Center of Gravity
Center of Gravity(CG): point located at the center of an object’s weight distribution. For symmetric object, it is located at geometric center. Ex. A ball, a wood block, a donut For irregularly shaped object, it is toward the heavier end. Ex. Baseball bat
If you slide a wrench horizontally, its center of gravity follows a straight line path.(fig 9-4 p120) If you toss the wrench like a projectile, its center of gravity follows a parabola like a projectile.
Center of mass: point of average mass distribution for an object. For almost all object on and near the earth, center of mass is the same as center of gravity. It is different if object is large enough for gravity to vary from one part to another. The center of mass of the solar system is outside the sun, not at its geometric center, making the sun wobble off center. (used to determine if other stars have planetary systems.)
Locating the center of gravity (read p 122 section 9.3) Toppling: if the CG of an object is above the area of support, it will remain upright. If the CG of an object extends outside the area of support, the object will topple. (p 124 fig 9-11)
Stability: you can not balance an object if: The support base is too small or inadequate (Ex. Balancing a pen from its tip) If its center of gravity is in unstable equilibrium meaning that any displacement lowers its center of gravity Read p 126 sect 9.5 An object is at: stable equilibrium if displacement raises its center of gravity and is at neutral equilibrium if its CG is neither rised or lowered with displacement.
Why don’t floating icebergs simply tip over? Why do pregnant woman tend to lean backward when walking? How do wrestlers stand to resist being toppled? If you carry a heavy load- such as a pail of water- with one arm, why do you tend to hold your free arm out horizontally? Why do you place your feet farther apart when you are riding a bumby bus or metro?
Center of gravity of people Our CG generally is 2 to 3 cm below or navel It is lower in women than in men (Why do you think it is different?) Why do you think vaulters bend there shape as shown below to clear the bar? Where is his CG located with this shape?
Center of Gravity