Not sure what this has to do with anything... full speed charge or steady canter the inertia might change but the mass remains the same and the mass of a horse + rider will knock more than 1 rank of infantry down even at a walk. Of course even at slower speeds horses can suffer broken legs even wearing full barding and I've even heard of horses running through a field hitting a stump breaking their sternum. A full speed running horse is not a very steady platform either for anything except archery. Just watch how much the jockeys move even balancing most of their weight over the stirrups. Most horses that would have been used by heavy cavalry probably have 2-3 gaits used for different activities.

The most difficult part of a heavy cavalry charge would be training the horses to maintain formation. Most horses when moving at anything more than a walk instinctively move forward or back of the horses around it. You can overcome that with training but it usually has to start young when you are mass producing horses for cavalry operations which use quite many. Trained horses and men could stay in formation and hit a wall of infantry. Whether that infantry maintained cohesion in the face of the charge directly related to how many cavalry casualties resulted and the training of the infantry after taking the charge would have a big effect as well if the infantry formation shattered or drew the cavalry in and isolated individual cavalrymen etc.

Most cavalry charges wouldn't be a single line of cavalry but some ranks as well. The first rank hits the first rank of infantry and either dies or not but either way knocks those infantry out of the way but loses momentum. The next rank would penetrate a little deeper... etc. A flat line which is so effective in MTW2 is probably one of the least effective formations for disrupting a steady infantry formation where a wedge would work much better but it would probably be ideal for chasing scattering infantry/skirmishers.