AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
An object falling through a fluid initially accelerates due to the force…
Full specification wording
Acceleration: An object falling through a fluid initially accelerates due to the force of gravity. Eventually the resultant force will be zero and the object will move at its terminal velocity.
When an object falls through a fluid such as air or water, gravity pulls it down, but the fluid exerts a drag force that increases with speed.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 21 of 21
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When an object falls through a fluid such as air or water, gravity pulls it down, but the fluid exerts a drag force that increases with speed. Initially the weight is larger than drag, so the object accelerates. As speed rises, drag increases until it equals the weight; the resultant force is then zero. At this point the object stops accelerating and falls at a constant maximum speed called terminal velocity. For example, a skydiver accelerates until air resistance balances weight, then falls at terminal velocity until the parachute opens. The same idea explains why a ball bearing reaches a steady speed in oil. Terminal velocity depends on shape, size and the fluid’s viscosity.
What earns the marks
- Describe that an object falling through a fluid initially accelerates because the force of gravity (weight) is greater than the drag force.
- Explain that as speed increases, the drag force from the fluid also increases.
- State that terminal velocity is reached when the drag force equals the weight, so the resultant force is zero.
- Recall that at terminal velocity the object moves at a constant speed in a straight line because there is no resultant force.
- Apply the idea to a skydiver or similar example, explaining how opening a parachute increases drag and reduces terminal velocity.
Common mistakes and how to put them right
Thinking terminal velocity means the object stops moving.
Put it right by stating that terminal velocity is a constant non-zero speed, not zero speed.
Believing that acceleration is constant throughout the fall.
Put it right by explaining that acceleration decreases as drag increases, becoming zero at terminal velocity.
Confusing weight and drag.
Put it right by stating that weight acts downwards due to gravity, while drag acts upwards opposing motion through the fluid.
Exam tips
- Use a free-body diagram or describe the forces to show how weight and drag change during the fall.
- When explaining terminal velocity, refer to the resultant force becoming zero and the object moving at constant speed.
- Link changes in shape or parachute opening to changes in drag and therefore to a new, lower terminal velocity.
Try the lesson
The start of the Acceleration: An object falling through a fluid initially accelerates due to the force of gravity. Eventually the resultant force will be zero and the object will move at its terminal velocity. lesson, made from checked teaching notes for this specification point.
Have a go first
A skydiver jumps from a plane and falls through the air. At the very start of the fall, what makes the skydiver accelerate downwards?
Listen
When an object falls through a fluid such as air or water, gravity pulls it down, but the fluid exerts a drag force that increases with speed. Initially the weight is larger than drag, so the object accelerates. As speed rises, drag increases until it equals the weight; the resultant force is then zero. At this point the object stops accelerating and falls at a constant maximum speed called terminal velocity. For example, a skydiver accelerates until air resistance balances weight, then falls at terminal velocity until the parachute opens. The same idea explains why a ball bearing reaches a steady speed in oil. Terminal velocity depends on shape, size and the fluid’s viscosity. A strong answer earns marks for these points. Describe that an object falling through a fluid initially accelerates because the force of gravity (weight) is greater than the drag force. Explain that as speed increases, the drag force from the fluid also increases. State that terminal velocity is reached when the drag force equals the weight, so the resultant force is zero. Recall that at terminal velocity the object moves at a constant speed in a straight line because there is no resultant force. A common slip: Thinking terminal velocity means the object stops moving. Instead, stating that terminal velocity is a constant non-zero speed, not zero speed. Exam tip: Use a free-body diagram or describe the forces to show how weight and drag change during the fall.
See it
Put it in order
Put these steps in the order they happen.
- 1. Object moves at terminal velocity
- 2. Resultant force is zero
- 3. Speed increases
- 4. Drag force equals weight
- 5. Object falls through a fluid
- 6. Initially accelerates due to force of gravity
- 7. Drag force from fluid increases
Key words
Say what each word means before you turn the card.
Drag
Try answering aloud or in your head before turning the card.
Terminal velocity
Try answering aloud or in your head before turning the card.
Weight
Try answering aloud or in your head before turning the card.
See every definition
- Drag
- A force from the fluid that increases with speed and acts upwards opposing motion through the fluid.
- Terminal velocity
- A constant maximum speed reached when the drag force equals the weight and the resultant force is zero.
- Weight
- A force acting downwards due to gravity, which is initially greater than drag so the object accelerates.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Acceleration: An object falling through a fluid initially accelerates due to the force of gravity. Eventually the resultant force will be zero and the object will move at its terminal velocity.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge