1、Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.
无外力的作用下，物体保持静止状态
2.The relationship between an object's mass m,it's acceleration a,and the applied force F is
F=ma (力、质量、加速度的关系)
Acceleration and force are vectors(as indicated by their symbols being displayed in slant bold font);in this law the direction
of the force vector is the same as the direction of the acceleration vector.
3.For every action there is an equal and opposite reaction.
作用力与反作用力的关系，方向相反，大小相等
