OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Describe the relationship between force and extension for a spring and…
Full specification wording
Forces in action: describe the relationship between force and extension for a spring and other simple systems (to include: graphical representation of the extension of a spring)
For a spring or other simple elastic system, the extension grows as the applied force grows, provided the elastic limit is not exceeded.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 9 of 21
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
For a spring or other simple elastic system, the extension grows as the applied force grows, provided the elastic limit is not exceeded. In this straight-line region the relationship is proportional: doubling the force doubles the extension, so force divided by extension stays constant and equals the spring constant k, with F = kx. A force–extension graph then plots as a straight line through the origin, and its gradient gives k. Beyond the elastic limit the line curves and the spring is permanently deformed, so the simple proportionality no longer holds. Learners should read values from such a graph, compare gradients and explain what a curved section shows.
What earns the marks
- States that extension increases as the applied force increases for a spring below its elastic limit.
- Identifies a straight line through the origin on a force–extension graph as showing a proportional relationship.
- Explains that the gradient of the straight section equals the spring constant k.
- Recognises that the line curves beyond the elastic limit because the spring is permanently deformed.
- Calculates extension by subtracting the original length from the stretched length before plotting or reading a graph.
Common mistakes and how to put them right
Plots the total stretched length instead of the extension.
Put it right: Subtract the original length first.
Reads the gradient as the reciprocal of the spring constant.
Put it right: Divide force by extension to obtain k.
Assumes the straight line continues forever.
Put it right: Note the curve beyond the elastic limit.
Treats a curved graph as showing a proportional relationship.
Put it right: Reserve proportionality for the straight section through the origin.
Exam tips
- Label both axes with quantity and unit before reading any values from the graph.
- Check whether the question gives total length or extension and convert if needed.
- Quote the gradient with its unit, N/m, when asked for the spring constant.
Try the lesson
The start of the Forces in action: describe the relationship between force and extension for a spring and other simple systems (to include: graphical representation of the extension of a spring) lesson, made from checked teaching notes for this specification point.
Have a go first
As the stretching force on a spring increases (within its elastic limit), what happens to the extension of the spring?
Listen
For a spring or other simple elastic system, the extension grows as the applied force grows, provided the elastic limit is not exceeded. In this straight-line region the relationship is proportional: doubling the force doubles the extension, so force divided by extension stays constant and equals the spring constant k, with F = kx. A force–extension graph then plots as a straight line through the origin, and its gradient gives k. Beyond the elastic limit the line curves and the spring is permanently deformed, so the simple proportionality no longer holds. Learners should read values from such a graph, compare gradients and explain what a curved section shows. A strong answer earns marks for these points. States that extension increases as the applied force increases for a spring below its elastic limit. Identifies a straight line through the origin on a force–extension graph as showing a proportional relationship. Explains that the gradient of the straight section equals the spring constant k. Recognises that the line curves beyond the elastic limit because the spring is permanently deformed. Exam tip: Label both axes with quantity and unit before reading any values from the graph.
See it
Key words
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Force and extension proportionality
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Spring constant and gradient
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Graphical representation of elastic behaviour
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Elastic limit and permanent deformation
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See every definition
- Force and extension proportionality
- Extension increases as the applied force increases, and in the straight-line region doubling the force doubles the extension, so force divided by extension stays constant.
- Spring constant and gradient
- Force divided by extension equals the spring constant k, and the gradient of the straight section of a force–extension graph gives k.
- Graphical representation of elastic behaviour
- A force–extension graph plots as a straight line through the origin in the proportional region, and beyond the elastic limit the line curves.
- Elastic limit and permanent deformation
- Beyond the elastic limit the line curves and the spring is permanently deformed, so the simple proportionality no longer holds.
Label the diagram
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3Box 3
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