18. You are standing in the middle of a subway car that is moving forward at constant velocity when another passenger accidently spills an enormous container of olive oil. Suddenly, the floor cannot exert any frictional forces on your feet. Because nothing else is touching you, you

 

  • remain in the middle of the subway car.
  • shift toward the side of the subway car (perpendicular to the direction of its velocity).
  • shift toward the back of the subway car (opposite its velocity).
  • shift toward the front of the subway car (in the direction of its velocity).