Newton's second law

例句：

1. 1. The force of gravity is the second law of motion.
   重力是运动学中的第二定律。
2. 2. The speed of an object in a straight line is directly proportional to the force acting on it.
   物体在直线上的速度与作用在其上的力成正比。
3. 3. In Newton's third law, for every action, there is an equal and opposite reaction.
   根据牛顿第三定律，每一个动作都有一个等大反向的反应。