Chapter 6 - Fluids and Solids

Question 1

Define the equation for density

Answer 1

Density = mass / volume

*density of a liquid is constant, they are incompressible

Question 2

Define the equation for specific density

Answer 2

Specific density = density of substance / density of water

*density of water often 1 so specific density = density of substance

Question 3

Define the equation for pressure and units for Pa and ATM

Answer 3

Pressure = force / area
Pressure = density x gravity x depth

Pascals 1 Pa = 1 N/m^2
1 atm = 1.01 x 10^5 Pa

Question 4

What is buoyance and its equation

Answer 4

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.

Question 5

What is relative density (specific gravity)?

Answer 5

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

Question 6

Define gauge pressure

Answer 6

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

Question 7

Define absolute pressure and its equation

Answer 7

Absolute pressure is pressure of a liquid relative to pressure in a vacuum.

Asbolute pressure = atmospheric pressure + gauge pressure

Question 8

Define viscosity, its equation and its relation to momentum

Answer 8

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

Question 9

Define the two equations for pressure

Answer 9

pressure = force / area or P = work / volume
pressure = diameter x gravity x density

• when velocity increases, pressure decreases

Question 10

Define the equation for Reynolds number and what it measures

Answer 10

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

Question 11

Define the two equations for surface tension and its relation to potential energy

Answer 11

Potential energy = surface tension x area

Surface tension = force of contraction of surface / length along surface

Question 12

The radius of a vessel decreases by 20%, the flow rate is the same, how much does the velocity change?

Answer 12

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%

Question 13

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

Answer 13

A1V1 = A2V2

0. 5 x 5 = 2 x ?
1. 5 = 2 x 1.25 or 2/5

Question 14

Density of water is 0.98 and mercury is 13.6. What percentage of the height of ice would be above the surface?

Answer 14

0.98 / 13.6 = 1/14 = 7%
but to get the percentage ABOVE the water
100 - 7 = 93%