AQA · A-Level · Physical Education
Course version: 7582 · Version from Sept 2016
The relationship between impulse and increasing and decreasing momentum in…
Full specification wording
Linear motion: The relationship between impulse and increasing and decreasing momentum in sprinting through the interpretation of force/time graphs.
Impulse is the product of force and the time over which it acts, so it equals the change in momentum.
Checked against the AQA 7582 specification (opens in a new tab) on .
- Point 7 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Impulse is the product of force and the time over which it acts, so it equals the change in momentum. On a force/time graph, the area between the force curve and the time axis gives the impulse. In sprinting, a large positive area during the drive phase increases forward momentum, while a negative area during the braking phase decreases it. A sprinter who applies a large force for a short contact time can still gain momentum if the net area is positive. The gradient of a force/time graph is not impulse; only the area is.
What earns the marks
- Impulse is defined as force multiplied by the time for which the force acts, and it equals the change in momentum.
- On a force/time graph, impulse is represented by the area between the force curve and the time axis.
- A positive area (force in the direction of motion) increases forward momentum during sprinting.
- A negative area (force opposing motion, such as during braking) decreases forward momentum.
- The net impulse is the algebraic sum of positive and negative areas, giving the overall change in momentum.
Common mistakes and how to put them right
Misunderstanding: the gradient of a force/time graph gives impulse.
Put it right: The gradient gives the rate of change of force, while the area under the graph gives impulse.
Misunderstanding: any area under a force/time graph increases momentum.
Put it right: Areas below the time axis represent negative impulse and decrease momentum.
Misunderstanding: impulse and momentum are unrelated quantities.
Put it right: Impulse equals the change in momentum, so a net impulse of 20 N s produces a momentum change of 20 kg m s⁻¹.
Misunderstanding: a short contact time cannot produce a large impulse.
Put it right: A large force over a short time can produce a large impulse, as in a sprint start.
Exam tips
- Always state that impulse is the area under a force/time graph, and show your method for calculating that area.
- When a graph has sections above and below the axis, calculate each area separately and then find the net area by subtraction.
- Use the correct units: force in newtons, time in seconds, impulse in newton seconds (N s) or kilogram metres per second (kg m s⁻¹).
- Link your answer back to sprinting by explaining how the net impulse changes the sprinter's forward momentum.
Try the lesson
The start of the Linear motion: The relationship between impulse and increasing and decreasing momentum in sprinting through the interpretation of force/time graphs. lesson, made from checked teaching notes for this specification point.
Have a go first
How is the impulse applied to a sprinter determined from a force/time graph of their sprint start?
Listen
Impulse is the product of force and the time over which it acts, so it equals the change in momentum. On a force/time graph, the area between the force curve and the time axis gives the impulse. In sprinting, a large positive area during the drive phase increases forward momentum, while a negative area during the braking phase decreases it. A sprinter who applies a large force for a short contact time can still gain momentum if the net area is positive. The gradient of a force/time graph is not impulse; only the area is. A strong answer earns marks for these points. Impulse is defined as force multiplied by the time for which the force acts, and it equals the change in momentum. On a force/time graph, impulse is represented by the area between the force curve and the time axis. A positive area (force in the direction of motion) increases forward momentum during sprinting. A negative area (force opposing motion, such as during braking) decreases forward momentum. A common slip: Misunderstanding: the gradient of a force/time graph gives impulse. Instead, the gradient gives the rate of change of force, while the area under the graph gives impulse. Exam tip: Always state that impulse is the area under a force/time graph, and show your method for calculating that area.
Key words
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Impulse
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Force/time graph interpretation
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Positive and negative impulse
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Net impulse
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See every definition
- Impulse
- The product of force and the time over which it acts, and it equals the change in momentum.
- Force/time graph interpretation
- The area between the force curve and the time axis gives the impulse, while the gradient gives the rate of change of force.
- Positive and negative impulse
- A positive area, where force is in the direction of motion, increases forward momentum, while a negative area, where force opposes motion, decreases it.
- Net impulse
- The algebraic sum of positive and negative areas, giving the overall change in momentum.
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