The thickness of the ice The temperature of the air above the ice The force of friction between the ice and the rock The gravitational force between the ice and the rock
I am hesitating between 4 options to answer the following question, which option should I choose?
Question: Kelly slides a flat rock across the smooth ice of a frozen pond. The rock slows down after several seconds. What causes the rock to slow down?
Possibilities:
- The thickness of the ice
- The temperature of the air above the ice
- The force of friction between the ice and the rock
- The gravitational force between the ice and the rock

The force of friction between the ice and the rock