AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Projectile motion: Knowledge that air resistance increases with speed.
Final Result: Air resistance increases with speed and opposes motion.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 6 of 7
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Final Result: Air resistance increases with speed and opposes motion. Air resistance is a resistive force exerted by the air on any object moving through it. Its magnitude increases as the object's speed increases, meaning faster objects experience larger drag. In projectile motion, this resistive force opposes the velocity vector, reducing both horizontal and vertical components. During ascent, drag acts downwards, increasing the downward resultant force and acceleration. During descent, drag acts upwards, decreasing the downward resultant force. As speed rises, drag rises, making the simple parabolic path an idealisation valid only when air resistance is negligible.
What earns the marks
- States that air resistance is a resistive force acting opposite to the direction of motion through the air.
- Explains that the magnitude of air resistance increases as the speed of the object through the air increases.
- Links increasing drag to changes in resultant force: during ascent, drag adds to weight increasing downward acceleration; during descent, it opposes weight reducing it.
- Applies the idea to projectile motion, noting that drag opposes velocity and so changes both horizontal and vertical motion.
- Recognises that the effect becomes more significant at higher speeds, so the ideal parabolic trajectory is only an approximation.
Common mistakes and how to put them right
Treating air resistance as a constant force regardless of speed.
Put it right by stating that drag grows as speed grows.
Saying air resistance acts in the direction of motion.
Put it right by stating it acts opposite to the velocity.
Assuming air resistance only affects horizontal motion.
Put it right by noting it opposes the whole velocity vector, so vertical motion is also affected.
Exam tips
- Use the phrase 'opposes motion' or 'acts opposite to velocity' whenever you describe drag.
- When comparing two speeds, state explicitly that the faster object experiences the greater air resistance.
- If a question asks about projectile shape, mention that drag reduces range and makes the path asymmetric rather than a perfect parabola.
Try the lesson
The start of the Projectile motion: Knowledge that air resistance increases with speed. lesson, made from checked teaching notes for this specification point.
Have a go first
A skydiver falls through the air. As her speed increases, what happens to the air resistance acting on her?
Listen
Final Result: Air resistance increases with speed and opposes motion. Air resistance is a resistive force exerted by the air on any object moving through it. Its magnitude increases as the object's speed increases, meaning faster objects experience larger drag. In projectile motion, this resistive force opposes the velocity vector, reducing both horizontal and vertical components. During ascent, drag acts downwards, increasing the downward resultant force and acceleration. During descent, drag acts upwards, decreasing the downward resultant force. As speed rises, drag rises, making the simple parabolic path an idealisation valid only when air resistance is negligible. A strong answer earns marks for these points. States that air resistance is a resistive force acting opposite to the direction of motion through the air. Explains that the magnitude of air resistance increases as the speed of the object through the air increases. Links increasing drag to changes in resultant force: during ascent, drag adds to weight increasing downward acceleration; during descent, it opposes weight reducing it. Applies the idea to projectile motion, noting that drag opposes velocity and so changes both horizontal and vertical motion. A common slip: Treating air resistance as a constant force regardless of speed. Instead, stating that drag grows as speed grows. Exam tip: Use the phrase 'opposes motion' or 'acts opposite to velocity' whenever you describe drag.
See it
Key words
Say what each word means before you turn the card.
Air resistance
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Drag
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See every definition
- Air resistance
- A resistive force exerted by the air on any object moving through it, acting opposite to the direction of motion.
- Drag
- A resistive force that opposes the velocity vector, reducing both horizontal and vertical components of motion.
Label the diagram
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