By Janel Laidman
What makes a spindle spin? What makes it spin for a long time? What makes it spin fast? Take a walk with me on the edge of physics to find out more!
What makes a spindle spin? Well, of course it's the energy you put into it with your fingers when you twist the shaft. The rules governing how this energy behaves can be explained by looking at something called the "angular momentum" of the spindle. The angular momentum describes the spin tendency of the spindle and is dependent on three quantities, the mass, the velocity and the radius of the spinning object.
You can think of it this way...
Intuitively we know that the heavier something is, the harder it is to get it moving and the harder it is to stop it (think of trying to move a big boulder or a bus). Interestingly, when this law is applied to rotational movement, the location of the weight in relation to the axis of rotation affects how hard it is to start and stop, so two rotating objects of the same overall weight can behave differently depending on their shape. We call the location of mass in relation to the axis the "moment of inertia" which you can think of as the "effective weight" of the spindle. If the weight is concentrated toward the center of the rotating object, near the shaft of a spindle, then the "effective weight" is smaller which means the spindle is easier to start and easier to stop. Conversely, if the weight is concentrated far from the center, near the rim of the whorl, then the spindle will be harder to start and harder to stop. This means that among spindles of the same overall weight, center weighted spindles stop sooner, and rim weighted spindles spin longer.
The location of the weight will also affect the speed of the rotating object. When the "effective weight" is large (rim weighted spindle) the speed of the rotating object is small. And conversely when the "effective weight" is small (center weighted spindle), the speed is large. Intuitively we know this because we have seen it in figure skating... when a skater does a scratch spin with her arms outstretched she spins at one speed, but when she pulls her arms in close to her body, the spin becomes much faster. What she has done is change her moment of inertia.
Many factors can affect the spin of the spindle and how long it will spin, most importantly the balance of the whorl and hook, and the resistance provided by the yarn itself, which will eventually slow and stop the spin. But all things being equal in the balance and yarn departments, a rim weighted spindle will spin longer and slower and a center weighted spindle will spin faster and shorter.
Which spindle is the best? That depends on what you want to use it for. Many spindles achieve a larger moment of inertia by having a larger diameter whorl. Others achieve it by scooping out wood near the shaft so the largest amount of wood is near the rim. Some fibers need more twist and you want your spindle to spin fast, so you might choose a center weighted spindle like a bead spindle or tahkli, other times you might want a lofty yarn with less twist and choose a nice long, slow spinner.
So next time you look at your spindle collection, see if you can identify which ones have a larger or smaller moment of inertia!
For those who care about the math:
L= angular momentum
m = mass
v= velocity
r = radius (this is the radial center of mass)
L = m v r
If r is bigger, then v has to be smaller to get the same L (m doesn't change in this example) So a bigger radius = a slower spin.
Alternatively
L = I ω
I = moment of inertia = mr2
ω = angular velocity = v/r
If r is bigger, then I is bigger and so the spindle is harder to start and harder to stop = longer spin.
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