All objects have a center of mass, and this is easily seen when an object is in rotational motion. Rotational motion describes an object spinning around an axis. In contrast, translational motion would be an object moving along a trajectory, in any direction, but not revolving. The center of mass is the point on an object in which it will rotate around with no constraints acting on it. For humans, our center of mass is simply in the center of our body, approximately in the torso area for most individuals. A lot of aerial silks moves incorporate rotation, often for a higher degree of complexity and a dramatic aesthetic.

The center of mass is also the balance point for an object. Shown in a move called hip key, my upper body tends to tilt downwards in this move, because I am suspended from a point below my center of mass.

[photo from N. Gyswyt]

Another important term within the physics of rotational motion is moment of inertia. An object’s moment of inertia is a measure of how much force needs to be applied to make the object rotate from rest. The moment of inertia is dependent on mass, and how that mass is distributed around an object’s center of mass. Mathematically, this can be expressed as the sum of each part of an object’s mass multiplied by the distance away from the object’s axis of rotation that part is located at.

Once we know an object’s moment of inertia, its rotational kinetic energy can be calculated. Rotational kinetic energy is determined by multiplying moment of inertia, or how difficult it is to spin an object, by the object’s rotational velocity.

It is easy to see how kinetic energy of rotation is related to kinetic energy of translation, as velocity is just replaced by angular velocity multiplied by radius. Also, this equation shows us the inverse relationship between moment of inertia and angular velocity. The larger the moment of inertia, the smaller resulting angular velocity. Demonstrated in a few transitions between positions in the silks, my moment of inertia varies from large to small. As I bring more of my mass closer to my axis of rotation, I speed up. I slow down if I extend my mass away from my axis of rotation.