单项选择题
A.瞬时性B.质点C.惯性系D.直线加速参考系
A.B.C.D.
A.Gm1m2(1/R2-1/R1)B.Gm1m2(1/R1-1/R2)C.Gm1m2(R2-R1)D.m1g(R2-R1)