natural center of mass shifts the center of mass back towards the horse's center due to the added weight. The rider also has a center of mass - usually a bit below the belly button for humans. Because of this high center of gravity, the rider can easily unbalance the horse and themselves - as many new riders find out. Leaning forward, lying back, shifting side to side all can affects the horse's center of mass.
Balancing!
My previous two RWAs focused on the physics of riding and exercising horses. This one will keep to that theme! The goal of this RWA will be to examine how horses balance.
After working with horses for years and now taking this class, I've come to wonder how exactly they balance. Of course it's necessary for all animals to have mechanisms for balance. Horses have the added task of balancing with riders on their backs, while sailing over jumps and as they pull carts and plows behind them. How do they stay balanced in each? What mechanisms of physics and biology come into play to keep them balanced?
Center of mass in horses
Balance and movement
The center of mass of a horse changes as it moves, especially as it changes gaits (walk, trot, canter and gallop). The pattern of a horses movement (which feet are on the ground when and in what combination) changes with each gait. The figure on the right shows a horse's base of support during the walk, trot and canter. Since it changes, the center of mass must change as well!
Jumping
Image courtesy of Clayton "Balance Basics," Veterinary Connection
Center of mass in stages:
White "X's" = the natural center of mass.
Red "X's" = the shifted center of mass
1) The approach.
As the horse approaches the jump, it places all four hooves and lowers its center of mass by lowering its head and neck.
2) The takeoff: period of time when front legs leave the ground until back legs leave the ground
In order to maximize jumping height, the horse "collects" its legs underneath itself thus centering its balance more in its center.
3) Flight: period in which the horse is in the air. During this time, the horse's center of mass moves like a parabola over the jump
4) Landing.
When the horse lands, its center of mass shifts forward entirely to its forehand. The rider, if pitched forward, can further cause the center of mass to move forward and unbalance the horse.
5) Re-balancing.
Once all of the horses hooves are on the ground the horse re-balances and its center of mass returns to its original position
Racing
In racing, a jockey leans forward in order to shift his center of mass in the direction of the horse's movement. In doing this, he reduces the weight on the horses back.
Dressage
Clayton, Hilary. "Balance Basics." Veterinary Connection (2005): 19-30. USDF Connection. Michigan State
University College of Veterinary Medicine, Apr. 2005. Web.
Dausend, Tonja. "The Horse's Balance." The Horse's Balance. N.p., 2005. Web. 27 May 2014.