In an elastic collision against a wall, where the angle of incidence is equal to the angle of deflection, why is it assumed that friction effected no impulse, and only the normal force did? I can understand if this was stated in the problem by saying "frictionless surface, or whatever" but this seems to be the case if a shoot a tennis ball against the floor or a wall too. Why is it that friction is almost negligible here? Is this a property of very-quick collisions, like in Kinetic Theory? Thanks.