以人在球体上走动引起球体向前滚动为例,利用牛顿定律和功能转换原理,说明人-球系统中内力与外力、动力与阻力之间的关系.尽管阻碍球体滚动的地面摩擦是人-球系统向前运动的必要条件,且作为外力与系统动量变化率相等,但从运动的能量来源和因果关系考察,人与球体之间的内力作用才是系统运动的根本原因,而系统动能的增加来自于人体克服重力所做的功.%With the instance of lion running ball, the relation among internal force, external force , dynamics and resistance is studied Based on Newtonian laws and the principle of energy and work. The friction which obstacles the relative motion between hall and land is the necessary condition for the motion of runner - ball system, and is equal to the ratio of system momentum in horizontal direction. However, the internal force between runner and ball in vertical direction is the real dynamic to drive the ball rotating and moving forward.The kinetic energy of the system comes from work provided by the runner against gravity.
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