Free Fall

Objects falling without air resistance (in a vacuum)

For object falling without air resistance, it will undergo free fall with a constant acceleration of 10 ms^(-2). 

This acceleration due to free fall (or acceleration due to gravity) does not depend on Material, Mass, and Size or shape. 
Motion of falling objects:

Initially, the only force acting on the falling object is its own weight.
All free-falling objects increase their speed at the same rate of 10m/s every second, that is, they experience constant acceleration of 10ms^(-2) (acceleration due to gravity).
All free-falling objects dropped from the same height will hit the ground at the same time with the same speed.

Object falling with air resistance

Air resistance always opposes the motion of a moving object.
It increases with 
  • Speed, 
  • Surface area of the object and 
  • Density of air. 
Motion of falling objects:

Initially, the only force acting on the falling object is its own weight and it accelerates downwards at 10 ms^(-2) (Diagram A).
As its speed increases, air resistance increases. Therefore, resultant force acting downwards is smaller, and hence accelerates at a rate less than 10 ms^(-2) (Diagram B & C).
When air resistance balances the weight, resultant force is 0 N and acceleration is 0 ms^(-2). Thus, the object falls with constant speed, which is called terminal velocity (Diagram D).
Note:
(1) The object accelerates at a rate of 10ms^(-2) initially, then, its acceleration gradually decreases to 0 ms^(-2) due to increasing air resistance.
(2) An object reaches terminal velocity only when the height is sufficient for it to fall.
(3) Terminal Velocity depends on
- Surface area of the falling object. The greater it is, the shorter the time it reaches terminal velocity and hence smaller terminal velocity.
- Mass of the falling object. The bigger it is, the longer the time it reaches terminal velocity and hence larger terminal velocity.

Parachuting / Skydiving

For parachuting, there are two terminal velocities:
(1) Before opening the parachute and 
(2) After opening the parachute.
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