Center of Gravity vs Center of Mass
The center of mass and center of gravity are two concepts frequently encounters in the study of physics. These are also concepts that are most confused between, and often people use them interchangeably, which is erroneous. This article will explain the difference between center of mass and center of gravity and let readers have a clearer understanding.
Center of mass of a rigid body is also called its center of gravity. However, this is true only in circumstances where gravitational forces are uniform. Since gravitational force of earth is taken to be uniform at all places, the center of mass and the center of gravity are effectively same. Center of gravity is defined as the average location of the weight of an object. In the case of earth, since gravitational pull is same at all places, each mass element would weigh the same so the center of gravity is identical to the center of mass. However, in a non uniform gravitational field, the center of gravity is not the same as the center of mass. Center of mass is a fixed property which is the average location of the mass of the body. It has nothing to do with gravity.
In the case of artificial satellites, gravitational pull is not uniform and in such conditions, center of gravity refers to the mean location of the gravitational pull acting on the body of the satellite. This obviously results in slight difference between its center of mass and the center of gravity.
The center of mass of a body does not coincide with its center of gravity and this is a property that is exploited by sport car makers to keep the center of mass as low as possible to make the car have a better balance. The concept of difference between center of mass and center of gravity is also exploited by high jumpers when they perform Fosbury Flop and bend their bodies in such a way so as to clear the high bar without touching it. They bend their bodies in such a way that they clear the bar despite their center of mass not clearing the bar.
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Center of Mass vs Center of Gravity • Center of mass and center of gravity are often taken as one in the study of physics because of the uniform gravitational pull of earth. • However, in non uniform gravitational fields, center of mass is away from the center of gravity • This fact is used by designers to make cars with a very low center of mass so as to give better balance.
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The sports car and high jumper analogies are incorrect. Those two objects ARE in a uniform gravitational field, so their COM and COG are the same.
A better analogy would be a very tall skyscraper (imagine a perfectly rectangular 100-story “box”). The center of mass of this skyscraper would be at floor 50. But the center of gravity would be slightly below, somewhere around the 49th floor, because the bottom of the skyscraper is pulled slightly stronger by the Earth than the top.
Thank you so very much for the building analogy. I’ve been reading center of gravity descriptions for about an hour, trying to understand why it isn’t the same as center of mass, and this is the first time I’ve caught a clue. Most descriptions fail miserably at the same point with comments like “This obviously results”, explaining nothing. Now I can visualize giant pancakes, large buildings, bags of water, landing on Star Destroyers, what have you, and it seems utterly “obvious”. I was kinda uncomfortable with that pole vaulter thing too, so my instincts are in the right place.