When I walk up a steep hill, why don’t I fall back down?
Because of a complex process that begins with fluid moving minute hairs inside your inner ears.
That, plus friction.
i.e., we assume no roll and no slip conditions.
Is this question related to a Staff Report?
I’m assuming this is not a response to a staff report, so I’m moving it to the General Questions forum where you’re likely to get more responses.
Didn’t notice where I posted it. There are some grades that I will fall down on, right?
Because you keep your center of gravity (=~hips) forward of your feet.
Only if you lose your footing or lose your balance. People climb vertical walls without falling down.
Depends on how much friction you have. Rock-climbing shoes have extremely grippy soles; with these, you can probably maintain traction on grades in excess of 60 degrees from horizontal. At some point your feet will not be able to dorsiflect any further, and you will be walking/standing on your toes rather than the entire sole of your foot; standing still will require a bit more effort to maintain balance, but walking - being the dynamic balancing act that it is - probably won’t be much more difficult.
Theoretically, if you have infinitely grippy shoes, you can climb/balance on a near-vertical grade using only your feet - provided there is enough room for you to lean forward (so as to position your center of mass vertically above your feets’ points-of-support on the grade) without hitting your head on the grade.
:dubious:
Where does my choice come into play? :dubious:
You chose to reply. 
I’ve chosen to add a descriptive thread title.
Gfactor
General Questions Moderator
For the same reason you don’t fall over on a level surface.
The way to remain upright is to keep your cg over your feet. The inner ear mechanism is a gyroscope (detects acceleration) and the body’s biofeedback system autonomously reacts to counteract any movement of the cg toward the edge of your foot pedestal. (You consciously override the system when you fall into bed or a chair.)
The key is the fact that the soles of the feet must have sufficient traction to provide the necessary offsetting moment, think banana peel, ice.
When you’re walking, you are consciously keeping your cg just forward of your pedestal and are assuming that your feet will encounter no slick spots. This is true whether it’s on the level or a grade.
A nitpick:
If your CG is forward of your point of contact with the ground, then you will be accelerating forward. If you are walking at a steady speed, then your CG is (on average) directly above your feet.
True. I should have added that on a grade, there is a component force that acts against the traction necessary to remain upright, the steeper the hill the greater the force. You slip and fall when that force becomes greater than the static friction of your “grippy” soles.
Infinitely grippy shoes would stick to the surface and you wouldn’t be able to move your feet.
I’m not talking about stickiness/adhesion that resists separation of the two surfaces. Just an infinite coefficient of static friction, such that the slightest normal force permits infinite friction force.
Soil and sidewalks offer plenty of chances for varied surface contact, increasing Friction/Grip.
You don’t keep your body at a 90 degree angle to the surface, but you do keep it more in line with gravity plus a slight lean in the direction you are moving (forward). Walking can be decribed as constantly falling forward and catching yourself. You just have to lift your feet a little higher on an upward grade.
Note that if the surface is too smooth, or the grade too steep you will fall back down when you can no longer maintain friction and/or a slightly forward center of gravity.
It also has to do with the fact that you have muscles all around your body, and you’ve learned how to activate just the right ones to counteract gravity.
(But lately, I’ve been discovering that beyond a certain age, you gradually start to lose that ability.)