Hello XXXXXXXXXX,
Friction opposes motion causing moving objects to slow down.
Friction wheels are added to a roller coaster to prevent movement to the side of the track.
A roller coaster is driven by changes in energy (no engine is needed). So the energy changes are the greatest factor affecting the design.
These links contain further information on rollercoaster physics.
http://www.funderstanding.com/k12/coaster/
http://www.glencoe.com/sec/science/webquest/content/rollercoast.shtml
Teacher
BSc, Dip tchg, Dip ICT Ed, Experienced teacher, published author