Sledge moving on a surface with friction

Question

A sledge moving on a frictionless icy surface with v₀=5 m/s, suddenly moves out onto the road. What is the distance the sledge will have moved on the road if the lenght of the slide is l=1 m, and the road's coefficient of friction is μ=0.5

Answer 1

The simple model of friction which is pretty successful does not depend on the area of contact. So you do not use the length (or width). If the surface is flat the friction depends on the the product of the weight of the sled and the coefficient of friction, μ. More generally the friction depends on the Normal force which is equal to the weight on a flat surface but gets smaller on an incline.