An airplane is flying from San Diego to Los Angeles. Many forces act on the plane, including gravity, air resistance, the thrust of the engine, and the lift of the wings. At some point during its trip the velocity of the plane is measured to be constant in time (which means its altitude is also constant). At this time, the total force on the plane:
- Is pointing downward
- Is pointing upward
- Is pointing backward
- Is zero.
- Is pointing forward