Negative Ease in knitting (both in flat rows and in rounds)

• Tomato Evony

Tomato Evony

I am a knitter and am interested using physics to explain the negative ease we see in knitting especially in rib [ (k1, p1) rib for example). I think some could be explained by the stretch-ability of the materials, e.g. wool, acrylic, but is there a movement of the yarn in between the "live" knots (the yarn form "knots" when they knit / purl) and if so I can see the yarn move when applying force on both ends, but how can they recoil back to original position when force is remove ?

I am a knitter and am interested using physics to explain the negative ease we see in knitting especially in rib [ (k1, p1) rib for example). I think some could be explained by the stretch-ability of the materials, e.g. wool, acrylic, but is there a movement of the yarn in between the "live" knots (the yarn form "knots" when they knit / purl) and if so I can see the yarn move when applying force on both ends, but how can they recoil back to original position when force is remove ?
Welcome to PhysicsForums.

Here are some photomicrographs of 2-strand 2-ply Peruvian Highland Wool yarn. It is unused and still in a ball. The fine marks on the ruler are 1/100 inch apart.

The first image is the relaxed yarn just unwound from the ball. Note that the individual fibers (hair) are crinkled, not straight, and rather tangled.

The second image is the yarn stretched by pulling on it. Here the fibers are straighter and closer together and the yarn diameter is smaller.

The third image is the yarn again relaxed. You can see it has regained much of its diameter and can probably detect that individual strands are again not straight.
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It is the tendency of the hair to recover its shape (crinkled) that causes the finished product to at least partially recover the original shape after being stretched. Each fiber acts somewhat like a spring.

Hope this helps.

Cheers,
Tom