Question

When two trains, going in opposite directions, are passing on tracks that are laid out close together, the train cars can often be seen to be leaning in toward one another where they are in proximity. How might the air passing through the narrow gap separating the two trains contribute to the observed attraction between their cars? (pick one)

a-/ When two trains pass each other in opposite directions and are close together, the air between the two trains is constricted in a narrow space. Therefore, the speed of the air increases, the pressure between the two trains decreases (by Bernoulli's principle), and the two trains lean in toward one another.

b-/ When two trains pass each other in opposite directions and are close together, the air between the trains is pushed in roughly equal and opposite directions so that the air becomes very still. Therefore, the speed of the air decreases, the pressure acting on the two trains increases (by Bernoulli's principle), and the two trains lean in toward one another.

c-/ When two trains pass each other in opposite directions and are close together, each train replaces the volume of air normally displaced by the other train. Therefore, the buoyant force acting on each train is roughly zero, the pressure between the two trains is roughly zero (by Archimedes' principle), and the two trains lean in toward one another.

d-/ When two trains pass each other in opposite directions and are close together, each train displaces a volume of air equal to its own volume. Therefore, the buoyant force acting on each train is twice as large as normal, the pressure acting on the two trains increases (by Archimedes' principle), and the two trains lean in toward one another.

e-/ When two trains pass each other in opposite directions and are close together, the trains create a column of air between them. The height of this column of air is the height of the train cars, much lower than the height of the atmosphere. So, (by the law of fluid pressure) the pressure on the outside of the trains is much greater than the pressure between the trains, and the trains lean in toward one another.

Answer 1

solution:

(A)

When two trains pass each other in opposite directions and are close together, the air between the two trains is constricted in a narrow space. Therefore, the speed of the air increases, the pressure between the two trains decreases (by Bernoulli's principle), and the two trains lean in toward one another.

explanation

As the fronts of the trains pass each other, they are compressing the air between them, which also adds to the amplitude of the wave (it's a compression wave after all), something that cannot occur without another train, or possibly a very close tunnel wall. The sides of the trains, as they are moving quickly, invoke the Bernoulli principle, resulting in low pressure between the trains to contrast with the high pressure around the fronts of the trains.