When an object falls, two forces are acting on it:
Falling objects will initially accelerate due to their weight and very little air resistance. As the velocity of the object increases, so does the upward force, which is air resistance.
This leads to falling objects reaching terminal velocity, which is the point at which the velocity of the object stays constant. This means it is not accelerating or decelerating. At this point, the resultant force is 0, which means the object is falling at a constant speed.
Below is a velocity-time graph that shows how the velocity of a skydiver changes as they step out of an aeroplane.
When the skydiver jumps out of the aeroplane, they are pulled down by gravity because they have mass.
A – They begin to accelerate.
B – As their velocity increases, so does the air resistance.
C – The skydiver reaches terminal velocity, which is the point at which the force of air resistance is equal to the force of weight.
D – The parachute opens, which drastically increases its surface area, increasing the air resistance. This leads to the skydiver decelerating.
E – The skydiver slows down until they reach their new terminal velocity, for their new surface area.
Keep in mind, that weight (the downward force) is the same at every point, as the skydiver’s mass and the gravitational field strength do not change.
Objects fall differently depending on how gravity and air resistance interact. For example, you may wonder why feathers float and take their time to reach the ground, whereas an apple just falls straight to the ground. This is because there is much greater air resistance when the feather is falling compared to the apple.
The apple can fall through the air at a faster rate. This is due to Galileo’s discovery, that objects which are denser (have more mass) will fall at a faster rate than objects which are less dense (or have less mass), as a result of air resistance.
When a feather and an apple are dropped at the same time, air resistance causes the feather to drop slower than the apple. However, in a vacuum (such as space), where there is no air resistance, the apple and the feather would fall at the same rate and reach the ground at the same time.