We walk slowly and place our feet carefully. I was bemused watching some people trying to get up the eroded scree path at speed. They slide quite a long way backwards on each step. The faster you try to walk, the bigger the impact speed on the ground. This means you foot has more momentum when it hits the ground. The rate of change of momentum is bigger. Newton's Second Law says that force is equal to rate of change of momentum, so your foot puts a bigger force on the ground. A bigger force on loose scree pushes it backwards. Newton's Third Law says that if Object A applies a forward force on Object B, then an equal backwards force acts on Object A. So you slide backwards. Crafty physicists tip-toe up and put a smaller force onto the scree. It doesn't slide backwards.
