Question: My ruler is metal and my friends ruler is plastic. My ruler is bent and his is not. Please can you explain why my ruler has not gone back to its original shape.

Bernard Ennis answered on 16 Nov 2015:

All materials deform when you apply a load on them. If the deformation is non-permanent i.e. the material retains its original shape when you remove the load, then this is known as elastic deformation. At some point the material will no longer be able to deform elastically and then one of two things can occur: either it will continue to deform or it will break. In the former case, the deformation is permanent and the material does not regain its original shape. This is called plastic deformation.

The amount of elastic deformation is metals is usually limited and when a certain load is reached (what we call the yield point) it starts to deform plastically. On the other hand, plastic materials exhibit much more elastic deformation, but the amount of plastic deformation is small, meaning that they break rather than yield.

In the case of your rulers, you’ve probably bent your ruler past the point where it will regain its shape, so it stays bent. Your friends’ plastic ruler may have been bent to the same angle, but because it hasn’t reached its elastic limit, it can regain its shape, otherwise it would have broken. Console yourself with the fact that you can bend your ruler double without it breaking, but your friend can’t.

Tip: if you want to use your ruler to launch projectiles (pieces of paper etc) then a plastic ruler is better for the reasons outlined above. This should not be taken as a stimulus for engaging in said activity is which is undertaken at your own risk. I hereby publicly and explicitly deny all responsibility for any damage to persons, property or otherwise arising therefrom.