# Why does the spatula twist when I throw it?

I was noticing this phenomenon today as I was putting the dishes away. I grabbed a rubber scraping spatula and casually threw it up into the air, giving it a bit of a flick so that the handle spun end over end allowing me to catch it by the handle again after one rotation. When I caught it, I noticed the rounded side of the spatula which started out on the left was now on the right.

I puzzled over this and gave it another throw, again only one single rotation of the handle, and the rounded end was now back on the left. Carefully I started throwing the spatula making sure to not apply any twist to the handle on release, yet the spatula continued to do a 180 degree flip as the handle rotated 360 degrees around.

Is this some unexplained mystery of physics? Some sort of weird gyroscopic effect?

Why does it do this?

Flat side of the blade into the wind, or spine?
If it’s flat side, I expect the utensil starts rotating so as to minimize wind resistance. That’ll get it halfway around, and the asymmetry of your catching method supplies the rest of the turn.

Hammers do the same thing. hold it with the head facing left and flip and it will twist 90 degrees and when you catch it the head will be facing right.

That is actually 180 degrees. I notice this when flipping hammers as well. Although I tend to flip them with the head facing up or down. When I catch it the head is facing the other way.

I see what you are saying, Squink, but the same thing would happen if I took a square peg 1 foot long and tossed it end over end. There would not be any less air resistance on one side of the peg, yet it would twist 180 degrees.

Are you throwing it up with the blade vertical? For me, the natural way to do it is to throw it up with my palm facing down, and catch it with my palm facing up, which means I’m now holding the spatula from the other side of the handle.

scr4:
No. I’m holding it with the blade horizontal. My palm is facing upward when I throw it, and upward when catching it, yet the spatula twists 180 degrees when I catch it.

My Classical Mechanics Professor called this the “remote control thorem” because you get the same effect when your watching TV and throwing the remote control up and down. I’ll see if I can dig up my notes and look up the actual name of the theorm and the reason for it later. From memory, the basic gist is that an object is stable about two axis, but will always be unstable about the third (I think the one with neither the largest or smallest moment of inertia) and so when you throw it, it will start to twist about that axis, thus you can’t avoid having it rotate about that axis when you throw it up in the air.

Plus hammers make lousy sails.
I tossed a spatula a few times, it reversed L to R each time, and there’s some funny twisting going on at the top of the arc.

I’ve heard of (and, strangely enough, noticed it with remote controls myself) the principle Simplicio mentions as well. A google turned up a non-physicist discussion of it here that may help: http://www.juggling.org/help/clubs/physics.html

ntucker:
Thanks for that excellent link. So from what I can gather, if I were to throw the spatula (or the book/hammer/square rod/etc.) perfectly straight up, it would not twist 180 degrees…
I’m not sure I buy that.

It’s the smallest. Just as something with a smaller second moment of inertia is easier to bend, something with a smaller first moment is easier to spin. Or rather, presents less resistance to spinning.

I just tried with a hammer and I was wrong. It only flips if you hold it sideways. If I hold the head straight up or down the hammer does not twist.

Non-rotation about the second axis is a unstable situation. Saying that it won’t twist if you throw it just right is strictly true, but it’s akin to saying that you can balance a sharpened pencil on its point if you set it just right. Ain’t gonna happen, and any perturbation would foil the perfection anyway.

Nope, I checked Goldstein, it’s the intermediate axis that is unstable. Remember, its not a question of what axis is it easiest to spin the object around in the sense of needing the least amount of torque (that would indeed be the one with the smallest moment, as you say), its around which axis is the motion least stable.

Concur - and this is a very good example to help understand the dynamics of the whole thing - the hammer head is conspicuously heavier than the claws.

Yes, and the twist starts with the head dropping. In the case of the spatula, which is lighter, there could be some aerodymamic stuff involved. I can’t try it out in my kitchen, because all my spatchy things are atypical. The only one with a flexy part slightly wider on one side also has a heavy stainless steel handle. Two others have a flap on only one side, and two are curved into a spoon shape.

ETA: We’re all talking about claw hammers here, I think. The effect doesn’t happen with a symmetrical hammer.

If you flip the spatula or remote with the opposite hand, you will see it flips the opposite way. When you flip it, you inadvertently add a bit of twist. It is just part of our natural motion when flipping. It takes much less force to get the ‘twist’ than the ‘flip’, so it is very difficult to flip with no twist.

I cannot find a link or cite, but I remember reading this about a deck of cards. It was the middle length axis (the “left to right”) of a deck of cards that would naturally rotate when flipped.

I don’t have one handy, maybe someone could test that…

That’s a misleading statement, if not simply untrue…

And that one’s not true.

And that one’s true, but misleading.

The question has already been answered by Simplicio and Pasta: it’s the the intermediate axis theorem at work. Understanding the physics is kind of tough, because it’s so non-intuitive. Spin around the “intermediate axis” (the principal axis that is neither “hardest” nor “easiest” to spin around) is unstable, so a small accidental motion in one of the other two axes results in the half-twist.

Here’s one explanation, referencing tennis rackets - which are symmetric, note.
Here’s another one with a movie (I think; I can’t see it myself)