Physics Lecture 4 - Newton's Laws Newton's Laws Isaac Newton developed the prescription for how to think about dynamics , the study of why things move as they do. The prescription is based on the three Laws of Motion as stated below. Mass is the measure of inertia, i.e. objects only change their motion in response to actions from external objects. What we call mass is the amount of opposition to changes in motion that an object possesses. The definition of force is that it equals the product of mass and acceleration.
Forces only occur as pairs.
That is, in order for object 1 to act on object 2 and change its motion, object 1 must exert a force on object 2. However, this action necessitates that object 2 also act to change the motion of object 1. The words can be replaced by mathematical equations that condense the length of the statement of these laws, but still, the deep ideas are in there. Forces are operationally defined. You know a force is acting if something which has a mass is changing its motion.