A passenger in a frictionless (!) turning cart slides outward. If an observer is looking from the moving cart, he thinks that a centrigual force pushes the passenger outward. But according to an observer at rest, the passenger moves in a straigth line at constant speed (while the cart is turning beneath him). An inward force of friction accelerates the cart toward center of the circle (centripetal acceleration); without this centripetal force, the passenger can't turn and continue in straight line because of its inertia.
Other animations by Yves Pelletier
The title says it all: this blog features physics videos found everywhere on the web: animations, demonstrations, lectures, documentaries.