The hallmark of an inertial reference frame is that any object which is subject to zero net force will travel in a straight line at constant speed. To illustrate this, consider the following: I am standing on a level floor at the origin of an inertial frame S and kick a frictionless puck due north across the floor.

a. Write down the x and y coordinates of the puck as functions of time as seen from my inertial frame . (Use x and y axes pointing east and north respectively.) Now consider two more observers, the first at rest in a frame S' that travels with constant velocity v due east relative to S , the second at rest in a frame S" that travels with constant acceleration due east relative to S. (All three frames coincide at the moment when I kick the puck, and S" is at rest relative to Sat that same moment.)
b. Find the coord i nates x', y' of the puck and describe the puck's path as seen from S'.
c. Do the same for S". Which of the frames i s inertial?