For the study of motion, we usually look at position versus time, and perhaps draw some inferences about velocity and acceleration. This is relevant to situations where we know position. In the real world, we might be more interested in velocity (via a speedometer, for instance), and use that to find distance traveled (by finding the area under the velocity vs. time graph). Sometimes, we might want to know the accelerations, and from that we can find the area under the acceleration versus time graph to find change in velocity. But how do you get acceleration data?
Fortunately, many smart phones have 3-axis accelerometers.
You can download apps that will display this information in real-time, or sample it and write it to a file. I like the Sensor Data app for its better capture facilities. If your computer and your iPhone are on the same network, you can grab the files via a browser on your computer (your phone is the web server in this case). These are CSV (comma-separated values) files, and can be directly opened in Excel.