因為我們必須重新定義動量，牛頓力學的動量是會隨坐標系而改變的，
兩個完全一樣的球反向做直線運動且速度相等，
從質心座標看動量為 0，但以球的觀點來看不為零，
所以 in general p = mv is not general。
we redefine momentum as p = mv[1-(v/c)^2]^-0.5
and by the def. of force: F = dp/dt
we can know if we want to accelerate a body(m =/= 0) is impossible,
because we need infinite force,
so we can't accelerate a body(m =/= 0) to 299792458 m/s .
someone will say if we reach the speed of light,the mass will be infinite,
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
we should be care that we redefine momentum, not mass,
so that statement is not good, I don't like it.