In earlier chapters we considered only “point objects.” That is, an object so small that it could be thought of as the tiniest ball. Now we want to expand our considerations to large objects.

First think of a baseball. If a baseball is tossed into the air, the ball follows a smooth parabolic path as described earlier in this book. If, however, a hammer is tossed its path appears much more complex. Why?

All objects are made of atoms. The mass of the object is the sum of the masses of all the atoms. And, the force of gravity on each atom is proportional to the mass of that atom. In a baseball the center may be made of different materials than the surface, but, if you ignore the laces, the ball is made up of the same materials regardless what the direction is. That is, the ball is spherically symmetric. The center of gravity in any object is defined as the average location of its weight. Because the mass is distributed evenly throughout a baseball, the center of gravity is located in the center of the ball. However, for an object such as a hammer, with a metal head and a wooden handle, the center of gravity is not directly in the middle. Since more mass is located in the metal head of the hammer, the center of gravity is closer to that point.

The laws of physics state that the center of gravity follows a parabolic curve when tossed in the air. Indeed, although the ball and the hammer do not appear to have similar motions, their centers of gravity do. If you watch closely both the center of a baseball and the center of gravity of a hammer, you will see that they both follow parabolic paths when thrown.

Because the force of gravity is proportional to the mass, the center of mass is at the same location as the center of gravity.