The Bending Moment of Asparagus

Reading Time: 2 minutes

Spring is almost here, and with it, one of my favorite spring vegetables: asparagus. But with asparagus always comes the conundrum of how to know how much to break off. The top part is tender, crunchy and delicious. The bottom part is tough and woody. So how do you know where the line is between the two so you don’t have to guess and risk eating tough asparagus or wasting tasty asparagus? Simple: Let physics do it for you!

The first thing to remember about asparagus is that it is actually a tender shoot that was in the active and rapid process of turning itself into a woody stem before it was picked. The bottom part was already well on the way through the process, while the top part was still actively growing and is quite tender. From a materials perspective, those two parts have drastically different properties. The woody part is tough and resistant to breaking. It has the ability to bend a little without snapping, but is stiff enough to resist the bending. The tender, upper part is brittle and snaps easily. Most importantly, the tough part is able to withstand shear forces while the upper part cannot.

For those of you who are a bit rusty on your high school physics, a bending moment is the twisting component of a force and is calculated by multiplying the magnitude of the force by the distance over which the moment acts. The interesting thing about bending moments is that they propagate through solid objects in the form of shear forces. For example, this concept is crucial in the design of structural beams as the engineer has to account for the shear forces on the beam.

This principle works nicely on our asparagus because the shear forces propagate through the woody portion of the stem and cause the tender portion to fail under shear stresses.

The only trick is to apply your forces correctly so that the maximum shear stress appears at the woody end of the asparagus and the minimum shear stress appears at the tender end. If you do it the wrong way, you’ll just break off the tip. So grip the tip in one hand, about an inch from the end. With your other hand grip the other end of the asparagus as close to the cut as you can. Hold the tip steady in one hand and bend the cut end, making sure that the axis of rotation is between your index finger and thumb. The asparagus will snap right at the junction between tender and woody.

Remember, science is the study of the very real world around us. A little understanding of the principles involved and how to apply them can lead to all kinds of little tricks that can make your life easier. Science!

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