People could not walk without friction. What friction are we talking about here, kinetic friction or static friction?

The coefficient of friction between a "safe" walking surface and your shoes is supposed to be about 0.6. Estimate the maximum acceleration you could attain under these conditions.

List all the forces acting on a car negotiating a turn at constant speed on a banked road. What should be the magnitude of the vector sum of all the forces acting on this car?

Estimate the coefficient of friction between skis and snow that would allow a skier to move down a 30 degree slope with constant speed if other factors were neglected. Is it reasonable to neglect other factors, such as air drag on the skier?