The acceleration of free fall, g

1. Near the surface of the Earth: $g$ gets smaller far from the planet, so the value only holds close to the ground.
2. Approximately constant: over the heights used in exam questions the field is treated as uniform, so $g$ does not change as the object falls.
3. Some questions tell you to use $10{m/s}^2$ instead; always use the value printed in the question.
4. The same number is the gravitational field strength, $9.8N/kg$: a field that pulls each kilogram with $9.8N$ gives each kilogram an acceleration of $9.8{m/s}^2$.
5. In free fall, all objects accelerate at the same rate whatever their mass: a larger mass is pulled by a larger weight, but is also harder to accelerate, and the two effects cancel exactly.

Falling with no resistance

1. With no air or liquid resistance, the only force on the object is its weight.
2. The weight is constant, so the resultant force is constant and downward, and the object falls with a constant acceleration of $g$.
3. The speed increases by about $9.8m/s$ every second: $0$, then $9.8m/s$, then $19.6m/s$, and so on.
4. On a speed-time graph this is a straight line of constant gradient from the origin, and it never levels off: with no resistance there is no terminal velocity.

Falling with air or liquid resistance

1. Air resistance (drag) acts upwards on a falling object, and it increases as the object goes faster.
2. At the moment of release the speed is zero, so the drag is zero; the resultant force is the full weight and the acceleration is at its maximum value, $g$.
3. As the object speeds up, the drag increases, so the resultant force (weight minus drag) decreases and the acceleration decreases, but the object is still speeding up, just less quickly each second.
4. When the drag has grown to equal the weight, the resultant force is zero, the acceleration is zero, and the object falls at a constant speed: its terminal velocity.
5. On a speed-time graph this is a curve that starts steep, gets less steep, and levels off into a horizontal line at the terminal velocity.
6. If a parachute opens, the surface area jumps and the drag becomes larger than the weight, so the resultant force acts upwards and the skydiver decelerates to a new, much lower terminal velocity that is safe for landing.
7. The same reasoning applies in a liquid: a ball bearing dropped into a tall tube of oil meets a large resistance even at low speed, so it reaches a small terminal velocity quickly and then falls at constant speed.