Physics: Mechanical Equilibrium & Newton’s First Law

Which of these objects is at mechanical equilibrium:

  1. A seagull standing on top of a fence post.
  2. A car moving due south at 40 kilometers per hour.
  3. A water drop from a kitchen tap, just before it hits the surface of the sink.
  4. A rain drop from the sky, just before it hits the ground.
  5. A car as it enters the freeway, traveling from a surface road where the speed limit is 60 km/h to the freeway where the speed limit is 100 km/h. The driver’s foot is pressing the accelerator.
  6. A can in a factory on a conveyor belt moving at 0.1 m/s.
  7. A bicycle wheel rolling in a straight line at 50 revolutions per second.
  8. A coin after it is dropped from someone’s hand and before it hits the ground.
  9. A rock thrown up in the air, at the very top of its journey through the air.
  10. The earth as it travels around the sun. (This is a trick question, something to think about. Assume that the earth travels around the sun in a circle, and assume that the earth’s speed is constant. The earth’s orbit is approximately circular and its speed is approximately constant – after all, it goes around pretty steadily once a year.)