假设分子的平动动能相同，设为零。
You assume that all molecules have the same translational energy. You set that equal to zero.
系统的平动动能就是零。
刚体的运动分为平动和转动，那么刚体的动能也就包括平动动能和转动动能。
The motion of a rigid body can be divided into two types: translation and rotation, so the kinetic energy of rigid body is made up of translational energy and rotational energy.
讨论滑动摩擦力作功，说明了滑动摩擦力可以使刚体的平动动能和转动动能之间相互转换，但总是作负功。
It is pointed out that translational kinetic energy and rotational kinetic energy mutually transferred by sliding friction force, but it only has positive acting.
讨论滑动摩擦力作功，说明了滑动摩擦力可以使刚体的平动动能和转动动能之间相互转换，但总是作负功。
It is pointed out that translational kinetic energy and rotational kinetic energy mutually transferred by sliding friction force, but it only has positive acting.
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