What does "Low strength but good strength-to-weight ratio" mean?

In summary, spider silk is incredibly strong and has a strength to density ratio that is similar to steel.f
  • #1
TL;DR Summary
I am studying the properties of polymers, I understand they are low strength but I do not understand what is meant by "good strength-to-weight ratio". What does that mean?
Good strength-to-weight-ratio
  • #2
I assume it means that an ant has low strength compared to e.g. our strength in absolute values, but compared to its own weight, the ant is incredibly stronger than we are. Spider webs are another example, or folded paper.
  • Like
Likes user12323567 and TeethWhitener
  • #3
It’ll depend on exactly what you mean by strength, but it’s generally either tensile, compressive, or shear strength. So taking elastic tensile strength as an example, the strength to weight ratio will be the Young’s modulus divided by the density of the material:
Polymers have a significantly lower density than, e.g., most metals, so that even though their Young’s modulus isn’t particularly high, their strength to weight ratio is.
  • Like
  • Informative
Likes Lord Jestocost, berkeman and Lnewqban
  • #4

  • #5
Copied from

"Spider silk is lighter than cotton and up to 1,000 times thinner than human hair, yet it's also incredibly strong for such a wispy material. This outsized strength is vital for spiders, who need their silk to withstand an array of destructive forces, from the frantic flapping of trapped insects to powerful blasts of wind and rain.

Still, for animals of our size, it's hard to grasp the proportional strength of spider silk unless we frame it in familiar terms. Comparing it with steel might sound absurd, for example, but on a per-weight basis, spider silk is stronger. It may lack the stiffness of steel, but it has similar tensile strength and a higher strength-to-density ratio.

"Quantitatively, spider silk is five times stronger than steel of the same diameter," explains a fact sheet from the University of Bristol School of Chemistry. It also draws comparisons with Kevlar, which has a higher strength rating but a lower fracture toughness than certain spider silks, according to the American Chemical Society (ACS). Spider silk is highly elastic, too, in some cases stretching four times its original length without breaking, and retains its strength below minus 40 degrees Celsius.

It has even been suggested — but not tested, obviously — that a pencil-width strand of spider silk could stop a Boeing 747 in flight. In a more natural flex, however, the Darwin's bark spider of Madagascar can stretch its dragline silk up to 25 meters (82 feet) across large rivers, forming the world's largest-known spiderwebs."

Suggested for: What does "Low strength but good strength-to-weight ratio" mean?