在经典力学中质量不变质点的机械运动规律由牛顿定律描述,而牛顿第二定律的两种表述形式f=ma和f=(d/(dt))(mv)是完全等价的,而牛顿定律作为力学原理已被无数实验所证实。但对低速度变质量的主体若某时刻的质量为m(t),所受的合外力为f,加速度为a,那么这些量之间是否仍具有fm=(t)a或f=(d/(dt))(m(t)v)的形式呢?本文对低速变质量物体的运动规律的探讨对此作了确切的回答。认为f=ma The law of mechanical motion of mass-invariant mass in classical mechanics is described by Newton’s law, whereas the two expressions of Newton’s second law, f = ma and f = (d / (dt)) (mv), are completely equivalent, Newton’s law has been verified by numerous experiments as a mechanics principle. However, for a body with low velocity and variable mass, if the mass of a moment is m (t) and the resultant combined force is f and the acceleration is a, then whether these bodies still have fm = (t) a or f = (d / (dt)) (m (t) v)) This paper gives an exact answer to the discussion about the motion law of low-velocity variable mass objects. Think f = ma