OK, let's get started. What does the Laws of Motion have to do with your ability as a horseback rider? The reason is that anytime your body is moved by an outside force of any kind, it affects your balance. Without balance, your brain panics your body into a rigid condition, which is often referred to as "the cradle position". So the bottom line is we humans don't function well in any activity or event, where something is making us out of balance, unless we can fix it. Let's fix it then!
Now, since riding a horse is the only sport activity that routinely ignores the Laws of Motion, let's pretend like you are trying a different sport other than horseback riding, just for an easier understanding of how this law works. You are now in the water, about to try water skiing. As you are bobbing up and down in the water you're in balance, putting your vest and skis on, the boat (momentum force) is just sitting still waiting for your que to start. During this time, there really isn't any "force" that is adversely affecting you and your balance. Once you give the signal to go though, everything changes. What is creating the change? The motor and the boat are now accelerating and momentum is being transferred to the skier. The boat is located in front of the skier, so the momentum is pulling on the skier from that direction. So by the Laws of Motion, the skier must lean away or back from this force in order to rise and and stay on top of the plane of water. Correct? Yes!
Now if you change back over to horseback riding, what should you do? Well, during a walk on a horse it's like the skier bobbing in the water, no momentum is being introduced that your brain and body can't handle very easily and essentially you're in balance. So it's easy to walk around on a horse for 99% of all humans. But, once the horse starts to trot or lope (canter), suddenly things change. What is actually changing? The horse (boat) is introducing momentum that the rider must do something with, or you will be out of balance. Now remember, most if not all humans, are going to end up by nature (panic) in the cradle position or forward when this motion happens. It's built into your brain to do this to save your life. Why do you have to save our life? Somewhere in your head, you know this animal can run off without your permission and that usually scares you to death. So now you are way too far forward, grabbing the horn or the neck or standing in the stirrups, yelling out WHOAAAAAAAA! Oh boy, do you need some help at this point! Usually a friend that is near us, or maybe if we are taking lessons, you'll hear them yelling out advice and instructions. Now why doesn't that work so well for us? Let's see!
In order to balance quicker and correctly, you must use the Law of Motion. So as the horse moves you up and forward, you should lean back just like the skier since the horse is pushing you forward. If you do this, your body will be in balance immediately and your brain will then relax and allow you to ride instead of cradling. If you have ever witnessed thousands of beginning riders like I have you will know how that looks.
All of the lessons I ever had, and the ones found on You Tube, give instructions whether it's English riding or Western, holding to this one thought: You must keep in some kind of "plumb" line from your ears to your heels, regardless of the motion of the horse. Why is it everyone wants you to remain in this upright alignment? I bet not one of them can really tell you why based on any facts, it's just what they were taught and so they pass it along. This is what makes riding a horse from the beginning so difficult!