Unit 2 Section 2 Hooke's Law

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What is Hooke's Law?

Hooke's Law: The extension of a stretched object is proportional to the load or force applied to it, up to the limit of proportionality.

Limit of Proportionality: The point beyond which force is no longer proportional to extension. Also known as hooke's law limit.

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Formula for Hooke's Law

F = k x /\L

F = force in N

k = spring constant in Nm^-1

/\L = extension in m

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What is the Elastic Limit?

Elastic Limit: The force beyond which a material will be permanently stretched.


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Investigating Extension

  • Object under test should be supported at the top using a clamp, with a ruler at the side of the clamp to measure its original length and extension.
  • Weights should be added one at a time to the other end of the object.
  • Do a trial investigation to work out the range and size of the weights needed.
  • You want to add the same size of weights each time and also add a large number of weights before the point the object breaks to find how extension should vary with the force applied.
  • After each weight is added the extension should be calculated.
  • Extension = new length - original length
  • Plot a load against extension graph
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Investigating Extension Diagram


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What is Plastic Behaviour?

Some atoms in the material move position relative to one another.

When load is removed, the atoms don't return to their original positions.

A metal stretched past its elastic limit shows plastic deformation.

Plastic: A plastic material is permanently stretched once the forces acting on it are removed.

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