力的时间分量

The component of force in the direction of the time-axis

用狭义相对论分析变质量系统的受力情况 ;在狭义相对论中 ,力有三个空间分量和一个时间分量 ,当物体质量发生变化时 。

The forces on the variable mass systems are analysed with special theory of relativity. On special theory of relativity, there are three components in the direction of the space-axes and one component in the direction of the time-axis in a force. When the mass of a body is changed, it would be acted on a force which is in the direction of the time-axis.