Newton’s Third Law


Chances are that you remember Newton’s Third Law from your high school physics class or, if you were dozing though high school and awoke to the nightmare of college physics, you may recall this famous law of motion in passing: For every action, there is an equal and opposite reaction.

Although Newton’s Third Law specifies equality and opposition in action and reaction, my interest in sharing this with you is to focus briefly on the broader concept of interaction force pairs.  How, you must be thinking, could this possibly relate to yoked prisms?  Consider a prism changing the path of light to be a physical/geometric optics force, and an individual viewing through the prism to be an interacting force.  The viewer and the prism are now an interaction force pair.

When I give my seminars or in-services to other professionals, I will call upon a volunteer from the audience who I will ask to stand and face opposite me, with arms relaxed to his or her side.  I extend my hands opposite the volunteer’s shoulders and exert a gentle force to both shoulders simultaneously.  What happens?


Typically the individual shows a subtle movement backward, but returns to the initial posture so quickly and subtly that the change is almost imperceptible.  I then repeat the motion, but with a harder push, and the individual may lose balance for a moment but quickly rebounds to the upright position.  Now, when I repeat it the third time, I pause just before the moment of impact to the shoulders, and what does the volunteer do?  Invariably anticipate the force that I am about to apply, and lean forward toward me to counteract it.  As WikiHow explains, it’s the same concept you use when you’re a passenger standing in a train or bus — you and the bus are now interaction force pairs, and to maintain upright posture you lean into the anticipated direction of the force created by the movement of the bus.


So what does this have to do with yoked prisms?  These paired prism effects exert an influence on your center of gravity.  Theoretically, when you look through pair of prisms that are bases down in both eyes, the floor moves upward and your posture should initially shift upward.  We might therefore conceive of child who is a toe walker, with center of gravity shifted forward, and think that yoked prisms bases down would transfer force further back toward the heels.  There is some evidence that in young healthy observers, bases down yoked prisms results in changes in gait due in part to the perception of increased height, the slant of the floor, and optic flow (take a look at this excellent overview by a group of optometrists from Univ. of New South Wales published in Investigative Ophthalmology and Vision Science last year).

Bob Sanet

But the concept of interacting force pairs is where Sir Isaac Newton meets Dr. Robert Sanet.  Bob is fond of this quote attributed to another optometric Bob, Dr. Robert Kraskin: “It’s not what a lens does to a person but what a person does with a lens”.  Bob and his wife, therapist extraordinaire Linda Sanet, demonstrate in their seminars that vertical yoked prisms can have seemingly paradoxical effects, rooted in a friendly dispute between the Kraskin and Kaplan schools of thought.  Does vertical yoked prism result in attentional and postural shifts of the individual in the same direction as the base of the prisms, or the opposite direction?  Well it all depends on whether the person leans into the direction that the prism shifts space, or away from it.  For several years now, after having a casual discussion with Linda and Bob about this, I’ve been fascinated by the individual responses of volunteers during my in-services as they walk with yoked prisms in place.  Curt Baxstrom has also aided my understanding of this in the context of yoked prisms and the vestibulo-ocular reflex.  Sir Isaac meeting Skeffington’s anti-gravity …

One thought on “Newton’s Third Law

  1. Either the body is in balance, or it dies. Balance is dynamic. Therefore, it becomes necessary for the body’s range of function to be expanded for greater flexibility of balance.


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