The simplest answer would be "acceleration due to gravity."
The exact value of this acceleration changes depending on which planet your on (for example).
Answer:
The force is 
Explanation:
From the question we are told that
The length of the box is 
The width of the box is 
The height is 
The pressure experience on one of the sides is mathematically represented as
Where A is the area of the box which is mathematically evaluated as

substituting values


This pressure is equivalent to the atmospheric pressure which has a constant value of 
This implies that

=> 
=> 
Because it generates enough momentum to keep the train going with out really having to speed up
I think centripetal force ☺
I believe the answer is F. This is because, F stands for Fluorine, which is an element and consists of one atom. A molecule consists of TWO OR MORE atoms held together by chemical bonds. F consists of only one atom, which is Fluorine. So the answer is C. Hope helped.