6. A car drives at steady speed around a perfectly circular track. (a) The car's acceleration is zero. (b) The net force on the car is zero. (c) Both the acceleration and net force on the car point outward. (d) Both the acceleration and net force on the car point inward. (e) If there is no friction, the acceleration is outward.

9. A spring has a force constant of 100 N/m and an unstretched length of 0.07 m. One end is attached to a post that is free to rotate in the center of a smooth. table, as shown in the top view in the figure below. The other end is attached to a 1kg disc moving in uniform circular motion on the table, which stretches the spring by 0.03 m. Friction is negligible. What is the centripetal force on the disc? Top View (a) 0.3 N (b) 3.0 N (c) 10 N (d) 300 N (e) 1000 N

4. A child has a ball on the end of a cord, and whirls the ball in a vertical circle. Assuming the speed of the ball is constant (an approximation), when would the tension in the cord be greatest? (a) At the top of the circle. (b) At the bottom of the circle. (c) A little after the bottom of the circle when the ball is climbing. (d) A little before the bottom of the circle when the ball is descending quickly. (e) Nowhere; the cord is pulled the same amount at all points.