In a roller coaster, a car has x joules of potential energy when it is resting at the top of the hill. the car moves downhill and reaches the bottom of the hill, where it moves quickly. assuming no friction, what does the law of conservation of energy predict about the car?

a)at the bottom, the car has x joules of potential energy.
b)at the bottom, the car has x joules of mechanical energy.
c)at the bottom, the car has more than x joules of kinetic energy.
d)at the bottom, the car has more than x joules of mechanical energy.

In an old-fashioned amusement park ride, passengers stand inside a 3.0-m-tall, 5.0-m-diameter hollow steel cylinder with their backs against the wall. the cylinder begins to rotate about a vertical axis. then the floor on which the passengers are standing suddenly drops away! if all goes well, the passengers will “stick” to the wall and not slide. clothing has a static coefficient of friction against steel in the range 0.60 to 1.0 and a kinetic coefficient in the range 0.40 to 0.70. part a what is the minimum rotational frequency, in rpm, for which the ride is safe? express your answer using two significant figures. f f = rpm