Hi sqq
Interesting to see a discussion of the physical principles of movement.
some points i would make
1)It's essential to get the physics absolutely correct to avoid mis -understanding. At a couple of points you've said "forces produced by acceleration". This is not correct. Acceleration is caused by forces, not vice-versa. When a car accelerates you feel a force in your back. But this is not a force 'caused' by the acceleration. Rather, this force is causing your acceleration (the friction with the seat contributes too, of course).Similarly, the force of gravity causes vertical acceleration, not the other way round. Sorry to be picky, but it's important.
2)The forces involved in human movement are very complex, that's why it takes a couple of years or more for a person to start to 'learn' them.
3)The concept of static balance is inappropriate in an activity, such as dancing, where the dancers are never motionless.

Someone mentioned that people walk down the street in a very good dance action. Well, not in this country they don't. Most people walk very sloppily, with the standing foot flat until the moving foot has been 'plonked' in front of it, and with absolutely no driving from the standing foot

If there is a big heavy ball on a smooth hard bed on a lorry the ball will move when the lorry accelerates, decelerates or curves. Accelerations produce the forces witch move the ball. I mean these forces acceleration produces. An opposing force is needed to keep the ball not to move on the lorry. At any constant speed the ball doesn't move, any acceleration moves.

We can think feet to be the wheels speeding up or slowing down velocity and upper body to be the ball affected by forces accelerations produce. If you push quickly with supporting foot your body forward you will feel the force bending your body backwards also naturally balancing the moments.

Yes, there are many complex simultaneous physical things happening while walking. My description is a simple one that can be understood and felt.

Forgot to write my Guest Name on previous.


sqq

"1)It's essential to get the physics absolutely correct to avoid mis -understanding. At a couple of points you've said "forces produced by acceleration". This is not correct."

Actually, it *is* correct.

Remember that each force must have an equal and opposite force. If we have an astronaught lying on her back in a rocket that is accelerating, the rocket is pushing on her back with a force that accelerates her. Because she is bein accelerated, her back is pushing against the rocket with an equal force. This is a force produced by acceleration, specifically by acceleration working against her mass inertia's "desire" not to be accelerated.

"2)The forces involved in human movement are very complex, that's why it takes a couple of years or more for a person to start to 'learn' them."

Took you less than two years to learn them the first time as a baby. Of course, it takes an adult longer to learn something quite comparable, but different due to its different requirements.

"3)The concept of static balance is inappropriate in an activity, such as dancing, where the dancers are never motionless."

Exactly. Though it's still worth paying attention to as an exercise, and even to understand specifically what you are not trying to do.

"Someone mentioned that people walk down the street in a very good dance action. Well, not in this country they don't. Most people walk very sloppily, with the standing foot flat until the moving foot has been 'plonked' in front of it, and with absolutely no driving from the standing foot"

If it is a soft action and not haphazardly sloppy, then it is a good dance action. Not necessarly a big one, but a good one for dignified but unambitious purposes - what skilled dancers would do while social dancing "small" for example.

As for the development of the back foot, bear in mind that there are to different actions. Steps that initiate rise need to see the weight roll through to the toe and the heel start to rise just before the free foot passes the standing foot. But in steps that do not initiate rise, the back heel will remain down until just an instant before the front foot is placed. Another thing you probably won't hear about from anyone but world-class masters...

Hi Anon.
I accpet most things you've said, but not the one about force and acceleration.
It's a very picky point, but Newton's Laws state quite clearly that objects move in a straight line,(...few other words..) unless acted upon by a force. There is another law about action and reaction which you've stated correctly.
However, in the example you've chosen, it is the force on the astronaut's back which causes her to accelerate (with the rocket).
although there is a force of reaction from the astronaut into the rocket, it is misleading to say that this is caused by the acceleration, even though, without the acceleration, it would not be there. It is there as a reaction to the force exerted on the astronaut, and this force causes her to accelerate. It's a subtle point, but can be important to distinguish between forces which cause movements, and reacions which arise from those movements.
We'll be on to centrifugal force next!
We'd better stick to dancing

"However, in the example you've chosen, it is the force on the astronaut's back which causes her to accelerate (with the rocket). although there is a force of reaction from the astronaut into the rocket, it is misleading to say that this is caused by the acceleration, even though, without the acceleration, it would not be there. It is there as a reaction to the force exerted on the astronaut, and this force causes her to accelerate. It's a subtle point, but can be important to distinguish between forces which cause movements, and reacions which arise from those movements."

The reaction *is* a force. Mathematically clean physics as we are discussing here is perfectly reversable. Causes cause effects, but it's equally correct to say that effects cause causes. (And I don't mean reversable in time - I mean simply that if A causes B, then B also causes A. Of course until you add losses such as friction to the model, time is perfectly reversable too)

"We'll be on to centrifugal force next!
We'd better stick to dancing"

Indeed better to stick to things that actually exist like dancing and centripitel force... Unless of course you want to accept forces caused by acceleration, of which your "centrifugal force" is a classic example.

Hi Anonymous.
I have an awful feeling that this discussion of Newton's laws won't benefit anyone's dancing. However - here goes..
You wrote:
""""If there is a big heavy ball on a smooth hard bed on a lorry the ball will move when the lorry accelerates, decelerates or curves. Accelerations produce the forces witch move the ball. I mean these forces acceleration produces. An opposing force is needed to keep the ball not to move on the lorry. At any constant speed the ball doesn't move, any acceleration moves.""""
The big heavy ball on the smooth surface will move (relative to the lorry) because there is no force acting on it. The ball actually continues to move at constant speed in a straight line(because there is no force on it in horizontal direction)) while the lorry moves inderneath it (the force from the friction of the tyres allows this.
The accelleration of the lorry doesn't produce any force on the ball.
When you push hard with the standing foot to accellerate the body forward, there is no force bending the body back. Where would this force come from? The muscles of the trunk must work to keep the upper body moving with the hips, and these muscles provide a force in the direction of acceleration, i.e. forward.
Forces in a moving body are indeed very complex.

What's wrong with picture two is that the moving leg is so far in advance that it will tip the body impractically off balance. The only way you can get your moving leg out that far ahead of your standing leg and still finish the step nicely is if you support the weight of the moving leg on the floor as you move it. Some people teach that, but it's a really, really bad idea!

Should clarify - the only way you can get your moving leg out that far and still keep going is if you slide it's weight on the floor. Ordinarily, by the time you have that much weight that far forward of the standing leg, your stride is over and you are beginning the process of arriving on the moving foot. What we see in the picture is likely the case of the body being partially on the moving foot during a lot of the travel of that moving foot, rather than only starting to arrive after the movin foot has stopped moving, which is of course the proper way.

Nothing to do with a rolling ball. Just that my rolling ball stays where it is and moves along with the wheels.
What I am saying is that moving backwards or forwards we walk away from the standing foot. We don't take it with us because we are standing on it. How can we. That goes for the knee as well.