for Review Test #2 Oct 20
Topics: Gravity - under, on, above the Earth, orbits, escape velocity Phases of matter and fluid mechanics, buoyancy Temperature, Heat and Thermodynamics - 3 laws Heat transfer and phase changes The Earth and global warming Special Relativity Methods Conceptual Review and Practice Problems Chapters 6 - 9 Review lectures (on-line) and know answers to clicker questions and homeworks. Go over practice test. Attend SI sessions. Bring: Banner ID and Two Number 2 pencils Simple calculator (no electronic notes) Reminder: There are NO make-up tests for this class
Test #2 Review
How to take a multiple choice test 1) Before the Test: Study hard (~2 hours/day Now through Sunday) Get plenty of rest the night before 2) During the Test: Draw simple sketches to help visualize problems Solve numerical problems in the margin Come up with your answer first, then look for it in the choices If you can't find the answer, try process of elimination If you don't know the answer, Go on to the next problem and come back to this one later TAKE YOUR TIME, don't hurry If you don't understand something, ask me. This is not meant to be a vocabulary test.
Gravitational Force
The gravitational force is always attractive The strength of the attraction decreases rapidly (as the square of) increasing distance
Gravity and Orbits
Gravity decreases as you climb out of the "potential well"
How much has the girl's weight dropped after she climbs a distance equal to the distance from the surface to the center of the Earth?
Review
Review
projectile orbits terms: apogee, perigee geosynchronous orbit
Orbital velocity
vorb = GM R
Escape Velocity
2GM R
vesc =
What shape is the orbit for a geosynchronous satellite?
1
Escape Velocity
Scaling
An ant can lift several times its own body weight
Speed at which an object can permanently escape from a body determined by strength of the gravitational force Earth: 11 km/s Moon: 2.4 km/s Sun: 42.5 km/s Explains why moon has no atmosphere!
Why can't an elephant do the same thing?
Objects grow heavier at a faster rate than they grow stronger
Strength related to 2-d crosssectional area Weight is related to volume
What happens to the area to volume ratio of an object as its size increases?
Archimedes' Principle
Temperature and Heat
An immersed object is buoyed up by a force equal to the weight of the fluid it displaces
What is temperature?
A measure of how warm or cold an object is with respect to some standard Related to the random thermal motion of the molecules in a substance; kT = KE = 1/2 mv2 The energy transferred between objects due to a temperature difference
What property of an object determines how much fluid a completely submerged object will displace?
What is heat?
The volume of the object
Regardless of shape, mass
For partially submerged object, it is the volume of the submerged part
Energy in transit (similar to work)
Boat
DEMO : Archimedes Principle
Thermal Expansion
Density of Water
Why do objects tend to expand when heated and contract when cooled?
As temperature increases, molecules jiggle faster and move farther apart
Important engineering consideration
Ex. Expansion joints in bridges
Golden gate bridge contracts more than a meter in cold weather
The unequal expansion of a bimetallic strip can operate a thermostat.
2
Three Laws of Thermodynamics
Clicker Question:
Which of the following must always increase for any physical process (like a chemical reaction)?
A: energy B: temperature C: entropy D: heat
1st law 2nd law 3rd law
Energy can neither be created nor destroyed.
The entropy of an isolated system always increases.
Nothing can be cooled to absolute 0 K.
What would the be entropy of a crystal at 0 K?
Clicker Question:
Suppose you buy some ice-cream at the store, and you don't want it to melt on the long bike-ride home. What can you do?
A: Leave it exposed to the air so the breeze keeps it cool. B: Wrap it up in your down jacket. C: Shield it from the sun, but don't otherwise cover it up. D: Nothing, how fast it melts only depends on distance.
Clicker Question:
Hydrogen and oxygen molecules in a gas sample have the same temperature. This means the hydrogen molecules, on the average, have the same ... A: kinetic energy, but more speed. B: speed, but less kinetic energy. C: speed, but more kinetic energy. D: kinetic energy and the same speed.
Phases of Matter
Changes of Phase
List the four phases of matter in order of increasing internal energy
solid => liquid => gas => plasma
What are the different phases of matter and some of the properties of each?
Solid: Fixed shape Liquid: Takes shape of container, incompressible Gas: Takes shape of container, easily compressible
List the processes that take you from a solid to a plasma
melting => evaporation => ionization
Gas and liquids both flow => both are "fluids"
Plasma: A gas that is electrified or "ionized"
List the processes that take you from a plasma to a solid
electron capture => condensation => freezing
3
Temperature and Phase Changes
Temperature is constant during a change of phase
The Temperature of the Earth
Absorbed heat goes into breaking molecular bonds => increased potential energy. Translational K.E. remains unchanged. (Left to Right) Extracted heat allows molecular bonds to form (decreased potential energy) at fixed temp. (Right to Left)
Neglecting other effects, at equilibrium the Earth would reradiate all of the energy received from the Sun and have an average temperature of -23o C.
A material absorbs energy when melting or vaporizing and expels energy when condensing or freezing. During a phase change, the energy goes into the change of state rather than into changing the object's temperature.
Why is the average temperature of Earth so much (about 40o C) higher?
The Greenhouse Effect
Postulates of Special Relativity
1st Postulate
Main greenhouse gases are...