# Question Video: Lubricating a Cylinder with Vertical Reciprocal Motion Physics

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. [E] The lubricant will flow toward the center of the piston more than in the opposite direction.

03:25

### 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.