Spider webs are strong in the sense that they're strong for their extremely small thickness. A strand of a spider web (called spider silk) is very thin, so we can easily break it, but if we could weave many strands together to a thickness of a climbing rope, this spider-web-rope would be many times stronger than the rope, even stronger than some types of steel. For comparison, the tensile strength (how much pulling force along the length of something you can apply before it breaks) of some steels are around 400 to 800 megapascals while silk from some spiders can have tensile strength of 1,000 megapascals. However, it is very difficult to get enough spider silk to make a thicker cable, and we do not yet have a way to use spider silk for practical purposes.

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It is important to think about how much spider web silk you are dealing with when you break through a usual spider web. Its a really really small amount, right? But what if you had a large amount of web? Or a web with strands that were an inch thick? This is why engineers describe materials with "absolute strength", which describes a material's ability to resist breaking regardless of how much you have of it.

Spider silk actually has strength that is pretty close to steel, but spider silk is also much less dense (density = weight of something per volume of that something). So if you had two ropes of equal weights, one made of spider web and one made of steel, the spider web rope would actually be five times stronger than the steel rope!

spider web has an extremely high "strength-to-weight ratio", that is, the amount of weight that a spider web can hold up without breaking is huge compared to the weight of the web itself. However, an animal the size of a spider can only hold so much web material in its body, so the total amount of weight that a spider's web can hold up is very small compared to the amount of weight that, say, your body can hold up.

It's the same principle of how a table built out of toothpicks for legs wouldn't be able to hold up much weight compared to a proper table. The toothpicks are just too small.

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The key to this is the definition of "strength". When someone uses the word strength, they typically mean the amount of weight that can be lifted, or the amount of force that can be supported. By this interpretation, spider webs don't seem all that strong; they can be brushed aside with only a very small force. However, in scientific contexts (such as in reporting strengths of materials, like spider silk) strength is force per area supporting that force. When we consider that spider silk has a diameter around 0.003 mm (roughly 10 times smaller than human hair), we can see that even a small force will give a very large strength. In fact,in terms of force per area, the strength of spider silk is greater than that of many types of steel.

Pieces of steel tend to be much larger than pieces of spider silk though, so steel seems stronger because it is easy to consider only the force (and not the force per area) taken to break a piece of steel. A discussion of the structure of spider silk which leads to this strength can be found in previous ScienceLine answers.

This is a really interesting question because people always mention that the silk that makes a spider’s web is roughly as strong as steel. If that is true, how come we can break it so easily? The truth is that spider’s silk is very strong but there just is not very much of it in a web. In fact, an average strand in a web is only one tenth the width of a human hair. Since there is so little silk it is easy for us to break it in the same way that it is easy to snap a twig in half but not a tree. If we had enough spider silk it would be as strong as a steel beam while weighing significantly less. Thank you for your question!

A spider’s silk is around 3 micrometers (0.003 millimeters) in diameter, so it is very thin. It is useful to think of a rope made of many threads as an analogy. A single thread could never hold up a person, but by intertwining many threads into a rope, it can easily support the weight of a human.

When we say how strong something is, we usually mean its tensile strength, which measures how much stress a material can withstand before breaking. A single strand of spider silk is strong for its size and can withstand the force of an insect that flies into it. If we could somehow scale up the spider silk to be 100 times thicker (0.3 millimeters) by constructing a rope of spider silk, it would have a strength comparable a 22 gauge steel wire.