Video Transcript
A cubic container holds water. The lid of the container can be
pushed downward, exerting pressure on the water in the container. Which of the following statements
most correctly describes how the pressure exerted by the water is changed when the
lid is pushed downward? (A) The pressure exerted on the
cube’s vertical sides is increased. (B) The pressure exerted on the
base of the cube and its vertical sides is increased. (C) The pressure exerted on the
base of the cube and its lid is increased. (D) The pressure exerted on the
base of the cube is increased. (E) The pressure exerted on the
base of the cube, its vertical sides, and its lid is increased.
To answer this question, we need to
recall something known as Pascal’s principle. This is also known as Pascal’s
law. And it states that the pressure of
a liquid is equally transmitted in all directions.
So we know we’ve got this cubic
container. And it’s filled up with water,
which we then compress with a lid. This of course results in an
increase in pressure of the water. This is because we’re trying to
compress it. We’re trying to squeeze the water
into a smaller space.
But we also know that water is an
incompressible fluid. Basically, we can’t compress it or
squeeze it into a smaller space like we’re trying to do. So if we’re exerting a force from
above with the lid, then the water must exert a force back so that it counters the
force that we’re putting on it. And now since the lid has a certain
area, that force is being transmitted all over the area of the lid. And this is basically what a
pressure is. It’s a force distributed over an
area. Hence, the pressure in the water
increases when we try to squeeze it with the lid.
However, Pascal’s principle tells
us that the pressure of a liquid is equally transmitted in all directions. So when we try to squeeze the water
with the lid, not only is it putting an increased pressure on the lid itself, but
it’s also putting an extra pressure on the vertical sides of the cube and the base
of the cube. This means that our final answer is
option (E). The pressure exerted on the base of
the cube, its vertical sides, and its lid is increased.
