OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(c) motion of objects falling in a uniform gravitational field in the…
Full specification wording
Dynamics: (c) motion of objects falling in a uniform gravitational field in the presence of drag
When an object falls in a uniform gravitational field with drag present, its acceleration is not constant.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 9 of 10
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When an object falls in a uniform gravitational field with drag present, its acceleration is not constant. At release, speed is zero so drag is zero and the object accelerates at g. As speed rises, drag increases, reducing the resultant force and therefore the acceleration. The object reaches terminal velocity, a constant maximum speed, when drag equals weight and the resultant force is zero. Before terminal velocity, the object accelerates at a continuously decreasing rate. A velocity–time graph shows a curve with its maximum gradient at t=0, which then continuously flattens as terminal velocity is approached. The same analysis applies to an object projected upwards, where weight and drag both act downwards.
What earns the marks
- Initially the object accelerates at g because drag is zero at zero speed.
- As speed increases, drag increases and the resultant force decreases, so acceleration decreases.
- Terminal velocity is reached when drag equals weight, so resultant force and acceleration are zero.
- At terminal velocity the object moves at constant speed in a straight line.
- A velocity–time graph for the fall has its steepest gradient at t=0 and continuously flattens until it becomes horizontal at terminal velocity.
Common mistakes and how to put them right
Thinking acceleration stays equal to g throughout the fall.
Put it right by noting that drag reduces the resultant force as speed increases.
Believing the object stops accelerating only when it hits the ground.
Put it right by identifying terminal velocity as the point where drag equals weight.
Thinking the velocity-time graph steepens initially.
Put it right by remembering the maximum acceleration, and thus maximum gradient, is at the instant of release.
Exam tips
- Sketch the velocity–time graph starting with a steep gradient that continuously flattens to a horizontal section to show terminal velocity.
- State the condition for terminal velocity as drag equals weight, giving zero resultant force.
- Use the terms 'resultant force' and 'acceleration' together to explain the changing motion.
Try the lesson
The start of the Dynamics: (c) motion of objects falling in a uniform gravitational field in the presence of drag lesson, made from checked teaching notes for this specification point.
Listen
When an object falls in a uniform gravitational field with drag present, its acceleration is not constant. At release, speed is zero so drag is zero and the object accelerates at g. As speed rises, drag increases, reducing the resultant force and therefore the acceleration. The object reaches terminal velocity, a constant maximum speed, when drag equals weight and the resultant force is zero. Before terminal velocity, the object accelerates at a continuously decreasing rate. A velocity–time graph shows a curve with its maximum gradient at t=0, which then continuously flattens as terminal velocity is approached. The same analysis applies to an object projected upwards, where weight and drag both act downwards. A strong answer earns marks for these points. Initially the object accelerates at g because drag is zero at zero speed. As speed increases, drag increases and the resultant force decreases, so acceleration decreases. Terminal velocity is reached when drag equals weight, so resultant force and acceleration are zero. At terminal velocity the object moves at constant speed in a straight line. A common slip: Thinking acceleration stays equal to g throughout the fall. Instead, noting that drag reduces the resultant force as speed increases. Exam tip: Sketch the velocity–time graph starting with a steep gradient that continuously flattens to a horizontal section to show terminal velocity.
See it
Key words
Say what each word means before you turn the card.
Terminal velocity
Try answering aloud or in your head before turning the card.
Free fall with drag
Try answering aloud or in your head before turning the card.
See every definition
- Terminal velocity
- A constant maximum speed reached when drag equals weight and the resultant force is zero.
- Free fall with drag
- Motion of an object falling in a uniform gravitational field in the presence of drag, where acceleration is not constant.
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 Dynamics: (c) motion of objects falling in a uniform gravitational field in the presence of drag
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