(3) An implication(暗示) what is known as inertial reference frame- those special reference frame in which Newton's law is valid Figure 2-1 A test to tell a reference frame is inertial or not Question: Try to give some examples to say why or why not a reference frame is an inertial reference frame.⑶ An implication（暗示） what is known as inertial reference frame — those special reference frame in which Newton’s law is valid. Question: Try to give some examples to say why or why not a reference frame is an inertial reference frame