Final answer:
Friction and air resistance provide the resistance that causes a roller coaster car to slow down, whereas gravity accelerates it downhill and inertia keeps it moving at a constant velocity.
Step-by-step explanation:
The resistance on a roller coaster causing the car to slow down is primarily due to friction and air resistance. Friction can occur between the wheels of the roller coaster and the tracks, which opposes the motion of the coaster. Air resistance is the force that opposes the motion of an object through the air and is more noticeable at higher speeds. While gravity can affect the speed of a roller coaster by pulling it down slopes, it is not what causes the car to slow down; it actually does the opposite by accelerating the car downhill. Inertia is the tendency of an object to resist changes in its state of motion, but it does not directly slow down the coaster; instead, inertia would keep the coaster moving at a constant velocity if unopposed by friction and air resistance.