Near the surface of the Earth, any object falling freely will have an acceleration of about 9.8 metres per second squared (m/s2). Objects falling through a fluid eventually reach terminal velocity. At terminal velocity, the object moves at a steady speed in a constant direction because the resultant force acting on it is zero. For example, a skydiver falling spread-eagled through the air reaches a maximum speed of about 53 m/s.
There are three stages as an object falls through a fluid:
The weight of an object does not change as it falls, as long as it stays whole.
The diagram shows what happens to the speed of a skydiver from when they leave the aircraft, to when they reach the ground after their parachute opens.
Before the parachute opens:
When the parachute opens, the air resistance increases. The skydiver slows down until a new, lower terminal velocity is reached.
Note that the skydiver does not go upwards when the parachute opens, even though this can appear to happen when a skydiver is being filmed. The illusion happens because the person with the camera opens their parachute later on, so falls downwards past the skydiver.
The diagram shows a velocity-time graph for an object falling through a fluid, eg air, water, oil.
The object accelerates at first because of the force of gravity. Its speed increases. The resultant force acts downwards because the frictional force acting against it is less than the weight of the object.
The object is still accelerating but its acceleration decreases as time goes by. Its speed still increases but by a smaller amount. The resultant force still acts downwards but is decreasing. This is because the frictional force acting against it is increasing as the speed increases, but is still less than the weight of the object.
The object is not accelerating any more. It has reached its terminal velocity and is falling at a steady speed. The resultant force is zero because the frictional force acting against it is now the same as the weight of the object.
The object does not stop falling once its resultant force is zero, unless it has hit the ground.