Why do we Fall Off of Horses? (Newton's 1st Law of Motion)

Why do we fall off of horses?

The answer is in Newton’s 1st law of motion

Lets start off with a definition of Newton’s 1st law of motion from from The Physics Classroom:

Newton's first law of motion is often stated as

An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

You can read their explanation at this link: http://www.physicsclassroom.com/Class/newtlaws/u2l1a.cfm

When you are riding a horse you have 2 objects or bodies in motion together, you and your horse.  Your horse is providing the force that gives you (and him) velocity (speed) moving you both forward.  The key to understanding falling off is the last line of the law “unless acted upon by an unbalanced force.”  When another force acts on your horse that is when you fall.

The force that your horse uses to stop is friction.  We will go into details on friction later, for now we can leave friction as the force created when your horse plants his feet on the ground rather than picking them up and pushing forward.  Friction is the “unbalanced force” from Newton’s law above.  It changes the horse’s motion from forward to stopped.  However, the rider is another “object in motion” which now stays in motion moving forward over the horse’s shoulders and neck and eventually on to the ground.

According to Newton’s law, when you fall off you should continue to move forward through the air at the same speed (or velocity) you were going.  But as you don’t see riders floating along through the arenas or down the trail you have probably guessed that there is another force we haven’t mentioned yet.  Well you are right!  That force is GRAVITY!

Gravity is the force pulling you down to the ground even as your velocity from your horse is still moving you forward.  

The part of Newton's law about keeping the same direction can also be a reason we fall.  I once fell off when the horse I was cantering shied away from the rail, suddenly jumping about 3 feet to the side. He didn't break his canter but I found myself still traveling along our original path with no horse under me.  It was a very "Wile E Coyote" moment as I looked down and realized I was not going to be able to stay up in the air, then I hit the ground.

So we have discussed why we fall, but not every sudden stop or spook results in a fall.  What keeps riders on?  Well let's go back to the idea of "unbalanced forces". When your horse changes his motion unexpectedly either by stopping or by moving to a new place he is exerting a force on his own body to make that change. As a rider you need to exert a force on your body to also make that change.

There are a couple of "staying on" forces that are commonly seen. One is using your legs as springs and the other is using your saddle or part of your horse to crash into.

Our legs are very effective springs, we use them when we run or skip ourselves and when we are in two-point or jump position on a horse.  Our knees are the best springs, but our ankles and hips can also help out.  When your horse stops suddenly, if you can keep your heels down and legs in position you may be able to let your joints stretch out like a spring and then pull you back into your seat, or at least back onto the horse.

Alternately you need to find something on your horse or saddle that can exert a force on you when you crash into it.  Western and Australian Saddles are best for this as they have higher pommels than English saddles.  This makes it easier for you to crash into it to stop your motion.  Of course sometimes this method can be almost as painful as falling (especially for boys).