Straight constant velocity does not produce any forces. Acceleration and deceleration do produce horizontal forces and those are needed on dancing. To every action there is an equal and opposite reaction.

When walking continuously there are two special balance points, when no horizontal forces are needed for balance of moments. At these points the balance point of the body approaches at constant velocity. At one point deceleration ends and acceleration begins and at another point acceleration ends and deceleration begins.

Energy of deceleration pushes the body forward, until the moving foot is passing the standing foot and the body begins to fall forward, end of deceleration. At that point must the horizontal force of pushing of the supporting foot begin to produce a forward acceleration which produces a backward force towards the body and moving foot to balance moments and avoid falling forward. Between deceleration and acceleration the velocity is constant and the balance point of the body is above the standing foot and no horizontal forces are needed. From a “time value” point of view the step begins at the moment of time when pushing of the standing foot begins at the strike of count one.

When the moving foot lands and the pushing of the old supporting foot ends, ends acceleration and deceleration begins. Between acceleration and deceleration the velocity is constant and the balance point of the body is between feet which both are on the floor at full stride and no horizontal forces are needed. Then the deceleration produces a forward force which pushes the mass of the body on the new standing foot until deceleration again ends and acceleration begins. Deceleration begins at the strike of count two.

Straightening of the knee of the moving foot produces the highest acceleration on the foot but the mass of the foot is low and the moment mass x acceleration x height counts. Upper body has more mass and height from the point of the moment which is the ball of the supporting foot.

All above supposing that there are no up and down movements which also are to use.

My main interest in dancing is physics of dance movements. It should be interesting to have a topic of physics for those interested in subject.

I've been following these discussions, and apart from the dancing issues, there are a lot of comments which aren't in line with the laws of physics. I think it's important to apply physics correctly to get a complete understanding of what's happening.
For example:
"""Then the deceleration produces a forward force which pushes the mass of the body on the new standing foot until deceleration again ends and acceleration begins"""
Decelleration (of the body in this case)cannot produce a forward force (on the body in this case). Decelleration would be caused by a force in the opposite direction (i.e. trying to push the body back since we're talking about forward movement here). Where does this force come from? the friction from the floor at the contact with the foot of the new standing leg. The body keeps moving forward because of momentum. An object can continue to have momentum even if there is no force on it.
Sorry to be so picky, but it's important in the long run

Suomynona. That was not Puzzled you were answering. Anyway, If I am not balanced before I step, what am I , sitting down maybe. Nobody would suggest and most certainly not me that the feet will come to a stop. Being that the third step of the Feather has a toe lower and that the step is smaller than the other two. It is practically a foot position only. There is plenty of time for a balance point as there is on every third step in our Basics. It might be worth deliberatly being off balance on your Feather just to prove the point that a recovery is possible on that balance point. That is if the weight is correctly over the RF. But be off balance and just blast your way through that part will create a disaster from that point on. This seems an interesting subject that Puzzled introduced and asked . How many balance points there are in a Forward Walk in the Foxtrot. 32 answers on 2 pages.

“Question is: What causes inertia, working as a force opposite to the force by which the particle is influenced?” by some famous physicist.

Braking with the landed foot(HT) is the force which decelerates the mass of the body and produces a forward force.

When the old supporting foot has pushed forward and is off the floor, new standing foot(HT) becomes the point about moment and the balance point of the body is behind it. Gravity tends to pull the body downwards behind the standing foot. Gravity and distance(x) from point about moment tend to rotate the mass of he body backwards. To have balance of moments must there be a forward force. The forward force is produced by braking with landed foot(HT) which slows down the velocity. The deceleration produces the forward force needed. The forward force x mass of body x height(y) of balance point of the body from the floor is equal and opposite to moment mgx and tends to rotate the body forward. Thus the moments mgx=may are in balance. As long as the balance point of the body is behind the landed foot(HT) and the pushed foot is off the floor there must be a forward force produced by deceleration.

The forward force needed can be demonstrated by standing on both feet at full stride. You can ask somebody to push you forward on the standing foot or yourself pull you forward on the standing foot. The decelerating force is equal and opposite to the force needed.

Momentum is mass x velocity. When walking the push of the standing foot increases momentum on every step by impulse force x time the pushing lasts. To avoid the velocity increase step by step must momentum also be decreased by an opposite and equal impulse on every step by braking with landed foot(HT) during the time the deceleration pushes the body on the new standing foot. Imagine while walking that there suddenly were not any friction under the landing foot and the body could not decelerate. You could never get on the foot. Constant velocity and momentum neither push nor pull.

My PC doesn't accept the word decelleration. English is my second and rarely used language.

Sqq
Are you saying that the points of balance are infinite when transferring the body weight from the back supporting foot to the new forward supporting foot? Also when I'm at the full extent of my forward stride the body weight will be more forward than central?

Yes and all the time when the moving foot is off the floor and when the both feet are on the floor the moments are in balance.

This has stood the test of time ever since the first book of Ballroom Technique was ever written. That is at the extent of your stride. The front heel is on the floor having not yet lowered to a flat foot. The back toe is on the floor with the heel raised. If you go to the Learning Section on this web- site. You will see a series of leg foot and body positions which can be printed. You will see that the body is in a split weight position before it moves over the front foot, when the knee bends to about 46 degrees between the foot to the knee. When the persons weight is right in between the legs.and is photographed. You would not know whether they are going forward or backward. Something that SQQ wrote about earlier. Don't lets forget that bend of the knee absorbs the propulsion created by the rear foot which involves the knee which just earlier was also bent to store and use the energy needed to go forward. Somebody is going to ask. Do we bend the knee on the first quick in the Feather Step. The answer is of course no. That is a toe with no lower. At the collection point. That is the weight arriving onto a bent knee . If you look down your weight is over the foot and the knee is slightly forward of the body but bent. How far the knee bends depends on how good a dancer you are.

Movements which can be done very very slowly only can be done at constant velocity or momentum. Dec- or acceleration makes all other possible.

Quickstep, the book has stood the test of time, yes. But much of what your write about and see in the videos here is not out of the book. It is recent development, some of it flawed. Compare an old video to a recent one and perhaps you can see the differences - not in the amount of movement, but in how the method of movement you are used to seeing no longer matches the original. Maybe you'll be lucky enough some day to see a true champion who moves in the traditional way, but at the modern volume. Or maybe you won't see one, or won't recongize what they are doing when you see it.

Hi Quickstep
Would you agree that the poise for the gentleman is forward at the commencement of a forward walk? If so, what does the gentleman gain by changing that poise at the full extent of the stride. There will be a point - which has been explained - that the body weight will be equally divided between the heel of the front foot and the ball of the back foot sometime during the walk. But surely the poise will still be forward.