- 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
- 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
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.
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
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
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)
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
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!