Point 17 of 18 — Chapter 4: HydrodynamicsQuestion bank: 1 questions (0 with pictures)
17 4.5 Applications of viscosity
Lubrication. Machine parts must be oiled from time to time. Lubrication
a) reduces the heat produced by friction, and
b) protects the parts from wear.
The lubricant must have high viscosity: it sticks to the moving parts and does not run off quickly. Water (low viscosity) would flow away from the parts at once.
Moving vehicles. When a car reaches its maximum speed, most of the engine's work is used to overcome air resistance (due to the viscosity of air) and tyre friction. At low or moderate speeds air resistance is proportional to the speed; above a certain speed it becomes proportional to the square of the speed, so fuel consumption rises rapidly. A skilled driver therefore limits the speed to save fuel.
Medicine — the blood sedimentation rate. A ball falling in a liquid is acted on by three forces: its weight (down), the buoyant force (up) and the viscous drag (up). The ball reaches a constant final (terminal) velocity that increases with the radius of the ball. In the same way, red blood cells in a blood sample settle at a rate that depends on their size:
In rheumatic fever the red cells stick together to form larger clumps (larger radius), so the sedimentation rate is higher than normal.
In anaemia the red cells break down and become smaller, so the rate is lower than normal.
📝 Question bank for this point
1 questions linked to this point only, 0 of them with pictures.
