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Science 7–10 · Year 7

Hooke's law: extension of a steel spring against load

Physical sciences — Forces (NSW Stage 4 focus area)

Practical, model not builtLow risk

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The idea

A spring stretches by an amount proportional to the force pulling it, until it is stretched past its elastic limit.

What you need

  • steel extendable springs with their coils separated, 2
  • retort stand, boss and clamp, 1
  • metre rule clamped vertically, 1
  • slotted mass set with hanger, 50 g slots up to 500 g, 1
  • pointer (straightened paperclip taped to the spring's lower hook), 1
  • G-clamp to fix the stand to the bench, 1
  • eye protection, 1 per learner

How to do it

  1. Clamp the stand to the bench. Hang the spring from the clamp with the metre rule beside it and record the pointer position with no load: this is the natural length reading.
  2. Hang the 50 g hanger and record the new pointer position. Extension = new reading minus the no-load reading.
  3. Add 50 g at a time up to 400 g, recording the pointer position each time. Then remove the masses one at a time and record the readings again to check the spring returns to its natural length.
  4. Convert each mass to force (F = m g with g = 9.8 N/kg) and each extension to metres, and tabulate force against extension.
  5. Plot extension (m) against force (N) and draw the line of best fit; find the gradient in metres per newton. Its reciprocal is the spring constant k in N/m.
  6. Repeat with the second spring and compare k values.

What you should see

Extension is proportional to force over the loaded range. For example, a spring with k = 25.0 N/m extends 3.9 cm under 0.98 N (100 g) and 15.7 cm under 3.92 N (400 g). Loading and unloading readings agree if the elastic limit was not passed; if it was, the unloaded spring stays longer than its natural length. The learner knows it worked when the graph is a straight line through the origin and the spring returns to its natural length.

What changes

What you change
force applied by the hanging masses (N)
What you measure
extension of the spring (cm)
What you keep the same
  • the same spring
  • masses added at rest, no bouncing
  • rule vertical and read at eye level

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Doubling the load more than doubles the stretch.
  • A spring always returns to its original length however far it is stretched.
  • Stiffer springs stretch more.

Safety card

Low riskLearners carry it out

Hazards

  • spring recoil into the eye if the load slips
  • masses falling

Controls

  • eye protection
  • keep total load under the spring's rated maximum
  • clamp the stand to the bench or weight its base

Note

No hazardous chemicals or naked flames are used. Complete the school's risk assessment for the activity before the lesson; the NSW Department of Education Science safety and compliance page points to CSIS 1.7 (Risk assessment – a pre-requisite for risk control) for how to carry it out.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Sources

The pages the author read to write this activity.

  1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
  2. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/content/stage-4/fa71c2a852
  3. spark.iop.org/investigating-simple-steel-springs
  4. spark.iop.org/collections/stretching-and-force
  5. instructional-resources.physics.uiowa.edu/1r1010-hookes-law-demo

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