As a waitress, its kind of important to learn how to balance multiple things on a tray while walking and without tipping the tray. Its hard to do so, and physics can help explain why. When loading items on a tray, they automatically apply a force down, which is their mass times the acceleration due to gravity. At the same time, my arm and hand have to push back with the same amount of force in order to keep the tray up. Also, when taking items off of a tray, its important to find that balance so that drinks don't go spilling everywhere.