Answers to Concept Checks

• 4.1
  1. Gauge pressure is equal to the pressure exerted by a column of fluid plus the ambient pressure above the fluid, minus atmospheric pressure. When atmospheric pressure is the only pressure above the fluid column, then gauge pressure equals the fluid pressure.
  2. Weight is density times volume and acceleration due to gravity.
  3. The SI unit of pressure is the pascal. Other common units include mmHg, torr, and atm.
  4. True. Density is directionless, and is thus a scalar quantity.
• 4.2
  1. Cohesion is the attractive force experienced by molecules of a fluid for one another. Adhesion is the attractive force experienced by molecules of a fluid for a different material (usually a solid).
  2. If adhesive and cohesive forces are equal, then no meniscus would form and the liquid surface would be flat.
  3. The displaced volume is equal to the volume of the block. The buoyant force is equal to the weight of the block, and is equal to the weight of the displaced fluid. By extension, the block and the fluid in which it is immersed must have the same density.
  4. False. A fluid with a low specific gravity can be used instead of water to determine volumes of objects that would otherwise float in water.
  5. The operator usually applies a force to the side with the smaller cross-sectional area. Because pressure is the same on both sides of the lift, a smaller force can be applied on the smaller surface area to generate the desired pressure.
• 4.3
  1. Dynamic pressure is the pressure associated with flow, and is represented by Static pressure is the pressure associated with position; static pressure is sacrificed for dynamic pressure during flow. A pitot tube is a device that measures static pressure during flow to calculate speed. Viscosity is a measure of the resistance of a liquid to flow. Laminar flow is flow in which there are no eddies and in which streamlines roughly parallel each other. Turbulence is the presence of backflow or current eddies.
  2. The continuity equation describes the relationship of flow and cross-sectional area in a tube, while Bernoulli’s equation describes the relationship between height, pressure, and flow. The Venturi effect is the direct relationship between cross-sectional area and pressure, and results from the combined relationships of the Bernoulli and continuity equations.
  3. Flow rate would increase when increasing either the radius of the tube or the pressure gradient, but would decrease with increasing viscosity or length of the tube.
• 4.4
  1. The continuity equation cannot be applied to human circulation. The presence of pulses, the elasticity of the vessels, and the nature of the pressure gradient preclude this type of analysis. Poiseuille’s law should instead be used for isolated segments.
  2. Total resistance increases as the air exits the body despite the increase in the diameter of the airways. This is because there are fewer airways in parallel with each other.
  3. In theory, there should be equal flow in the venae cavae and the main pulmonary trunk. In reality, the flow in the venae cavae is actually slightly less than in the pulmonary trunk because some of the blood entering the right side of the heart is actually from cardiac (coronary) circulation, not systemic circulation.