Leah watches a toy car move across a table at a constant speed. The car moves across the table in one direction and falls off the other side. Leah decides to try another situation. She watches the car move across the table at a constant speed, then when it reaches the other side of the table, her friend Joey pushes the car in the other direction. The car moves back to Leah. Based on Leah's informal experiment, what forces cause the car to change direction?