Question

Because of inertia: your body (more precisely the center of mass of you+legs above the foot) is moving forward when your foot catches on something. The foot contacting the ground is a localized, external impulse at the contact point; it can stop the foot, but it does not instantly remove the forward momentum of the whole body.

Answer 1

Yes — that’s exactly right. A short way to put it: stopping the foot is a localized impulse; it can change the momentum of the foot (and apply forces up the leg), but it does not instantly remove the forward momentum of the whole body (the center of mass). Because the body’s mass is distributed above the contact point, that localized force produces a torque about the center of mass (or about the contact point), so instead of the whole body coming to a stop you tend to rotate forward about the foot and keep moving.

Two useful viewpoints:
- Linear momentum/impulse: the ground delivers an impulse at the foot; to stop the body’s center-of-mass forward motion you need an impulse equal and opposite to the body’s momentum. A single brief contact at one point usually cannot deliver that impulse to the whole mass without causing rotation or large internal forces.
- Torque/rotation: with the COM ahead of the support point the ground force has a moment arm and creates a forward pitching moment, so the body rotates about the foot (you lurch or fall) unless other forces/steps redistribute momentum quickly.

That’s also why recovery strategies work: moving the other foot forward, bending knees, lowering the COM or using the arms changes the impulse distribution and moments so the COM can be slowed without pitching over.