Slide 7 of 12
Notes:
A practical example here is the film of oil that separates two moving metal surfaces in a car engine -- such as the piston surface and the wall of the cylinder it moves in. It’s important for engine performance and endurance to keep that layer of oil at just the right viscosity.
Viscosity usually decreases as the temperature increases. Did you ever put cold maple syrup in the microwave, or run hot water over the bottle before you used it? Remember how “thick” it seemed when cold and how “watery” when hot?
The so-called “weight” of motor oil is really a specification of its
viscosity and how that viscosity varies with temperature. Oil with
SAE 10 rating has, at a given temperature, lower viscosity than SAE 40
oil. Oil rated as 10W40 is supposed to have the low viscosity of SAE 10
oil at low engine temperature and the high viscosity of SAE 40 at high
engine temperature.
