Ask yourself. “What does the author want me to know about the topic?” What does the author want me to know about motion?
Read the first last sentences of the paragraph. The last sentences appears main idea? Perhaps.
Once you feel sure that you found the main idea, test it. I read the paragraph to see if the last sentences could act as summary of the other sentences and if the example explains the main idea. It does . the topic is motion and the paragraph describes a ball being moved over time. The other sentences repeat “moved” and “time.” The last sentences includes both of those ideas.