What it does
Every wire stretches when you hang a load on it. For small loads it stretches in proportion to the load and springs back when you take it off: Hooke’s law. Load it more and it goes past its elastic limit, flows like soft clay, keeps a permanent stretch, thins at one spot and finally breaks. This tool lets you do all of that safely with wires of steel, copper, aluminium and rubber, and shows where you are on the stress-strain curve as you go.
The second mode is Searle’s experiment, the school practical for Young’s modulus: add loads half a kilogram at a time, read the stretch on the micrometer, and the tool fits the best straight line and works out Young’s modulus from its slope. Pick the mystery wire to find an unknown modulus yourself.
It follows the school topics on the mechanical properties of solids: stress, strain, Hooke’s law, the stress-strain curve and the elastic moduli (Class 11 Physics), with Searle’s apparatus from the practical list. To read a curve from a real test file use stress-strain curve analyser; for forces and machines see simple machines lab, and for pressure in liquids hydraulic lift.
How to use it
- Pick Pull a wire or Searle’s experiment.
- Choose the wire, its length and its thickness. A thicker wire has more area, so the same load gives less stress.
- In Pull a wire, drag the load. The dot on the curve and the facts show the stress, the strain, the stretch, and what happens if you take the load off. “Zoom on the start” shows the straight Hooke’s law part, which is too small to see on the whole curve. “New wire” gives you a fresh one after a break.
- In Searle’s experiment, tap Add 0.5 kg and Take 0.5 kg off to take readings, and read Young’s modulus from your slope. “Start again” clears the table.
Worked example
Open the page: a steel wire 2 m long and 1 mm thick with 10 kg hanging on it. The area is π × (0.0005 m)² = 7.854 × 10⁻⁷ m², the force 10 × 9.8 = 98 N, so the stress is 98 ÷ 7.854 × 10⁻⁷ = 124.78 MPa. With Young’s modulus 200 GPa the strain is 124.78 × 10⁶ ÷ 200 × 10⁹ = 6.24 × 10⁻⁴ and the stretch is 6.24 × 10⁻⁴ × 2 m = 1.248 mm. That is inside the elastic limit (250 MPa, about 20 kg here), so the wire springs back.
Now raise the load to 25 kg: the stress is 311.94 MPa, past the elastic limit, and the stretch jumps to about 63 mm. Lower the load to 0: the wire stays 59.70 mm longer for good. The wire breaks above 33.7 kg (420 MPa).
Tips
- Strain has no unit: it is a length divided by a length.
- Young’s modulus depends on the material, not on the size of the wire. The breaking load depends on both.
- In Searle’s experiment, keep the loads well inside the elastic limit, or the points stop lying on a line.
FAQ
Why does Searle’s apparatus use two wires? The second, unloaded wire carries the spirit level and the scale, so a change in room temperature or a sagging support stretches both and cancels out.
Why does the curve go down before it breaks? It plots the force over the wire’s starting area. Near the end the wire thins at one spot (necking), so it carries less force.
Are these real values? They are rounded teaching numbers that behave like each material. Real metals vary with how they were made.
Is anything stored? No. It all runs in your browser.
Where the numbers come from
- Stress = F/A, strain = ΔL/L, Hooke's law (stress = Y × strain), the shape of a ductile metal's stress-strain curve (elastic limit, strain hardening, necking and fracture), unloading along a line of slope Y (permanent set), rubber's curve (the simple neo-Hookean model) and Searle's method Y = g L/(A × slope): standard school physics found in any textbook.
- The material values (Young's modulus, elastic limit, greatest stress, strains) are made-up, rounded teaching numbers that behave like the named material, not data for any real grade. g = 9.8 m/s².
- Checked by hand: a 2 m, 0.5 mm steel wire (Y 200 GPa) stretches 2.496 mm under 5 kg; Y comes back exactly from the slope of noise-free readings; the curve is continuous and its inverse matches it to 1 part in a million.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.