The Upper Body Leading

[ProgressNotPerfection]

Or a video!

Ahahaha - my favorite is the 4-legged yellow one. That was adorable.

 

[lucia_l]

PNP:
Yes. And all the words jumbled like that is precisely why we shouldn't start interrupting teachers to explain that "energy can't be directed, it's a scalar". It's the sort of precision we need in some contexts, but not on the dance floor! And now, on to the physics word salad which is going to get worse! After this, I'll shut up on this thread and if Warren wants to continue, we can move to the physics word thread. I'll also try the hidden text formatting. (We'll see if it works.) (If you can't avoid looking, notice there are adjectives. There are types of energy!! )

To Warren or anyone who wants to read more of the physics argument:
[hidden=Hidden text]

Spoiler: Hidden?

"Come from" doesn't fix the problem. Mechanical energy doesn't 'comes from' mechanical energy! Both gravitational potential and kinetic energy are both mechanical energy. That mechanical energy has to 'coe from' somewhere.

No on to "Work" and your point about body rise. Yes. Your CG moves up and down quite a bit. But the normal force is applied at the surface of your foot. The surface of your foot does not rise while the normal force is applied. ( The reason I left a trivial amount open as a possibility is even concrete actually flexes a little.) Your CG moving around change your amount of gravitational potential energy. But that (mostly) results in transforming some gravitational potential energy to kinetic energy. And when you move back up the reverse happens. Both of those are mechanical energy. (And with luck, that transformation is non-dissipative, which means you can keep going just like a roller coaster might! )

In fact, it can be worth having you think of a log rolling without slipping down a hill. A friction force definitely acts at the surface of the log. If it did not, the log would slip down the hill (and never start rotating.) No work is done by the friction at the contact point between the log and the hill surface. The logs CG moving down a is not the relevant Δx in F dot Δx. No mechanical energy is created by this. Gravitational potential energy is transformed into kinetic energy.

Now to a 1st law of thermo example: If you draw a control surface around your whole body and move with it, put on little arrows for "external work on the system" and "Heat added to the system", there is no positive external work from the foot on the floor. 1st law with U 'internal energy-- pretty much chemical-- ME mechanical energy, W work, Q heat, we have Δ(U+ ME) = W + Q. And we know W< 0 because there either is no displacement at the point of application of the force (surface of your foot) or your foot slide and the friction force acts in the opposite direction of the slide so W on the system is negative. (Yes. I will dispense with discussing Q.)

The source of increased ME cannot be positive external work because that can't be done by the forces acting on foot in contact with floor. (In the case of a body, the ΔME (mechanical energy) 'comes from' ΔU (internal energy). Not W. ME can then happily trade off between kinetic, gravitational potential and some elastic.)

You bring up the fact that the force on the floor has consequences. I agree. The action of the foot does generate an IMPULSE which changes MOMENTUM, Δp. But that's not work, W, on the system of your body. It is simply the case that we can accelerate or change momentum without having any work done at all!

On the body parts: Yes. Work is a scalar. I mean "equal in magnitude", "opposite in ± sign". I don't mean "opposite in direction". Yes. The - sign means the work extended by one part of the body is absorbed by the other. That was my point, so I'm glad we agree on that.

[/hidden]

Now to the part that's back to being closer to OPs discussion.
I don't know which type of advice or wording of the advice works best. It may depend on context and what the student is doing at the moment.

I have stood there while teachers tell leads to use their chest and also say things like use energy. This is generally accompanied by a demonstration of the action. Not a single student has ever literally pushed their chest against mine and bumped me there! The advice is generally accompanied by pointing out that I will sense their lead and move in response (which generally does happen.) Initiation in the chest with a nice locked in frame is much nicer than pitching shoulders back or forth or kicking me with their legs and so on. So generally, whatever the series of words means, it does seem to communicate something useful.

 

[Warren J. Dew]

It's the sort of precision we need in some contexts, but not on the dance floor!

Sure. A forum about dance isn't the dance floor, though. In fact, I'd argue that the original post is as precise as any in the thread; it's the accuracy that some of us are concerned about.

To Warren or anyone who wants to read more of the physics argument:

[hidden=Hidden text]

Spoiler: Hidden?

No on to "Work" and your point about body rise. Yes. Your CG moves up and down quite a bit. But the normal force is applied at the surface of your foot.

The vertical force is applied throughout the body column. Thus, it acts on the part of the body that is rising. You've proven that the energy does not come from the floor, but I didn't say it came from the floor.

Your CG moving around change your amount of gravitational potential energy. But that (mostly) results in transforming some gravitational potential energy to kinetic energy.

The CG moving vertically in a rise that results from just pushing the nonmoving body up involves very little kinetic energy at all. Transformation between gravitational potential energy and kinetic energy is what happens mostly in body swing, not in an initial rise from a still position or in a driving step, which is what I was talking about.

And with luck, that transformation is non-dissipative, which means you can keep going just like a roller coaster might!

Agreed with respect to body swing, which isn't what I was talking about.

Now to a 1st law of thermo example: If you draw a control surface around your whole body and move with it, put on little arrows for "external work on the system" and "Heat added to the system", there is no positive external work from the foot on the floor. 1st law with U 'internal energy-- pretty much chemical-- ME mechanical energy, W work, Q heat, we have Δ(U+ ME) = W + Q. And we know W< 0 because there either is no displacement at the point of application of the force (surface of your foot) or your foot slide and the friction force acts in the opposite direction of the slide so W on the system is negative. (Yes. I will dispense with discussing Q.)

So? Nowhere did I discuss any energy originating from outside that control volume.

The source of increased ME cannot be positive external work

Why do you think I'm talking about external work? One's foot is not outside the control volume.

[/hidden]

I have stood there while teachers tell leads to use their chest and also say things like use energy.

I have had one pro-am instructor who told me to put my chest on her. Fortunately, she wasn't the type to be sloppy about words like "energy" - or maybe she just didn't agree with what the original poster is getting at.

 

[lucia_l]

Other than AT, I've pretty much always been advised body connection is lower than the chest. If I'm in proper frame in smooth, the guys chest isn't going to be touching mine!

Mind you, I'm not always in proper frame.

But your ballroom-competitive experience is definitely longer and more extensive than mine.
I'm going to take a stab at how I would the OP paragraph that seems to have the largest number of physics terms. It seems to me it's better to explain how it could be changed to better align with physics than to just say "you used energy or power wrong!"

​

Spoiler: Physic semantics involving force/energy etc.

Here's the original.

Among them, the upper body is the driving force of the movement, just like the engine of a car, the legs and feet are the supporting power of movement, just like the wheels of a car, and the hips play the role of connecting the upper body with the legs and feet, just like the main body of the car, which are just the parts driven by the power of movement without any direct help to the movement.​

Here's my stab.
Among them, the upper body provides some potential energy that can be converted to energy of motion which manifests as change in speed. Like a car’s tires and axle, the force at the feet and legs directs the motion. Like the engine of a car, the internal work performed largely in the muscles of the legs, further increases speed, while the hips and core hips and core play the role of connecting the upper body with the legs and feet and ensuring that changes in momentum result in controlled motion in the desired direction.

By initiating the motion by moving the upper body to a slight imbalance in the proper position, and holding an appropriately organized frame, the body's reflexive responses will encourage correct operation of the lower bodies engines and drive train.

I'm not sure my change helps dancers. It makes a bit happier in terms of engineering!

 

[Captain Price]

The upper body leading is a principle, not a style. My point is that dancers have to use as much as energy required from the upper body in order to dance perfectly. That does not mean that your leg, feet etc. do not need to be very strong. In fact, in most cases, dancers use his energy more from leg than from the upper body, but the speed and distance of movement is always determined by how much energy to use from the upper body. If the dancer has not use as much as energy required from the upper body because of his incorrect understanding or way of dancing, his dancing will never be perfect.

It would be useful to exemplify what is meant by "upper body leading" in the context of some simple figures. As far as I understand the idea, it amounts to swaying in the direction of movement, for example in the Tipsy, to accelerate the movement. But in most cases, the sway is against the direction of movement as the conclusion of a swing, so this principle is seldom used in standard technique.

 

[Captain Price]

Another case that I can think of is swaying slightly to the L (lady R) when opening to PP, e.g. on step 3 of Open Telemark. This is not part of standard technique

 

[Julius Zhou]

It would be useful to exemplify what is meant by "upper body leading" in the context of some simple figures. As far as I understand the idea, it amounts to swaying in the direction of movement, for example in the Tipsy, to accelerate the movement. But in most cases, the sway is against the direction of movement as the conclusion of a swing, so this principle is seldom used in standard technique.

Yes, what you said about the sway above is right. As I said in my previous post of The Technical Details of 1-3 of Natural Turn in Waltz: "The sway makes the body shape very beautiful, but its original purpose is not for the beauty of the body shape, and it is purely for maintaining a good body balance. Since the dancer swings his body very strongly to one side within a split second for a huge step, the great power to that side should be balanced by a strong sway of the body to the other side." (also see: Natural Turn in Waltz question)

The upper body leading means in general that the distance of movement as well as the speed of movement is purely decided by the use of the energy of the upper body, and the legs only offer supporting for the moving upper body, achieved by increasing their muscle tone.

 

[Julius Zhou]

Another case that I can think of is swaying slightly to the L (lady R) when opening to PP, e.g. on step 3 of Open Telemark. This is not part of standard technique

I think the sway used here is mainly for balance too, and by stretching the side on the new standing leg to make a slight sway to L for man and to R for lady, the couple will gain a better balance.