Fluids (Physics: Chapter 4)

Question 1

What is pressure and what is the SI units for pressure?

Answer 1

Pressure is the amount of force applied per area. The SI unit for pressure is pascals (Pa).

• 1 Pa = 1 N/m²

Question 2

What is the relationship between weight and density?

Answer 2

The equation for weight is F = mg. Density (rho) is equal to mass/volume. So the relationship between density and weight is F = (rho) x V x gravity

Question 3

What is are the other units of Pressure and what are their converstion units?

Answer 3

The other (non-SI units) of pressure are torrs, atm, and mmHg.

• 1 atm = 760 torrs = 760 mmHg = 1.013 x 10⁵ Pa

Question 4

Is density a scalar or vector quantity?

Answer 4

Density is a scalar quantity because it is directionless.

• Pressure is also a scalar quantity as it is applied uniformly throughout a container.

Question 5

What is absolute pressure (also known as hydrostatic pressure)?

Answer 5

Absolute pressure is the total pressure exerted on an object submerged within a fluid.

• The equation is: P = P₀ + pgz​
  
  • P = absolute (hydrostatic pressure)
  • P₀ = pressure at surface of fluid
  • p = density of fluid
  • g = acceleration due to gravity
  • z = depth of submerged object

Question 6

What is gauge pressure?

Answer 6

Gauge pressure is the pressure differential between atmospheric pressure and the absolute pressure within a container.

• Equation: P_gauge = P - P_atm = (P₀ + pgz) - P_atm
• If P₀ = P_atm then P_gauge = pgz

Question 7

What is Pascal’s Principle?

Answer 7

Pascal’s Principle states that for incompressible fluids, a change in pressure will be transmitted undiminshed to every portion of the fluid and to the walls of the containing vessel.

Question 8

What is an isobaric process and how does it relate to hydraulic systems?

Answer 8

An isobaric process is one in which there is no change in pressure. Hydraulic systems are isobaric systems in order to manipulate forces to lift heavier objects.

Question 9

What is the equation to calculate the area of a circle, since it is necessary to know in order to calculate in hydraulics?

Answer 9

The area of a circle equation is:

A_circle = π x r²

Question 10

What is Archimede’s Principle?

Answer 10

Archimede’s Principle states that the buoyant force is equal to the weight of the fluid displaced by a partially or completely submerged object.

Question 11

What can specific gravity tell us about the percentage of an object that will sink or float?

Answer 11

If an object has a specific gravity greater than 1. Then an object will sink in water. If the object has a specific gravity equal to 1, the object will be completely submerged but will not sink. If the object has a specific gravity less than 1 (ex: .877 specific gravity), then the object will be submerged by that ratio (ex: 87% submerged).

• If the fluid an object is submerged in is not water, substitute the density of the new fluid in for the density of water to determine the relative specific gravity.