Terminal Velocity and Air Resistance

Terminal Velocity

When an object falls, two forces are acting on it:

  • Weight – Downward force caused by gravity.
  • Air resistance – Frictional force that acts in the opposite direction to the moving object.

Falling objects will initially accelerate due to their weight and very little air resistance. As the velocity of the object increases, so does the air resistance, which acts as an upward force.

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 due to their 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, drastically increasing the skydiver’s surface area and therefore, 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.

  • There is an increase in air resistance when you increase the surface area of the object.

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.

The Effects of Air Resistance

Galileo discovered that in the absence of air resistance, all objects, regardless of their mass or density, fall at the same rate. However, in real-world conditions with air resistance, denser or more streamlined objects may fall faster than less dense or less streamlined ones.

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.

Therefore, the apple can fall through the air at a faster rate.

When dropped simultaneously, air resistance causes the feather to fall 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.