Multiple choiceQuestion 1 · Recall
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By the end you can explain how a tensile force and a compressive force deform an object, and use the words load, extension, compression and limit of proportionality correctly.
Hang a mass on a vertical wire and the wire gets longer. The force pulling it is a tensile force: two forces pulling outwards, along the wire, at each end.
Push down on a short pillar and it gets shorter. The force squashing it is a compressive force: two forces pushing inwards, along the pillar.
Both are examples of deformation — a change of shape or size caused by a force. In this topic we only study deformation along one direction (the length), because that is what the syllabus measures.
Extension and compression are both lengths, so they are measured in metres (often quoted in mm for a wire).
What matters is the direction of the forces relative to the object.
A rubber band and a sponge behave in the same two ways — the rubber band when you pull it, the sponge when you press it. The material does not decide which happens; the direction of the forces does.
Worked example: Worked example: reading extension off a measurement
A vertical wire is 1.800 m long with no load. A 4.0 N weight is hung on it, and the wire is then measured as 1.804 m long.
State the load and the extension, in mm.
For small loads, doubling the load doubles the extension. A graph of load against extension is a straight line through the origin.
Push the load far enough and the line starts to curve. The extension still increases, but no longer in proportion to the load.
The point where the straight line ends is the limit of proportionality. Beyond it you can no longer use "load is proportional to extension", so any calculation that assumes a straight-line graph stops working there.
Every spring and every wire has one; a very stiff steel wire simply reaches it at a bigger load than a slinky does.