# 5.6.14 Terminal Velocity

### Freefall

• In the absence of air resistance, all objects fall with the same acceleration
• This is called the acceleration due to gravity:

In the absence of air resistance, Galileo discovered that all objects (near Earth’s surface) fall with an acceleration of about 9.8 m/s2

• This means that for every second an object falls, its velocity will increase by 9.8 m/s
• The symbol g also stands for the gravitational field strength, and can be used to calculate the weight of an object using its mass:

weight = mass × g

### Terminal Velocity

• When a skydiver jumps out of a plane, two forces act:
• Weight (due to gravity)
• Air resistance (due to friction)
• The resultant force on the skydiver decreases as they fall. This is illustrated in the image below:

Debbie initially accelerates downwards due to her weight. The upwards air resistance increases as she falls until it eventually grows big enough to balance the weight force

• Initially, the upwards air resistance is very small because the skydiver isn’t falling very quickly
• As they speed up, the air resistance increases, eventually growing large enough to balance the downwards weight force
• Once air resistance equals weight, there is no longer any resultant force
• Therefore, the skydiver’s acceleration is zero – they now travel at a constant speed
• This speed is called their 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 ____________.

(c) The faster it falls, the larger the force of ______________ which acts on it.

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

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

#### Exam 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 upwards 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’ – these are two different concepts!

### Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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