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.
Formula for Hooke's Law
F = k x /\L
F = force in N
k = spring constant in Nm^-1
/\L = extension in m
What is the Elastic Limit?
Elastic Limit: The force beyond which a material will be permanently stretched.
- 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
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.