Springs: (a) tensile and compressive deformation; extension and compression

The idea

When a material is subjected to opposing forces, it can deform. Tensile deformation occurs when forces stretch the material, producing an extension (increase in length). Compressive deformation occurs when forces squash the material, producing a compression (decrease in length). For example, a spring pulled at both ends experiences tensile deformation and its extension is the increase from its natural length. A spring pushed at both ends experiences compressive deformation and its compression is the decrease from its natural length. Extension and compression are both measured in metres and are vector quantities in the sense that they have direction along the line of the force, but in this course they are usually treated as magnitudes with a sign convention. The deformation is elastic if the material returns to its original shape when the forces are removed, and plastic if it does not.

What earns the marks

• Tensile deformation is caused by forces that stretch a material, producing an extension.
• Compressive deformation is caused by forces that squash a material, producing a compression.
• Extension is the increase in length from the natural length; compression is the decrease in length from the natural length.
• Elastic deformation is reversible; plastic deformation is permanent.
• The direction of the deforming force determines whether deformation is tensile or compressive.

Common mistakes and how to put them right

• Confusing extension with compression.

  Put it right by checking whether the material is stretched or squashed.

• Thinking that compression always means the material gets shorter than its natural length.

  Put it right: Compression is a decrease relative to the natural length, but the material may still be under compression while shorter than its original length.

• Assuming all deformation is elastic.

  Put it right by distinguishing between elastic and plastic behaviour.

• Forgetting to measure extension from the natural length, not from an arbitrary starting point.

  Put it right by using the unstretched length as the reference.

Exam tips

• Read the question carefully to identify whether the material is being stretched or squashed.
• Use the terms extension and compression correctly in your answers.
• If a graph of force against extension is given, note that the gradient gives the spring constant for a spring obeying Hooke’s law.
• Remember that extension and compression are measured in metres, and convert from cm or mm if necessary.