The steel ball in a pinball game rolls around a flat, tilted surface. If you flick the ball straight uphill, it gradually slows to a stop and then begins to roll downhill. Which direction is the ball accelerating (a) as it rolls uphill? (b) as it rolls downhill?

Why: As the steel ball rolls uphill, the velocity gradually decreases and finally the ball stops, thus, the acceleration will be in the opposite direction of motion (that is downhill). After the stop, the ball will start rolling downhill by an increasing velocity, thus, the acceleration in this case will be in the same direction as the motion (downhill again).