Thread: From a physics stand point where is the best place to sit on a horse.

1. Hi, I am new to the forum but was glad to find it. I am a saddle maker by profession. In the world of the horse one of the classic debates is about where the best place is to sit on the horses back. There is a debate going on right now about the horses center of gravity. Some new research is indicating it is much further back than previously thought. Center of Mass is also being discussed but is being shown much further forward on the horses body. My questions are : Is the center of gravity the same as the center of mass? In a system as complex as a moving horse is it a given the rider should be above the horses center of gravity?

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3. Center of gravity means the same thing as center of mass, unless you are dealing with a non-uniform gravitational field, but that doesn't apply here. I'm pretty sure the center of mass of the horse will be somewhere in the middle of his body, so I don't know how the rider would get below that, unless he was slung underneath.
Is there some particular reason for the rider to be over the center of mass? I would think that physiological factors would be more important, like if the horse were bearing more weight on the shoulder or on the spine.

Now if the horse is turning quickly, then it might help to have the weight over the center to make it easier to pivot. I'm just guessing about what the debate might be about.

4. Yes, center of mass and center of gravity mean the same thing.
(Assuming of course that the strength of gravity is not different on different parts of the body you are finding the center of mass for)

5. Harold, Looks like I did not state my second question correctly I should have said should the riders center of gravity be aligned with the horses? But then your comment about being below the horse brings more questions to mind. If you have a horse with a center of gravity and human with a center of gravity and the human sits on the horse are you dealing with two centers of gravity or do they then form a single object with a new center of gravity?
Dan, can you assume the strength of gravity is the same on all parts of a horse? The head is more massive than the hoof.

6. A small argument can be made that as densities of materials differ.. so to the center of mass / gravity can be not where it seems..
From the world of Motor cycle racing.. I do understand the issues of direction changes and gravity mass.. Can you sit in the middle of mass..NO. Good luck ?

7. Yes the head is bigger than the hoof, but the horse's butt is bigger than its head so it all balances out in the end.

You are dealing with units of mass instead of the masses of separate objects.
So when you calculate how much gravity is working on every gram of matter the force is the same per unit of mass.

(Hmmm, Harold got the first answer in while I was typing the second one. I must learn how to type with more than one finger some day.)

8. Originally Posted by David Genadek
Harold, Looks like I did not state my second question correctly I should have said should the riders center of gravity be aligned with the horses? But then your comment about being below the horse brings more questions to mind. If you have a horse with a center of gravity and human with a center of gravity and the human sits on the horse are you dealing with two centers of gravity or do they then form a single object with a new center of gravity?
Dan, can you assume the strength of gravity is the same on all parts of a horse? The head is more massive than the hoof.
I think the proper way to say it would be that you want the rider to be directly above the horse's center of gravity. You can define the center of gravity of the horse, and the center of gravity of the rider separately, and you can define the center of gravity of the whole system. If the rider's center of gravity is vertically in line with the horse's then the presence of the rider would not change the center of gravity of the horse+rider system, except to raise it up a little bit.

If we are talking about thoroughbred racing around a circular track, then the horse is always turning. Part of his effort, then is rotating around one half circle as he goes around each turn of the track, then straightening back out on the straightaway. The turning effort is resisted by the moment of inertia, which is minimized by keeping the mass as close as possible to the axis of rotation.
List of moments of inertia - Wikipedia, the free encyclopedia
I have a hard time believing this would make much difference, but when the horse might be winning in a photo finish, who knows?

9. On the horses back. Otherwise you fall off.

10. The moment of inertia looks like it applies to a rigid structure. "Mass moments of inertia have units of dimension [mass] × [length]2. It should not be confused with the second moment of area, which is used in bending calculations." A horses body bends through a coupled motion of the spine so would that change the picture in relation to the COG/COM?
In one conversation I had one of the participants was a mechanical engineer and he felt the center of motion was of greater value than the COG but I would think they are the same thing?

11. To get the best performance out of a horse, you don't sit. Watch a jockey in a horse race. They aren't sitting. They stand in the stirrups, knees bent. The jockey uses his legs as shock absorbers to reduce the impact of his weight as much as possible. He also crouches down to reduce air resistance.