Video Transcript
A piston undergoes horizontal
reciprocal motion within a cylinder in an engine, as shown in the diagram. Where there is contact between the
piston and the cylinder, friction is produced. The friction can be reduced by
coating the cylinder with a lubricating fluid. If the lubricating fluid used has a
low viscosity, which of the following most correctly explains why parts of the
cylinder will not retain sufficient lubrication? (A) The lubricant will flow from
the lower parts of the cylinder toward the upper parts of the cylinder more than in
the opposite direction. (B) The lubricant will flow from
the upper parts of the cylinder toward the lower parts of the cylinder more than in
the opposite direction. (C) The lubricant will flow from
the open end of the cylinder toward the closed end of the cylinder more than in the
opposite direction. (D) The lubricant will flow from
the closed end of the cylinder toward the open end of the cylinder more than in the
opposite direction. And finally (E) the lubricant will
flow toward the center of the piston more than in the opposite direction.
Since we can’t fit all the answer
options on screen, let’s condense options (A), (B), and (C) so we can see them all
together. Answer option (A) essentially said
that the lubricant flows from the lower parts of the cylinder to the upper
parts. On our diagram, that would look
like the lubricant flowing this way. Answer option (B) basically said
that the lubricant flows from the upper parts of the cylinder toward the lower
parts. We can show that motion using this
pink arrow. Option (C) basically said that
lubricant flows from the open end of the cylinder toward the closed end. We could show that overall motion
using this green arrow.
These arrows on our diagram help us
summarize what the different answer options claim. Let’s make arrows for options (D)
and (E) then. Option (D) says that the lubricant
will flow more from the closed end of the cylinder toward the open end. We show that using the blue
arrow. And then with red arrows, we can
depict answer option (E), where the lubricant flows more toward the center of the
piston than in the opposite direction.
Let’s now recall that here we have
a cylinder with a piston that moves left and right in it. At the points of contact between
the piston and the cylinder, friction is produced. This friction can be minimized by
using a lubricant. And we’re told that in this case
the lubricant used has a low viscosity. A low-viscosity fluid is one that
is not thick like honey or motor oil is thick. One example of a low-viscosity
fluid is water. So whatever the lubricant used
here, since it does have a low viscosity, we can think of it moving through the
cylinder like water would move.
In general, water is not a
thick-enough fluid to stick to any surfaces, either the walls of the cylinder or the
faces of the piston. Rather, a low-viscosity fluid will
tend to flow downward towards this bottom part of the cylinder. And it won’t be able to be moved
very much at all in the opposite direction toward the top of the cylinder because
that would go against the acceleration due to gravity. And it would also require a
stickier lubricant than the one we have. All that to say we do expect the
lubricant in this cylinder to move in the direction of this pink arrow, towards the
lower parts of the cylinder. Note that this sort of motion
corresponds to answer option (B).
All the other arrows we have in our
diagram corresponding to other answer options indicate the motion of a fluid that
would move opposite the direction of gravity or one that was viscous enough that it
could stick to the different surfaces involved in this system. For our final answer then, we’ll
say that the lubricant flows from the upper parts of the cylinder towards the lower
parts of the cylinder more than in the opposite direction.
