This simulation explores the effect of frictional force in a scenario where two objects stacked on top of each other are subjected to a an external force. Can you figure out the relationship between Ma and uab so that the present does not slide off the sled?

此程式模擬摩擦力影響堆疊的兩物間受外力的相對移動關係。你能找出Ma 與 uab間存在怎樣的關係時可以讓禮物不會從雪橇上滑落嗎?