Center of Gravity (COG)  Definition--The Point located at an object’s average position of the weight  In other words…. The center of an object’s weight  Symmetrical object’s, like a baseball the C of G would be in the exact center of object  However other oddly shaped objects will find COG in any number of positions, depending on weight distribution  COG

C.O.G.  When objects rotate freely they must rotate about an axis through the COG  Basically treat the object as if all its weight is concentrated at that one pt.  In class demos…

C.O.G. --Balancing  For an object to balance, and not topple… support must be directly below C.O.G.

Where C.O.G. is located  Generally found in the middle of all the weight…  Does not even have to be within, the object itself  Ex. boomerang  Will be located toward one side of an object where most of its mass is focused…  Ex. Weebles COG gravity

Weebles Wobble, but they don’t fall down???  Weebles have very low COG  Whenever rolling it will roll to a stop when its COG is as low as possible  This occurs when it is standing upright  Also occurs for inflatable toy clowns  Objects with a low COG are less likely to topple because of this principle  Higher COG is, the easier to topple

Balancing Stuff  Again, all that has to happen to balance, is for a support to be directly beneath COG

Advantage of low COG  Athletic advantages  wrestling—harder to takedown  Football – “ “ “  Both easier to drive power through their legs  SUV’s …. Tip over all the time b/c COG is too high  ESUVEE ESUVEE  Farmer’s tractors  Much more control in all vehicles w/ low COG

 Deadliest Catch  Pots on deck and freezing ice make boat top heavy… more likely to roll and sink  Ballast tanks at bottom help lower boat’s COG

Animals  Low COG High COG

T. Rex & Tails

Humans - Where is our COG?  Just below our belly button  Notice, support always below COG  Bipedalism??  Only mammals w/ this ability to walk on 2 legs  Because of Evolution and how our legs changed to balance between steps is why we are only mammals to walk

 Because our legs/hips evolved so that our support base (feet) were close together allows us to be bipedal  Apes and our early ancestors hips were constructed differently with a wide set base.  Impossible to walk bipedal Hip protruding from joint…. Creates inward angled femurs… Which makes feet close together…. Providing a stable/efficient base for walking upright

How to find COG….

Picking up Chair Demo  Boys vs. Girls  Why girls can do this but boys can’t??  Different weight distribution of body types

Principles of Stability: I. Other things being equal, the lower the CG, the greater will be the body’s stability II. Greater stability is obtained if the base of support is widened in the direction of the line of force III. For maximum stability the line of gravity should intersect the base of support at a point that will allow the greatest range of movement within the area of the base in the direction of the forces causing motion

Principles of Stability: IV. Other things being equal, the greater the mass of a body, the greater will be its stability V. Other things being equal, the most stable position of a vertical segmented body is one in which the CG of each weight-bearing segments lies in a vertical line centered over the base of support

Principles of Stability: VI. Other things being equal, the greater the friction between the supporting surface and the parts of the body in contact with it, the more stable the body will be VII. Other things being equal, a person has better balance in locomotion under difficult circumstances when the vision is focused on stationary objects rather than on disturbing stimuli

Principles of Stability: VIII.There is a positive relationship between one’s physical and emotional state and the ability to maintain balance under difficult circumstances IX. Regaining equilibrium is based on the same principles as maintaining it.

Mobility  Mobility & stability have an inverse relationship  A critical point is the the change form a position of stability to a state of mobility & vice versa

Mobility  To initiate a step, line of gravity must be shifted forward of the base of support, swing then moves forward to reestablish a base of support  Often is sport, it is necessary to alter stability intentionally to become mobile

Mobility Fig 14.14a

Mobility  Often is sport, it is necessary to alter stability intentionally to become mobile  Putting the body into motion  Ability to start, stop, or change direction quickly depends on manipulating the stability of the body

Mobility  Both speed and direction of desired mobility are used to determine the nature of the change in stability required to initiate motion

Mobility  To enhance the speed of a start, the line of gravity should be as close as possible to the edge of the base of support  A quick stop requires a large base of support, lower CG, and move the line of gravity away form the leading edge of the base of support