Forces, Movement & Changing Shape

What terminal velocity is

• Terminal velocity is the largest steady speed a falling object reaches once the air pushing up on it has grown enough to balance its weight
• At terminal velocity:
  • the upward push from air resistance matches the downward pull of weight exactly in size
  • the resultant force is zero
  • the object stops accelerating and falls at a constant velocity

The four stages of a falling object

• Stage 1, the moment of release. Only weight acts (air resistance is zero because there is no relative motion through the air). The resultant force is the full weight pulling downwards, so the object accelerates downwards at the acceleration of free fall, g
• Stage 2, early fall. The object now has a small downward speed, so air particles begin to collide with it and a small upward air resistance appears. Weight still wins, so the object continues to accelerate downwards, but at a slightly reduced rate
• Stage 3, as the speed climbs, air resistance climbs with it (more air particles hit per second, and harder). The resultant force shrinks, so the acceleration shrinks. The object is still gaining speed, but more slowly
• Stage 4, the speed reaches the point where air resistance equals weight. Resultant force is now zero, acceleration is now zero, and the object falls at a constant terminal velocity

Example — completing a sentence

Example — fill in the blanks: "An object falls from a plane. (a) At first the object ______. (b) The size of the upward force called ______ grows as it gains speed. (c) Eventually the object ______ at a steady terminal velocity once that force has grown enough to match the object's ______."

• (a) accelerates downwards, because weight is the only sizeable force at first
• (b) air resistance, because collisions with air particles produce this upward push, and they happen more often as the object moves faster
• (c) the object falls at a steady terminal velocity once air resistance has grown enough to match the object's weight