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Question
A spring stretches by 5 cm under a 20 N load. Find its spring constant.
Solution
Convert.
Apply F = ke.
Answer
400 N/m.
Question
A spring of k = 500 N/m is stretched 8 cm within its elastic limit. Calculate the elastic PE stored.
Solution
Convert.
Apply formula.
Answer
1.6 J.
Hooke's law
When to use
Within limit of proportionality only. Recall (not on equation sheet).
Elastic PE
When to use
On AQA equation sheet.
The extension of an elastic object is directly proportional to the force applied, within the limit of proportionality.
A measure of stiffness; force per unit extension (N/m).
Deformation that fully reverses when the force is removed.
Permanent deformation; the material does not return to its original shape.
Mistake
Using new length instead of extension.
Why it happens
Common confusion.
How to avoid it
Extension = new length − natural length. Always subtract.
Mistake
Forgetting to square e in elastic PE.
Why it happens
Quick substitution.
How to avoid it
Write the formula in full: ½ × k × e × e.