Required Practicals / Edexcel / Practical 9
9 AS CP9

Force-extension relationships (CP9)

Investigate force-extension relationships for a spring, rubber band and polythene strip.

Apparatus

  • Helical spring, rubber band and polythene strip
  • Slotted masses and hanger
  • Metre rule and pointer
  • Retort stand and clamp
  • Graph paper

Safety

  • Stand clear of hanging masses. Use a tray below to catch fallen masses.
  • Rubber bands and polythene can snap back; wear eye protection.

Method

  1. Clamp each material vertically. Attach a pointer to the lower end and note its starting position on the rule.
  2. Add masses in 100 g steps. After each addition, record the total load F and the extension x.
  3. For loading and unloading, record separate sets of readings for the rubber band.
  4. Plot F vs x for all three materials on the same axes.
  5. For the spring, check Hooke's Law: F = kx (straight line through origin). Identify the elastic limit.

Key Variables

Independent Applied force F
Dependent Extension x
Controlled Temperature; Rate of loading (add masses slowly)

Analysis and Results

  • Spring: linear region (Hooke's Law, $F = kx$); spring constant $k = \text{gradient}$. Beyond the elastic limit, line curves.
  • Rubber band: non-linear; loading and unloading curves are different (hysteresis). Area between curves = energy dissipated as heat.
  • Polythene strip: initially elastic, then plastic deformation (permanent extension on unloading).
  • Area under F-x graph = elastic potential energy stored.

Common Errors

  • Not distinguishing between elastic limit and limit of proportionality (they are different points on the graph).
  • Only recording loading data for rubber band and missing hysteresis.
  • Overloading the spring past its elastic limit on the first trial.

Exam-style questions on this practical. Click Show mark scheme to reveal the answer after attempting each question.

Q1 3 marks

A spring obeys Hooke's Law up to an extension of 8.0 cm, where the force is 4.0 N. Calculate the spring constant and the elastic potential energy stored at the elastic limit.

Q2 3 marks

Explain the shape of the loading and unloading curves for a rubber band and state what the area between the curves represents.