8)If an airplane is moving at a constant velocity of 400 m/s for 20 s, what is its acceleration during this period?
0 m/s^2 b/c no change
10)You try to move a couch across the floor but it doesn't move. What is the other force acting on the couch?
Static friction
11)You finally get the couch in Problem 10 to move, What other force is acting on the couch now?
Kinetic friction