Dat Torque

Hip circles are punks, you guys. Your trapeze bar/lyra wants to crush your hip bones and eat all the skin on your forearms if you even get around the bar at all. If you do, it will try to rip all your clothes off. Next post I’ll give you a progression, but first let’s look at what’s happening in most people’s dead zone: that point at which you get two or three progressively slower rotations and not a single one past that.

If you’ve read some of my other posts, particularly the one about controlling forward rotations, you know that rotating like a stick makes you go faster and rotating like a lever (or a stick with a joint in the middle. Or a broken stick.) makes you go slower. In a hip circle, you’re moving your weight off center from the point you’re rotating around so you can whip it to generate momentum and go faster. Lookit!

Don’t freak out. Imma say some physics stuff, but it’ll be fine, I super swear.

So imagine one of those three pronged spinnie gates at the entrance to the subway. You know, these:

If you walk up to one and hit it with your hand like BAP, it spins because it’s glued on (or whatever) to the post. It does not go flying across the room. You hitting the thing like BAP is a force, which goes in a straight line because that’s how a force, and hitting, works. The part of the force that causes the thingie to spin is the torque. If you hit the very end of the spinnie thing, farthest away from the post, it spins around waaay faster. That’s because there’s more torque. There’s more torque because you applied the force farther away, or at a greater radius away, from the post (the point around which the thing spins). So, torque equals radius times force. See how not scary that was?

Okay, now you’re the spinnie thing. First, let’s make it super simple to see what force and torque are doing. Imagine that we’re looking at you on the trapeze from the side. You are a very pretty pink line.

The force is just how hard you’re being pulled to the ground, which is your mass times gravity, WHICH is just your weight. The part of your weight that causes you to spin is the torque. Like if gravity came up to you and was all BAP on the back of your head. At this point you might be wondering where the other half of your body is. Don’t worry, I know you have legs! I’m just drawing the part of you that’s causing the torque. To start rotating, you need a lot of torque, so you either have to get heavier in like one second or get your weight as far away from the point you’re rotating around (the trapeze) as possible.

“What if I put my arms over my head?”

Faceless internet person, that would be genius!!

In the first half of the rotation (from 12 o’clock to 6 o’clock), gravity is your friend and will help you get so much torque! As you start to come up though (from 6 o’clock to 12 o’clock), gravity works against you, so it’s creating torque in the wrong direction! No!! How do you make torque small? You either have to get super light in like one second, or get your weight as CLOSE to the point you’re rotating around as possible.

“What if I curl into the tiniest of balls?”

Genius again, nameless stranger!!!

The anti torque is so small! Look how teeny his arrow is!

Keep the torque small from 6 o’clock to 12 o’clock (stay in a ball shape).

And as soon as you’re into the second half of the rotation where gravity can help you again (from 12 o’clock to 6 o’clock), get your weight as FAR AWAY from the axis of rotation as possible to make your torque super big again.

Now that we have a basic idea of generally what should happen, let’s look a little more closely at what the upper body (dark pink line) and lower body (light pink line) are doing at various points. At the beginning the upper body can use gravity to increase torque, so the upper body should be as long as possible. The legs are long here because if they weren’t you’d fall off the bar (in a perfect system you’d make them really short or not have them).
As the upper body reaches 6 o’clock, it gets as short as possible. The lower body also gets as short as possible until it reaches 12 o’clock to make the anti torque as small as you can.

“Hold the phone, Charlie! If there’s no torque but there IS anti torque, why don’t I fall backwards? You are saying the crazy things.”

That is true, very perceptive and somewhat precocious internet blog reader acrobat! The thing that keeps you going is your angular momentum. Don’t over think this part: momentum in a circle works just how you think it should: it’s the amount of oomph you have to keep going around. The anti torque will rob you of angular momentum (slow you down), but so long as your angular momentum is greater than the anti torque, you’re fine and you’ll keep spinning. See why it’s important to reduce anti torque as much as you can? 

Physicists are kind of wackadoodle calling angular momentum L, but most of them do good work and are relatively benign so we just let them have their ridiculous letter abbreviations.

As the lower body passes 12 o’clock, it gets as LONG as possible to generate the most torque. The upper body stays short because it’s still generating anti torque. See all the torque? The upper body can’t get very long when it passes 12 o’clock because you’re holding your legs, but it should be as long as possible. Rinse, repeat!

Now, let’s look at it with real humans! One of my students graciously agreed to let me use the following video stills for the side by sides in this post: thank you, Wren!! Starting position is super good: arms are extended  above the head (Wren’s shoulder flexibility is lovely, but mine leaves a bit to be desired…), legs are as close to horizontal as they can be with balance maintained.

At first glance, you’d think that it would be an advantage to keep the body as straight as possible for as long as possible, because sticks spin faster than levers. That’s true, but only if the weight of the legs never rotates backwards. The knees are a good indicator of which direction the weight in the legs is actually going. In the top image, the knees are above horizontal (the blue line). That’s fine if they never dip back to the line. In the bottom image, the knees haven’t moved from their starting position at slightly below horizontal. 

As the upper body dives forward, the arms drop. Remember the part where we want to make the upper body short as we near 6 o’clock? Dropping the arms does exactly that. Check out the knees in the upper panel: they’ve dropped down to the blue line. That means that there’s weight pulling the performer BACKWARDS even this early in the skill. Note the knees again. In the bottom panel they’re still in the same position even though the legs are starting to bend, while the knees in the upper panel are a bit lower still.  Remember how you want to make yourself short from 6 o’clock to 12 o’clock to reduce the torque in the wrong direction? Any weight you can throw in the direction you’re spinning will help counter that anti-torque by increasing your momentum. You can pick up a little extra momentum by keeping your legs straight then bending the knees really fast as your upper body nears 6 o’clock. You can see how fast the legs have bent by how blurry the feet are. Also, don’t forget about your giant 10 pound head! If you want to pull your weight in, chin to chest, ya’ll!!Again, look at the blur of the feet to get an idea of speed. Also, note how active the arms are in the lower panel. They’re pulling the chest in to the knees as hard as possible.As soon as the hips pass 12 o’clock gravity is on your side again, which means you want the lower body to be as long as possible. Many performers focus too much on the chest and straighten the legs too late. It’s about the butt, friends! Notice again how hard the arms are pulling on the legs in the bottom panel. If tension in the arms is maintained from 6 o’clock to 12 o’clock, the weight is kept closer to the point you’re rotating around, which reduces anti torque, which helps you maintain momentum until gravity is on your side again. With even a little laxity in the arms, the upper body stretches out and momentum is lost. Legs stay straight all the way from 12 o’clock to 6 o’clock. In the upper panel, the legs haven’t yet straightened at 9 o’clock, which means there’s a lot of unharnessed torque! Also, as soon at the upper body passes 12 o’clock, it gets as long as possible, too. There’s only so much of that that’s possible when you’re holding onto your legs for dear life, so it comes in the form of elongated neck and elongation through the upper arms (elbows press down as chest reaches up). Here, the legs in the top panel have straightened just past 9 o’clock, but the neck is short, the upper arms are compressed rather than long, and the arms aren’t pulling the chest toward the thighs. In the lower panel there’s a slight increase in speed as the radius is as long as possible to create the largest torque. The speed is faster here than in the previous image (compare the blur in the face and hair). Emphasis on elongating the upper arm: pressing through the elbows and stretching the crown of the head outward.
Emphasis on keeping very firm tension in the arms to keep the chest close to the legs.Emphasis on neck lengthening.As the legs near 6 o’clock and start to bend, the upper body is still well between 12 o’clock and 6 o’clock. As such, the upper body should be stretching as long as possible for the biggest radius and the biggest torque. Note the difference in neck length and the difference in speed blurring between the upper and lower panels. If the previous steps have been done well, momentum is now working for you: the momentum of the upper body pushes the lower body up and tucking the feet in generates even a bit more momentum in the right direction. The arms are very active and the angle between the femurs and the spine is quite acute. Here’s a still from the top of the third rotation for both performers. This is generally the point at which the momentum from the initial drop has run out and all that’s left is the momentum you can generate on your own.

Now that you have an idea (beaten to death) of what physics is doing to ruin your fun/make you have all the fun, go play, video, and see if you can ID your tendencies. Now that your brain gets what’s supposed to happen, in the next post I’ll take you through a progression to get your body to freaking do it already!

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