Question Video: Lubricating a Cylinder with Vertical Reciprocal Motion Physics

A piston undergoes vertical 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 mainly stick only to the upper face of the cylinder. [B] The lubricant will mainly stick only to the lower face of the cylinder. [C] The lubricant will flow toward the center of the piston more than in the opposite direction. [D] The lubricant will flow from the open end of the cylinder toward the closed end of the cylinder more than in the opposite direction. [E] The lubricant will flow from the closed end of the cylinder toward the open end of the cylinder more than in the opposite direction.

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Video Transcript

A piston undergoes vertical 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 mainly stick only to the upper face of the cylinder. (B) The lubricant will mainly stick only to the lower face of the cylinder. (C) The lubricant will flow toward the center of the piston more than in the opposite direction. (D) The lubricant will flow from the open end of the cylinder toward the closed end of the cylinder more than in the opposite direction. (E) The lubricant will flow from the closed end of the cylinder toward the open end of the cylinder more than in the opposite direction.

Taking a look at our diagram, we see here this cylinder with a piston that moves up and down inside it. On its perimeter, the piston is in contact with the walls of the cylinder, and we’re told that as the piston moves, friction is produced. To minimize this friction, a lubricant, for example, motor oil, is put into the cylinder. We aren’t told specifically what the lubricant is, but we are told that it has a low viscosity. An example of a fluid with a fairly low viscosity is water. Water has a very low resistance to flow, and so does whatever lubricant we’re using. We want to choose which of our five answer options describes why it is that this low-viscosity lubricant doesn’t lubricate all the parts of this system effectively.

Starting from the top of our list, notice that answer options (A) and (B) talk about the lubricant sticking to a surface. Since we’re told though that the lubricant being used is low viscosity, we know it will tend not to stick to any surface. That would only be possible for a high-viscosity lubricant. This means we can cross off answer options (A) and (B). Option (C) says that the lubricant will tend to flow more toward the center of the piston than in the other direction. As we look at the cylinder and the piston though, we see that there’s no reason why they should be so. There’s nothing about the shape of this system that would make the lubricant tend toward the center of the piston.

We can also cross off answer choice (C). Option (D) says the lubricant will flow from the open end of the cylinder, that’s this end up here, toward the closed end, that’s this end at the bottom, and that its overall motion in this direction, top to bottom, will be greater than the motion of the lubricant from bottom to top. To get an idea of whether this explanation makes sense or not, imagine pouring a cup of water into the open end of the cylinder. Due to gravity, we know that that water will tend to move towards the closed end, the bottom of the cylinder.

And then, the fact that water is a low-viscosity fluid means that as the piston moves down and up, it’s unlikely to bring much of the water up. Therefore, we would expect that the overall motion of this water, which we can think of as a lubricant, is more from the open end towards the closed end of the cylinder. Answer option (D) looks like it might be our answer. But just to confirm that it is, let’s look at option (E).

This option says the lubricant will tend to flow from the closed end of the cylinder, that’s this end at the bottom, toward the open end, the end at the top, more than in the other direction. Answer option (E) is claiming that the lubricant will tend to move up against the force of gravity more than it moves down. As we think though about pouring a low-viscosity fluid into the cylinder, such as water, we see that that won’t happen. Rather, the overall motion of a low-viscosity lubricant will be in the direction indicated in answer option (D).

So we do choose option (D). The reason that not all of the parts in this system are sufficiently lubricated is that the lubricant will flow from the open end of the cylinder toward the closed end of the cylinder more than in the opposite direction.

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