4. Fluids Flashcards
True or false: Fluids can exert sheer forces, but not perpendicular forces.
False.
It is the opposite. For example, you can fall and hit water and it will hurt (perpendicular).
(Equation) What is the equation for Density?
ρ=density
m= mass
V= volume
(Equation) Weight of a fluid.
Fg=ρVg
or Fg=gm
Note= the difference in these equations is that in the first one, you don’t know the mass but you know density (taken from the density equation of p=m/v), and in the second one you do know the equation.
Fg= weight
ρ = density
V=volume
g= acceleration due to gravity (9.8 or 10m/s2)
(Equation) What is the equation for specific gravity?
ρ=density of substance over the specific gravity of water
** The value of SG is going to be unitless. It just tells you if it will float or not. If it is greater than 1, it will sink in water. If it is less than 1, it is less dense and it will float.
How do you define pressure and what type of quantity is it (scalar or vector)?
Pressure is defined as a measure of force per unit area; it is exerted by a fluid on the walls of its container and on objects placed on the fluid
It is a scalar quantity
(Equation) What is the equation for pressure?
P=pressure
- Units for pressure= pascal (Pa) which equals
- 1Pa= 1 N/m2=1atm= 760 torr= 760 mmHg= 1.1013x105Pa =110,130 Pa
- F=force
- A= area
What is the definition of the absolute or hydrostatic pressure?
Absolute (Hydrostatic) pressure is the sum of all pressures at a certain point within a fluid (remember, includes gas and liquids); it is equal to the pressure at the surface of the fluid (usually atmospheric pressure) plus the pressure due to the fluid itself
P=Po + ρgz
- P=absolute pressure
- Po= incident or ambient pressure (does not always mean ambient pressure, however, in general, 1 atm)
- ρ= density of the fluid
- g= acceleration due to gravity
- z= depth of the object
(Equation) What is the equation for hydrostatic or absolute pressure?
P=Po + ρgz
- P=absolute pressure
- Po= incident or ambient pressure (does not always mean ambient pressure, however, in general, 1 atm)
- ρ= density of the fluid
- g= acceleration due to gravity
- z= depth of the object
(Equation) What is the equation for gauge pressure?
It’s important to note that because the Incident or ambient pressure generally equals the atmospheric pressure, the equation just results to Pgauge= ρgz
The longer version of this is:
Pgauge=P-Patm= (Po +ρgz)-Patm
- Pgauge= pressure of the gage
- P= total pressure
- Patm= pressure outside the closed system
- Po= incident or ambient pressure (does not always mean ambient pressure, however, in general, 1 atm)
- ρ= density of the fluid
- g= acceleration due to gravity
- z= depth of the object
What is the definition of the gauge pressure?
Gauge pressure is the name for the difference between absolute pressure and atmospheric pressure.
In liquids, gauge pressure is caused by the weight of the liquid above the point of measurement.
Another way to think of this is that gauge pressure is the amount of pressure in a closed space above and beyond the atmospheric pressure.
In general this is:
Pgauge= ρgz
The longer version is:
Pgauge=P-Patm= (Po +ρgz)-Patm
- Pgauge= pressure of the gage
- P= total pressure
- Patm= pressure outside the closed system
- Po= incident or ambient pressure (does not always mean ambient pressure, however, in general, 1 atm)
- ρ= density of the fluid
- g= acceleration due to gravity
- z= depth of the object
How does gauge pressure relate to the pressure exerted by a column of fluid?
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.
What is the relationship between weight and density?
Fg (weight)= ρVg
Units for ρ= mass/volume. -> essentially you are multiplying density (m/v) * volume to get the mass, which then you can calculate the weight, just like solids.
So if you multiple this by volume, it gives you the mass.
This is no different than m*g=F. Same equations, over and over, just a little different.
What is the SI unit for pressure? What are other common units of pressure?
SI Unit: Pascal= Pa
Other Units: 1 atm= 760 torr= 760 mmhg= 110,000 Pa
True or False: Density is a scalar quantity
True: Density is directionless.
In hydrostatics, what is Pascal’s Principle?
Pascal’s principle states that a pressure applied to an incompressible fluid will be distributed undiminished through the entire volume of the fluid
Real World: When the air pressure changes above a large body of water, the water level rises or falls to re-establish pressure equilibrium between the air and the water. The surface of a water body directly below a high-pressure air pocket forms a very small but measurably valley of water. A low pressure air system has the opposite effect, creating a hill of water.
(Equation) What is application of Pascal’s law to hydraulic machine and mechanical advantage?
P= pressure
F1=downward force on piston 1
A1= Surface area of piston 1
F2= the upward force on piston 2
A2= the surface area of piston 2
Similar the the equation F=W=P∆V in that P=W/∆V
What is Archimedes’ principle in regards to buoyant force?
Archimedes’ principle governs the buoyant force.
When an object is placed in a fluid, the fluid generates a buoyant force against the object that is equal to the weight of the fluid displaced by the object.
In other words, a body wholly or partially immersed in a fluid will be buoyed upwards by a force equal to the weight of the fluid that it displaces. This is another definition which they say is the same but doesn’t make sense to me (why it is the same): any object placed in a fluid will cause a volume of fluid to be displaced equal to the volume of the object that is submerged. The force is always directed upwards
What is the direction of the buyant force?
It is always against gravity, so pointing upwards.
To which side of a hydraulic lift would the operator usually apply a force- the side with the larger cross-sectional area, or the side with the smaller cross sectional area? Why?
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.
(Equation) What is the equation to calculate the rate of flow through a pipe or confined space?
- Q= flow rate (volume flowing per time)
- r= radius of the tube
- ∆P= the pressure gradient
- η= viscosity of the fluid
- L=length of the pipe
Key Point: The MCAT really will not test on this equation. However, it does want you to know that the flow will increase if the radius increases (by an exponent of 4). So it’s important to know that a slight change in the radius may have a significant effect on the pressure gradient.
True or False: Flow rate can change depending on the cross-sectional area of the tube.
False: Flow rate is always constant.
Key Concept: While flow rate is constant in a tube regardless of the cross-sectional area, linear speed of a fluid will increase with decreasing cross-sectional area.
The flow stays the same. However, fluids have higher speeds through narrower tubes.
Another way to think about it: Flow will always be the same through the pipe. It may be faster, it may be slower, if it is constricted, but it will always be the same. For example, if you have a restricted area of the pipe, the area that the fluid can flow through will be reduced, however to compensate, the speed will increase.
In regards to the conservation of energy for a flowing fluid, what does Bernoulli’s equation tell us?
Bernoulli’s equation is an expression of conservation of energy for a flowing fluid. This equation states that the sum of static pressure and the dynamic pressure will be constant between any two points in a closed system.
Key Point: in the end, Bernoulli’s equation is nothing other than a statement of energy conservation: more energy dedicated toward fluid movement means less energy dedicated toward static fluid pressure. Also, the inverse is true: more static pressure means less movement.
It tells us that when the liquid speeds up, its pressure goes down.
But the flow is always the same
(Equation) What is Bernoulli’s equation for static pressure and dynamic pressure?
- P= absolute pressure of the fluid
- ρ= density of the fluid
- v= linear speed
- g= the acceleration due to gravity
- h= the height of the fluid above some datum
Pressure + KE + Potential Energy= Pressure + KE + Potential Energy
Key Point: in the end, Bernoulli’s equation is nothing other than a statement of energy conservation: more energy dedicated toward fluid movement means less energy dedicated toward static fluid pressure. Also, the inverse is true: more static pressure means less movement.
If they are the same height you can ignore the ρgh because they will be the same on both sides
It tells us that when the liquid speeds up, its pressure goes down.
In tubes, what is the Venturi effect?
Essentially, if the fluid has a higher velocity, it has lower pressure.
What is a pitot tube?
is a device that measures static pressure during flow to calculate speed
