Work is how much you have to push or pull a roller coaster in order to move it for a certain distance. Work is done if a chain, pulled by a motor, pulls the roller coaster up to the top of a hill on the track. It takes work to lift a roller coaster to the top of the first hill in order to overcome the force of gravity. The more mass the coaster has, the more work required to pull it to the top. The further a roller coaster must climb the more work that is required to get the coaster to the top of the hill.
Work is done when the force of gravity pulls the coaster down a hill.