圆盘

一个有唯一名称的刚性圆盘，有以下参数：

- position: The X/Y position of the center of mass of the disk, relative to the center of the world.
- angle: By how much the disk is rotated in positive direction, that is clock-wise.
- velocity: The velocity of the center of mass of the disk. Unit: meters/second.
- angular velocity: How fast the disk is spinning in positive direction, that is clock-wise.
- mass: Total mass of the disk. This influences forces with which the disk interacts with other items.
- inertia: How difficult it is to change the disk's rotation.
- momentum: A fundamental quantity in mechanics that is conserved in the absence of external forces. A product of the mass and the velocity.
- angular momentum: Measure of the extent to which the disk will continue to rotate unless acted upon by an external torque.
- kinetic energy: The work needed to accelerate the body of a given mass from rest to its current velocity.
- radius: Defines how big the disk is.