The average force of the fluid molecules on a wall per unit area is called what?

The average force of the fluid molecules on a wall per unit area is called what?

• A. Viscosity
• B. Pressure
• C. Temperature
• D. Density

Answer: (B)

Pressure is the average force that fluid molecules exert per unit area on a surface. This comes from molecules colliding with the wall and transferring momentum; when those impulses are averaged over the contact area, you get a pressure value. In a fluid at rest, this pressure acts uniformly in all directions, which is why it’s a scalar quantity describing how much push the fluid conveys to surfaces. Viscosity, by contrast, measures a fluid’s resistance to shear and is tied to velocity gradients, not a normal force per area. Temperature relates to the average kinetic energy of the molecules, not the force the fluid exerts on a boundary. Density is mass per unit volume, not a measure of contact force per area. Hence the term that fits the description is pressure.

Pressure is the average force that fluid molecules exert per unit area on a surface. This comes from molecules colliding with the wall and transferring momentum; when those impulses are averaged over the contact area, you get a pressure value. In a fluid at rest, this pressure acts uniformly in all directions, which is why it’s a scalar quantity describing how much push the fluid conveys to surfaces.

Viscosity, by contrast, measures a fluid’s resistance to shear and is tied to velocity gradients, not a normal force per area. Temperature relates to the average kinetic energy of the molecules, not the force the fluid exerts on a boundary. Density is mass per unit volume, not a measure of contact force per area. Hence the term that fits the description is pressure.