A horse pulls a sleigh at constant velocity. Suppose the horse wants to speed up. If it pulls forward on the sleigh with a larger force to try to make the net force on the sleigh nonzero, the sleigh will simultaneously pull back on the horse with an equally larger force (Newton's third law). Then how is it possible for the horse and sleigh to ever speed up? Compare the magnitudes of the horizontal forces acting on the horse and on the sleigh if they both have a nonzero net force in the forward directions.