Jody is planning to see how far she can run. In order to track her distance she is using the tree as a reference point.
You can tell that Jody is moving because the tree seems to be getting farther away. But the tree is not able to move so we know that Jody is the one in motion.
After her run, Jody sits down to rest and has an apple for a snack. She is no longer in motion since the tree is staying the same distance away as she rest. But if we were to make the apple in her hand a reference point it wouldn't work since it is moving with her.