BasicsBasics• We need to review fundamental We need to review fundamental

information about physical properties information about physical properties and their units. These will lead us to and their units. These will lead us to two important methods: Conservation of two important methods: Conservation of Mass, and Conservation of Energy.Mass, and Conservation of Energy.

Scalars and VectorsScalars and Vectors• A scalar is a quantity with a size, for

example mass or length

• A vector has a size (magnitude) and a direction.

http://www.engineeringtoolbox.com/average-velocity-d_1392.html

VelocityVelocity• Velocity is the rate and direction of

change in position of an object.

• For example, at the beginning of the Winter Break, our car had an average speed of 61.39 miles per hour, and a direction, South. The combination of these two properties, speed and direction, forms the vector quantity Velocity

http://www.engineeringtoolbox.com/average-velocity-d_1392.html

Vector ComponentsVector Components• Vectors can be broken down into components

• For example in two dimensions, we can define two mutually perpendicular axes in convenient directions, and then calculate the magnitude in each direction

• Vectors can be added

• The brown vector plus

the blue vector equals

the green vector

Vectors 2: Acceleration.Vectors 2: Acceleration.• Acceleration is the change in Velocity during

some small time interval. Notice that either speed or direction, or both, may change.

• For example, falling objects are accelerated by gravitational attraction, g. In English units, the speed of falling objects increases by about

g = 32.2 feet/second every second, written g = 32.2 ft/sec2

SI Units: SI Units: Kilogram, meter, secondKilogram, meter, second

• Most scientists and engineers try to avoid English units, preferring instead SI units. For example, in SI units, the speed of falling objects increases by about 9.81 meters/second every second, written

g = 9.81 m/sec2

• Unfortunately, some data will be in English units. We must learn to use both.

http://en.wikipedia.org/wiki/International_System_of_Units

Système international d'unités pron system’ internah’tionana doo’neetay

Data and Conversion FactorsData and Conversion Factors

• In your work you will be scrounging for data from many sources. It won’t always be in the units you want. We convert from one unit to another by using conversion factors.

• Conversion Factors involve multiplication by one, nothing changes

• 1 foot = 12 inches so 1 foot = 1 12 “

ExampleExample

• Lava is flowing at a velocity of 30 meters per minute down Kilauea. What is this speed in feet per minute?

• Steps: (1) write down the value you have, then (2) select a conversion factor and write it as a fraction so the unit you want to get rid of is on the opposite side, and cancel. Then calculate.

• (1) (2)• 30 meters x 3.281 feet = 98.61 feet

minute meter minute

Chaining Conversion FactorsChaining Conversion Factors• Lava is flowing at a velocity of 30 meters per minute

from a vent atop Kilauea. What is this speed in feet per second?

• 30 meters x 3.281 feet x 1 minute = 1.64 feet

minute meter 60 seconds sec

Momentum (plural: momenta)Momentum (plural: momenta)• Momentum (p) is the product of velocity and mass, p = mv• In a collision between two particles, for example, if there is

no frictional loss the total momentum is conserved.

• Ex: two particles collide and m1 = m2, one with initial speed v1 ,

the other at rest v2 = 0,

• m1v1 + m2v2 = constant

ForceForce• Force is the change in momentum with respect to time.• A normal speeds, Force is the product of Mass (kilograms)

and Acceleration (meters/sec2), • So Force must have SI units of kg . m

sec2

• 1 kg . m is called a Newton (N)

sec2

StaticsStatics

• If all forces and Torques are balanced, an object doesn’t move, and is said to be static

• Discussion Torques, See-sawF=2

F=1

-1 0 +2

F=3

The forces are balanced in the y direction. 2 + 1 force units (say, pounds) down are balanced by three pounds directed up.The torques are also balanced around the pivot: 1 pounds is 2 feet to the right of the pivot (= 2 foot-pounds) and 2 pounds one foot to the left = -2 foot - pounds

PressurePressure

• Pressure is Force per unit Area

• So Pressure must have units of kg . m

sec2 m2

• 1 kg . m is called a Pascal (Pa)

sec2 m2

DensityDensity• Density is the mass contained in a unit volume

• Thus density must have SI units kg/m3

• The symbol for density is pronounced “rho”

• Very important is not a p, it is an r• It is NOT the same as pressure

A Conversion Factors TrickA Conversion Factors Trick

Suppose you need the density of water in kg/m3. You may recall that 1 cubic centimeter (cm3) of water has a mass of 1 gram.

1 gram water x (100 cm)3 x 1 kilogram = 1000 kg / m3

(1 centimeter)3 (1 meter)3 1000 grams

water = 1000 kg / m3

Don’t forget to cube the 100cmDon’t forget to cube the 100cm

Conservation of Mass – No Storage

Mass flow rateConservation of Mass : In a confined system “running full” and

filled with an incompressible fluid, the same amount of mass that enters the system must also exit the system at the same time.

1A1Vel1(mass inflow rate) = 2A2Vel2( mass outflow rate)

What goes in, must come out.

Notice all of the conditions/assumptions confined (pipe), running full, incompressible fluid (no compressible volatiles), same elevation (no Pressure differences).

Volcanic pipe full of magma

Mass Flow Rate for a vertical nozzle

Lava is incompressible, so the

density does not change and 1=

2. The density cancels out,

1A1V1 = 2A2V2

so A1V1 =A2V2

Notice If A2 < A1 then V2 > V1

Here A2 < A1 .Thus lava exiting a smaller opening has a higher

velocity than at inflow

1A1V1(mass inflow rate) = 2A2V2( mass outflow rate)

Consider lava flowing out an opening where the vent cross-sectional area is less than the magma chamber.

A2

A1

V2

Just before the exit, assume (for now) P2 = P1, 2 = 1

exit

V1

Flow RateFlow Rate

• For mass conservation with constant density, the flow rate is defined as

• Q = Velocity x Area

• Units are meter/sec x meters 2

• Thus Q is Volume/time units m3/sec

EnergyEnergy• Energy is the ability to do work, and work and

energy have the same units• Work is the product of Force times distance, • W = Fd Distance has SI units of meters

• 1 kg . m2 is called a N.m or Joule (J)

sec2

• Energy in an isolated system is conserved • KE + PE + Pv + Heat = constant

N.m is pronounced Newton meter, Joule sounds like Jewel. KE is Kinetic Energy, PE is Potential Energy, Pv is Pressure Energy, v is unit volumeAn isolated system, as contrasted with an open system, is a physical system that does not interact with its surroundings.

Pressure Energy is Pressure Energy is Pressure x volumePressure x volume

• Energy has

units kg . m2

sec2

So pressure energy must have the same units, and Pressure alone is kg . m

sec2 m2

So if we multiply Pressure by a unit volume m3 we get units of energy

m3

Kinetic EnergyKinetic Energy

• Kinetic Energy (KE) is the energy of motion

• KE = 1/2 mass . Velocity 2 = 1/2 mV2

• SI units for KE are 1/2 . kg . m . m• sec2Note the use of m both for meters and for mass. The context will tell you which.

That’s the reason we study units.Note that the first two units make a Newton (force) and the remaining unit is meters, so the units of KE are indeed Energy

Potential EnergyPotential Energy

• Potential energy (PE) is the energy possible if an object is released within an acceleration field, for example above a solid surface in a gravitational field.

• The PE of an object at height h is

PE = mgh Units are kg . m . m

sec2

Note that the first two units make a Newton (force) and the remaining unit is meters, so the units of PE are indeed EnergyNote also, these are the same units as for KE

KE and PE exchangeKE and PE exchange• An object falling under gravity loses

Potential Energy and gains Kinetic Energy.• A pendulum in a vacuum has potential

energy PE = mgh at the highest points, and no kinetic energy because it stops

• A pendulum in a vacuum has kinetic energy KE = 1/2 mass.V2 at the lowest point h = 0, and no potential energy.

• The two energy extremes are equalStops v=0 at high point, fastest but h = 0 at low point.Without friction, the kinetic energy at the lowest spot (1) equals the potential energy at the highest spot, and the pendulum will run forever.

Conservation of EnergyConservation of Energy• We said earlier “Energy is Conserved” in a closed

system. • This means

KE + PE + Pv + Heat = constant • For simple systems involving fluids without friction

heat losses, at two places 1 and 2

1/2 mV12 + mgh1 + P1v = 1/2 mV2

2 + mgh2 + P2v

Usually we try to eliminate some of the terms.

If both places are at the same pressure (say both touch the atmosphere) the pressure terms are identical

• A basaltic fountain on Kilauea volcano reaches a height of 53 Meters. What was the exit velocity at the vent? P1 =P2 = Patm =0

Patm =0 is a standard called gauge pressure

At the vent (1) the height is zero, so there is only kinetic energy, KE = 1/2mV2

At the top, h = 53 meters, the particles stop briefly before falling back to earth. There is only potential energy, Pot.E. = mgh, at (2).The masses are the same, so they cancel.

Ex. 2 - Conservation of Energy ProblemEx. 2 - Conservation of Energy Problem• A tall volcano fills its neck with magma to an elevation of

h1 = 1000 m above a weak area. The pressure forms a rupture in the weak area. Define the height there as h2 = 0 meters.

• How fast is the lateral blast?

• The rupture has area A2 = 10000 m2 , small compared to the magma chamber surface with area A1 = 3 x 107 m2. Therefore assume V1 ~ 0

Step 1.Step 1.Calculate Pressure at depthCalculate Pressure at depth

• Pressure can be calculated

as P = gh

• For an Andesitic magma

with density = 2450 kg/m3

• If the surface of the magma chamber is at atmospheric pressure, what is the pressure 1000 meters below the surface?

See the handout

Step 2Step 2Calculate the velocityCalculate the velocity

• Calculate V3 using conservation of energy with the calculated pressure of magma P2 entering the pipe and exiting to atmospheric Pressure P3 via the rupture.

V2 ~ 0, P2 P3 ~0 = P atm

V3 = ?

Specific EnergySpecific Energy

• It will be convenient to separate out the mass. Since the “mass in” is the same as the “mass out”, we will be able to cancel them when necessary. The conservation of energy terms are:

HandoutHandout

• We will walk through the steps in a handout, an then do similar problems in class and for homework.