Terminal Velocity (Oxford AQA IGCSE Physics)
Revision Note
Written by: Leander Oates
Reviewed by: Caroline Carroll
Terminal Velocity
Objects falling through fluids will experience two forces
Weight
Drag
At the moment the object is released, the only force acting on it is weight
It has not yet begun to move through the fluid
It is not being supported by any surface
The resultant force causes the object to accelerate
As the object accelerates, the drag force takes effect
The object collides with the particles in the fluid
The drag force increases with speed
As the object accelerates, speed increases so the drag force increases
The greater the speed of the object, the greater the drag force
Eventually, the drag force becomes balanced with the weight force
There is no resultant force
The object falls at a constant velocity
This is terminal velocity
Worked Example
A marble falls through a glass cylinder of oil.
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 the marble ______________ when the force of friction equals the weight.
Answer:
Part (a)
The weight of an object is the force of gravity which acts on it.
The force of weight 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 large initially, so it accelerates
Part (c)
The faster it falls, the larger the force of friction which acts on it.
The drag force is due to friction between the marble and collisions with oil particles
Part (d)
Eventually, the marble falls at a steady speed when the force of friction equals the weight.
When the upward drag increases enough to balance the downward weight force, the resultant force on the object is zero
Terminal velocity is reached
Parachutes
Parachutes can be used to reduce the terminal velocity of an object allowing it to land safely
Forces acting on a skydiver
When a skydiver jumps out of a plane, initially the only force acting on them is weight
The resultant force acts in the downward direction
The skydiver accelerates
As the skydiver accelerates, their speed increases so air resistance increases
The resultant force acts in the downward direction with a smaller magnitude
The skydiver continues to accelerate but at a slower rate
Air resistance increases until it is equal to the weight
The forces are balanced
There is no resultant force
Terminal velocity is reached
The parachute is deployed increasing the surface area of the skydiver
The parachute collides with many more air particles
Air resistance increases greatly
The force of air resistance is now greater than the force of weight
The resultant force acts in the upward direction
The skydiver continues falling in the downward direction
The skydiver is decelerating
As the skydiver decelerates, their speed decreases
Therefore air resistance decreases
Therefore the resultant force decreases
Air resistance decreases until it is equal to the weight
The forces are balanced
There is no resultant force
A new slower terminal velocity is reached
Velocity-time graph of a skydiver
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