As the roller coaster cars go down the first hill, they go (1.) (faster and faster/slower and slower). This change in speed means their (2.) (velocity/reference point) also changes. When the cars climb the next hill, they (3.) (speed up/slow down). This causes their (4.) (velocity/reference point) to change again. When the cars hit a curve, their (5.) (velocity/reference point) changes again! (6.) (Acceleration/Speed) is a measure of the change in (7.) (velocity/motion) during a period of time.