Chapter 6 - Fluids and Solids Flashcards
Define the equation for density
Density = mass / volume
*density of a liquid is constant, they are incompressible
Define the equation for specific density
Specific density = density of substance / density of water
*density of water often 1 so specific density = density of substance
Define the equation for pressure and units for Pa and ATM
Pressure = force / area Pressure = density x gravity x depth
Pascals 1 Pa = 1 N/m^2
1 atm = 1.01 x 10^5 Pa
What is buoyance and its equation
Fb = volume x density of fluid displaced x gravity
= mass x gravity
Buoyance is the upward force equal to the weight of fluid displaced that an object experiences in water.
An object floating must at least be displacing its own weight.
What is relative density (specific gravity)?
relative density (or specific gravity) = height below surface of liquid
eg. if specific gravity =0.9 then 90% of that object is immersed in water.
* less dense objects float
Define gauge pressure
GP = Patm +
difference in pressure of in an object compared to outside the object. Eg. gauge pressure of tyres refers to difference in pressure between tyres and in atmospheric pressure
Define absolute pressure and its equation
Absolute pressure is pressure of a liquid relative to pressure in a vacuum.
Asbolute pressure = atmospheric pressure + gauge pressure
Define viscosity, its equation and its relation to momentum
Viscosity is the resistance to flow of layers of liquid. The greater the transfer of momentum, the more energy lost
Viscosity is a measure of efficiency of momentum transfer
↑ viscosity = ↑ transfer of momentum = ↓ mechanical energy
V = (force/area) / velocity/distance
Define the two equations for pressure
pressure = force / area or P = work / volume pressure = diameter x gravity x density
- when velocity increases, pressure decreases
Define the equation for Reynolds number and what it measures
R = velocity x diameter x density / viscosity coefficient
Measures if a fluid flow is streamline or turbulent
R<2000 is streamline
R>2000 is turbulent
Increase in R is increase in turbulance
decreasing temp causes increase in viscosity and a decrease in R
Define the two equations for surface tension and its relation to potential energy
Potential energy = surface tension x area
Surface tension = force of contraction of surface / length along surface
The radius of a vessel decreases by 20%, the flow rate is the same, how much does the velocity change?
A1 V1 = A2 V2 = constant If the diameter decreases, velocity (pressure) will increase Area of a vessel is pi x radius^2 assume A + V = 1 therefore a decrease means radius2 = 0.8 0.8^2 = 0.64 area or A2 A1 V1 = A2 V2 1 x 1 = 0.64 x V2 1 x 1/0.65 = 1.5 therefore velocity will increase by 50%
Fluid flows into a pipe with a diamter 0.5m at a rate of 5m/s. What is the rate at the other end with a diamter of 2m
A1V1 = A2V2
- 5 x 5 = 2 x ?
- 5 = 2 x 1.25 or 2/5
Density of water is 0.98 and mercury is 13.6. What percentage of the height of ice would be above the surface?
0.98 / 13.6 = 1/14 = 7%
but to get the percentage ABOVE the water
100 - 7 = 93%