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Episode 57 – Center of Gravity

The Definitions of Mass, Gravity, and Weight.
These terms are loosely used so we need to define them to eliminate confusion.

Mass on earth is equivalent to weight because the Earth has a uniform constant gravitational force.  In other words the amount of force the earth pulls on something is proportional to the mass of that object and we call that force weight.    However, once that object is sent into space and is weightless, the object still has the same mass.  It would still require the same force to accelerate the object in space as it does on earth if the object is being accelerated in the horizontal direction on earth.

Gravity is the force pulling something toward earth.  The gravitational force depends upon the mass of the object.

The correct measurement of an object’s mass is kilograms in international units (SI) or slugs in the English (US) units.  The correct measurement of the gravitational force on an object is pounds in the English units and Newtons in the SI units.

Center of Gravity:    The more precise term for this is center of mass.  However, in common language it is usually called center of gravity.  The Center of Gravity of an object is a point where the mass X distance on either side of that point are equal.

  • On a one dimension object it is equivalent to a balanced lever.
  • On a two dimension object and we draw a line through the object’s center of gravity.  In that plane. the object will be balanced.

The center of gravity is important subject for several reasons.  First, it combines that whole object into one point so calculations are much much easier.   Second, if we are somehow moving this object the center of gravity will help us understand  and describe the movement of the object. The third is understanding stability of an object.

An example, picking up a load with a crane and the crane is not centered over the center of gravity.   Once the object is lifted, the object will move.

Another example is standing dominoes up on the ends.  A little force on the domino causes the center of gravity to be outside of the two edges at the bottom of the domino… the domino will tip over.

An example of an object with very low center of gravity and therefore a very stable object is  the one shown in this diagram.   This diagrams the little toy man at the top of the toy “tip-it” which is show on this old advertisement commercial.



Credits:  The top picture is my creation,   the bottom picture is from Wiki-Commons article Center of Mass.



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