Terminal Velocity

Terminal velocity is the fastest speed that an object can reach when falling
Terminal velocity is reached when the upward and downward acting forces are balanced
- The resultant force on the object reaches zero
- The object no longer accelerates and a constant terminal velocity is reached

Falling objects

Falling objects experience two forces:
- Weight
- Air resistance

The force of air resistance increases as the object's speed increases
This is because the object collides with air particles as it moves through the air
- The faster the object is travelling, the more collisions it has with the air particles
The weight of the object does not change
This is because
- The mass, $m$ , of the object does not change
- The acceleration of freefall, $g$ , does not change

Reaching terminal velocity

Skydiver in freefall reaching terminal velocity

Debbie the skydiver, downloadable IGCSE & GCSE Physics revision notes

The skydiver initially accelerates downward due to the force of weight. The upward force of air resistance increases as they fall until eventually it equals the weight force and terminal velocity is reached

At the instant the skydiver steps out of the plane, the support force of the plane is no longer acting on the skydiver, but they are not yet falling, so the only force exerted them is the weight force
- There is a downward acting resultant force on the skydiver
- The resultant force is equal to the weight force
- The skydiver accelerates downward at maximum acceleration

As the skydiver begins to fall, the force of air resistance is very small because the skydiver's speed is small
- There is a downward acting resultant force on the skydiver
- The resultant force is equal to the weight force minus the force of air resistance
- The skydiver accelerates downward but the acceleration decreases

As the skydiver accelerates, their speed increases, so the force of air resistance increases
- There is a downward acting resultant force on the skydiver
- The resultant force is equal to the weight force minus the force of air resistance
- The skydiver accelerates downward but the acceleration continues to decrease

As the skydiver's acceleration decreases, their speed increases at a slower and slower rate
- Eventually, the skydiver reaches a speed at which the force of air resistance is equal to the force of weight
- The forces are balanced, so the resultant force is zero
- The skydiver no longer accelerates and a constant velocity is reached
  - This is terminal velocity

Worked example

A small object falls out of an aircraft. Choose words from the list to complete the sentences below:

Friction Gravity Air pressure

Accelerates Falls at a steady speed Slows down

(a) The weight of an object is the force of __________ which acts on it.

(b) When something falls, initially it ____________.

(d) Eventually it ______________ when the force of friction equals the force of gravity acting on it.

Answer:

Part (a)

The weight of an object is the force of gravity which acts on it.
- The weight force is due to the Earth's gravitational pull on the object, so weight is due to gravity

Part (b)

When something falls, initially it accelerates.
- The resultant force on the object is very large initially, so it accelerates
- This is because there is a large unbalanced force downwards (its weight) - the upward force of air resistance is very small to begin with

Part (c)

The faster it falls, the larger the force of friction which acts on it.
- The force of air resistance is due to friction between the object's motion and collisions with air particles
- Air particles try to slow the object down, so air itself produces a frictional force, called air resistance (sometimes called drag)

Part (d)

Eventually it falls at a steady speed when the force of friction equals the force of gravity acting on it.
- When the upwards air resistance grows enough to balance the downwards weight force, the resultant force on the object is zero
- This means the object isn't accelerating - rather, it is moving at a steady (terminal) speed

Examiner Tip

The force of gravity on an object is called its weight. If you are asked to name this force, use this word: don't call it 'gravity', as this term could also mean gravitational field strength, and so might be marked wrong. Additionally, remember to identify air resistance as the upward force on a falling object. This force gets larger as the object speeds up, but the weight of the object stays constant. Don't confuse 'air resistance' with 'air pressure' as these are two different concepts!

Terminal Velocity (Edexcel IGCSE Physics: Double Science)

Revision Note