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Blog about centre of gravity and how this relates to the SPA, pendulum bob experiment. Centre of mass. 1) CG of norm pendelum bob 2) CG of bob+lump on top 3) CG of bob+rotiprata of plasticine. =D
The center of mass of a system of particles is a specific point at which, for many purposes, the system's mass behaves as if it were concentrated. The center of mass is a function only of the positions and masses of the particles that comprise the system. In the case of a rigid body, the position of its center of mass is fixed in relation to the object (but not necessarily in contact with it). In the case of a loose distribution of masses in free space, such as, say, shot from a shotgun, the position of the center of mass is a point in space among them that may not correspond to the position of any individual mass. In the context of an entirely uniform gravitational field, the center of mass is often called the center of gravity — the point where gravity can be said to act.

The center of mass of a body does not always coincide with its intuitive geometric center, and one can exploit this freedom. Engineers try to design a sports car's center of gravity as low as possible to make the car handle better. When high jumpers perform a "Fosbury Flop", they bend their body in such a way that it is possible for the jumper to clear the bar while his or her center of mass does not.[1]

The so-called center of gravity frame (a less-preferred term for the center of momentum frame) is an inertial frame defined as the inertial frame in which the center of mass of a system is at rest.
(http://en.wikipedia.org/wiki/Center_of_mass)

The above is quoted from wikipedia and is about the center of mass, which I was redirected to when I searched for "center of gravity". But, many people that I have talked to have claimed that center of mass does not equal to center of gravity. Personally, I think that the center of mass is very different from the center of gravity. Hence, I looked for more information on other websites.

http://www.grc.nasa.gov/WWW/K-12/airplane/cg.html says: "The center of gravity is a geometric property of any object. The center of gravity is the average location of the weight of an object. We can completely describe the motion of any object through space in terms of the translation of the center of gravity of the object from one place to another, and the rotation of the object about its center of gravity if it is free to rotate. If the object is confined to rotate about some other point, like a hinge, we can still describe its motion. In flight, both airplanes and rockets rotate about their centers of gravity. A kite, on the other hand, rotates about the bridle point. But the trim of a kite still depends on the location of the center of gravity relative to the bridle point, because for every object the weight always acts through the center of gravity. "

That means that the center of gravity is, in fact, the point of which an object/geometrical shape balances, and it is entirely different from the center of mass of an object, which is the point of which mass is said to be acting on.

So, how does this relate to our SPA project?? Well, I think that we should roll our plasticine into a flat, big piece before wrapping it around the pendulum bob. This way, we can make sure that the center of gravity for the pendulum bob is the same as before. Also, we can make sure that the balance of the pendulum bob is kept constant.