Physics required practicals

Force and extension

AQA 4.5 · RP6

GCSE Physics (8463) · Required practical 6 — method, variables, the marks examiners report students losing.

Verified against AQA 8463 (2026 spec)

Investigate the relationship between the force applied to a spring and its extension, and find the spring constant (F = k x e).

Apparatus

  • Spring, and a clamp stand with boss and clamp
  • Metre ruler fixed vertically, with a pointer on the spring
  • Slotted masses and a mass hanger (each 100 g weighs about 1 N)
  • Set square to read the ruler squarely
  • Safety goggles and a counterweight or G-clamp so the stand cannot topple

Method

  1. 1Clamp the spring at the top and record its unstretched length using the pointer against the ruler.
  2. 2Hang a 1 N weight on the spring and record the new length; extension = new length − original length.
  3. 3Add weights in equal steps, recording the length each time, and work out the extension for each force.
  4. 4Take the weights off and check the spring returns to its original length (that it is still elastic).
  5. 5Plot a graph of force (y-axis) against extension (x-axis).

Variables

Independent

Force applied to the spring (weight added)

Dependent

Extension of the spring

Control

  • The same spring throughout
  • The same starting point and measuring method
  • Temperature

Results & processing

  • Extension = stretched length − natural length; plot force against extension.
  • The graph is a straight line through the origin while the spring obeys Hooke's law; the gradient is the spring constant k. Beyond the limit of proportionality the line curves.

Where students lose marks

Plotting length instead of extension.

Fix: Extension = stretched length − natural (unstretched) length; subtract the starting length before plotting.

Reading the ruler at an angle (parallax).

Fix: Use a pointer and a set square, and read the ruler with your eye level with the pointer.

Adding too much weight.

Fix: Going past the limit of proportionality permanently stretches the spring, so it will not return to its original length; keep the loads sensible.

Improve the method

  • Use a pointer and set square to read the ruler accurately.
  • Add small, equal increments of force and check the spring returns to its original length each time.
  • Repeat the readings and take a mean extension for each force.

Try it — exam-style

Medium
2 marks
ORIGINAL

A spring extends by 0.04 m when a force of 2.0 N is applied. Calculate the spring constant.

Easy
1 mark
ORIGINAL

On a force-extension graph, what does the point where the straight line starts to curve represent?

Questions are written in the style of past AQA papers — never copied from them.

Drill it properly

Physics equations

F = k x e and the forces equations you need for this practical.

AQA GCSE Physics flashcards

Hooke's law, elastic behaviour and the limit of proportionality, drilled.

Stuck on force and extension?

Hooke's-law questions hinge on extension vs length and reading a graph gradient — I drill both, and your first lesson is free.

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