sand has its own unique coefficient of friction, as does every surface. when i went to the beach, i was pulling a cooler. there was a lot of friction between the sand and the wheels of the cooler, so pulling the cooler took a lot of effort. i could distinguish that sand had a greater coefficient of friction because it was so much harder to pull the cooler on sand than on concrete. i struggled with friction to get my iced tea to the beach and sand chair. i had to pull with more force in order to counteract the friction between the sand and the wheels.