Physics 104 - How Things Work.
Greg Sullivan
Fall, 2000
Introduction of who I am.
 Professor here
 Research in Particle Astrophysics
o Elementary particles & fields
Purpose of course:
 How things work
o Scientific bases of how things work
o From outside to inside and the underlying scientific
principles
Grading:
 Homework
15%
 2 - 1 Hr exams 20% each
 Term paper
20%
 Final exam
25%
o Comprehensive
Homework:
 Practice
o OK to help each other but….
Term papers:
 Look at the web page
Exams:
 Closed book ~75 minutes
 Stress concepts
o Some short answer
o Some numerical
1
Don’t want you to remember a bunch of formulas (some). I want you
to know how things work not remember ho they work.
Ask Questions
 Cliché – The only stupid question is the one you don’t ask.
TA:
Yung-Fu Chen
yfuchen@physics.umd.edu
physics 1322
Book:
 How Things Work – The Physics of Everyday Life
o Louis A. Bloomfield
Hand out the syllabus, policy, & schedule.
Physics.
What is Physics??
 Try to break down the complexity of the world into simple rules.
 A few basic laws
o Periodic table
o Example of the soccer game
 Watch the game and try to figure out the rules from
what happens on the field
DEMO – C3-02: Table Cloth trick
 Why do the dishes stay there
when I pull out the table
cloth?
o What are the rules?
o Do they apply
everywhere?
2
Issue:
 Language
o Often physics terms have a meaning different then in
normal usage. (e.g. conservation of energy)
So, in order to set a foundation for the entire course, and for your
basic understanding, we will start with
The Laws of Motion
Begin Section 1.1
DEMO – Throw tennis balls around room
Falling Balls
 What are the rules?
 Are there rules?
Questions:
 Does tennis ball & baseball follow the same rules?
 Under what conditions do they have the same path?
 Horizontal motion vs. falling motion?
 How does weight affect the motion?
History:
 Aristotle
350BC
o VF
 Heavier fall faster
 Galileo
1600
o All fall equal (tower of Pisa)
3
DEMO
- C4-33: FREE FALL IN VACUUM - FEATHER
AND
BALL
- C4-34: GALILEO'S EXPERIMENT - MASSES IN FREE FALL
The motion of an isolated ball
 No gravity
 Correct answer eluded people for thousands of years
 Galileo’s law of inertia
Inertia: A Body in motion tends to remain in motion; a body at rest
tends to remain at rest.
Falling ball more complicated
 Accurate description need several physical quantities
o Position
o Speed
o Velocity
o Mass
o Acceleration
o Force
Location of ball:
 Position
o Distance & direction from reference point
 3 spatial coords wrt a reference
4
DEMO - A2-01: CARTESIAN COORDINATE AXES
o e.g. 30 miles north of Campus
o 10 miles east of my house
 could both be same location!
Position is an example of a vector quantity.
 Both magnitude and direction
If a ball is moving, then its position is changing.
 Velocity
o How quickly the position is changing
o Speed the ball is moving & direction it is heading.
speed 
distance
time
o Velocity is vector quantity with speed & direction
 50mph due north
Newton - ~1660
Armed with this terminology:
Inertia stated as
Newton’s 1st Law:
An object that is not subject to any outside force moves at a
constant velocity, covering equal distances in equal times (speed)
along a straight line path(direction).
5
DEMO - C4-04: F=MA WITH ULI AND FORCE PROBE
use demo to show graph of constant velocity motion
Why is dV/dt = 0?
 Because of mass
o Inertial Mass
 What takes more force?
o To stop a heavy or a light object?
DEMO - C3-04: INERTIA - LEAD BRICK AND HAND
C3-12: PENCIL AND PLYWOOD
6
OK Back to falling balls again
DEMO - C2-06: BALL DROP ON ROPE - EQUAL AND UNEQUAL
INTERVALS
What happens?
 Are equal distances covered in equal times?
 What does the 1st Law tell us?
o There must be an out side force!
 Gravity pulling down!
DEMO - C2-07: FREE FALL - EQUAL TIME INTERVALS
When something pushes on the ball, its velocity changes
 It accelerates
7
Acceleration:
 Change in velocity per unit time
acceleration 
change in velocity
time
 Vector quantity like position & velocity
o Magnitude & direction
 Direction along the direction of push (force)
DEMO - C4-04: F=MA WITH ULI AND FORCE PROBE
show position vs velocity vs acceleration graphs
What objects are accelerating?
1. car going straight down the road
2. constant speed on the beltway
3. object in orbit (satellite)
4. putting breaks on at 55mph
5. falling ball
6. thrown baseball
If we push an object its velocity changes, that is it accelerates
Direction of the change in velocity is along the pusj.
8
A Simple rule that relates the push (force), the acceleration, and the
inertia (mass).
Newton’s 2nd Law
The force exerted on an object is equal to the product of that
object’s mass times its acceleration. The acceleration is in the same
direction as the force.


F ma
or by rearranging the formula:


F
a 
m
acceleration is proportional to Force divided by the inertia(mass) and
is along the direction of the force.
DEMO - C4-02: AIR TRACK - A = F/M (with ULI)
NOTE: at start of 2nd lecture use computer with internet
connection displayed on projector to access the phys 104 class
website http://umdgrb.umd.edu/sullivan/physics104.html and to
join hypernews.
9
Units:
SI Units (metric)
 length (distance) meters , m
 time seconds, s
 mass kilograms. Kg
So, for example
 velocity = m/s
 acceleration = m/s/s or m/s2
 Force – kg m/s2 = 1 Newton ,N
o 1 N ~ force created by 10 US quarters in your hand
Weight & Gravity:
Why does an object weight something?
 Gravity
 Pull of gravity from earth
o Moon & sun too far away
 More subtle – ocean tides
 Remarkable thing about gravity
o Weight is proportional to mass


w mg
Where g is a constant that is determined by the local strength of
gravity. The constant g is called the acceleration due to gravity.
 Determined by the properties of Earth.
o Radius, mass
o Doesn’t depend on the object being considered
g = 9.8 m/s2 (32 ft/s2) on earth’s surface
Objects will weigh something different on another planet or moon
On moon weight = 1/6 that of earth
gmoon = 1/6 gearth
10
Back again to the Falling Balls:
Only force on the falling ball is its weight.
 How much will it accelerate?
  

minertial  a  F  w  mgravity  g


minertial  a  mgravity  g
 
ag
The acceleration of the object due to the force of gravity is
independent of the mass of the object.
 Remember the demo with the feather & ball
 Newton & Einstein got famous for this
Consider a baseball and a bowling ball:
Although the bowling ball weighs more (has more force due to
gravity), it also has more mass (inertia) which resists the force more.
So, the greater inertia exactly cancels the greater force and they
both end up with the same acceleration, and therefore the same
motion.
Now we can examine the motion of any falling ball near the earth’s
surface. Any ball will accelerate downward at a constant rate of
9.8 m/s2.
 What about velocity & position of falling ball? (1D)
Velocity
V = V0 + a t
Example: throw a ball up at 20 m/s (~45mph).
V = 20m/s –9.8m/s2 x t
When does it stop?
V=0 = 20 –9.8 x t
t ~ 2s
What does this mean? What is a at this point?
11
What about position?
Distance = <vel> x time
<vel> = ½(V0 + Vf) = ½ (V0 + (V0 + at))
<vel> = initial velocity + ½ acceleration x time
present position = initial position + Distance
X = X0 + V0 * t + ½ * a * t2
Second order in time
 Parabola
Summarize falling ball: (draw this geometrically as in book)
T(s)
0
1
2
3
A(m/s2)
-9.8
-9.8
-9.8
-9.8
V(m/s)
0
-9.8
-19.6
-29.4
P(m)
0
-4.9
-19.6
-44.1
Recall V & X graphs from Air-track demo with ULI
falling Ball
0
0
-5
-5
-10
-15
Position(m)
Velocity (m/s)
-10
-15
-20
-20
-25
-30
-35
-25
-40
-30
-45
-50
-35
0
0
0.5
1
1.5
2
Time (s)
12
2.5
3
3.5
0.5
1
1.5
2
Time (s)
2.5
3
3.5
Projectile Motion:
DEMO – Throw the ball around
Ball has 2-D motion
 Up & down (called this X above, now call this Y)
 Direction it is thrown (X)
Gravity only acts in Y direction
 ax = 0 , ay= -9.8 m/s2 = g
X = X0 + V0x * t
Y= Y0 + V0y * t + ½ * g * t2
DEMO - C2-25: FUNNEL CART
DEMO - C2-21: BALLS DROPPED AND SHOT
13
DEMO - C2-22: MONKEY AND HUNTER
DEMO - C2-24: WATER DROP PARABOLA
End Section 1.1
14
RAMPS:
Try pushing something up a ramp.
 Heavy object sitting on table
o Large force
 Put it on an inclined plane
o Less force to move it!
DEMO: B2-03: EQUILIBRIUM OF FORCES - INCLINED PLANE
You can use a ramp to lift very heavy objects with relatively little
force!
 Mechanical advantage.
How does this work?
Need to understand addition of forces.
 What forces are acting me as I stand here?
o Gravity down
o Why don’t I accelerate?
 Force of floor acting up
Is the force always the same?
 No, it always just cancels out weight
 Reaction force
15
Newton’s 3rd law:
For every force that one object exerts on a second object, there
is an equal but oppositely directed force that the second object exerts
on the first object.
DEMO: C5-19: ACTION AND REACTION - INSTRUCTOR AND
CART
Examples
 Boat on water
 Person on ice
 Recoil of a gun
We’ll come back to this later in Rockets.
Consider a collision between a car and a truck:
Mcar = 1 ton
Mtruck = 10 ton
Ftc = -Fc t
Mcac = Mtat
ac = -Mt/Mc * at
ac = -10 at
Summarize Newton’s 3 Laws.
16
Addition of Forces:
Draw figure of object with forces from gravity(weight) and the reaction
force of the floor.
 Force is a vector
DEMO: B2-02: SUM OF FORCES - SPRING SCALES
Give examples of summing of forces.
 Pushing you east and north, you go northeast
DEMO: B2-16: VECTOR ADDITION WITH ROPE AND STUDENTS
DEMO: C5-31: AIR TRACK - SAILING UPWIND
17
Work & Energy:
The capacity to make things happen is energy, and the process of
making them happen is called work.
Work & Energy are physical quantities
 They are measurable!
Physical definitions are different then common English usage.
 Energy
o Is not exuberance of a 5yr old
o Capacity to do work
 Work
o Not the activities you do to get money
o Process of transferring energy
Energy is what is transferred, and work does the tyransferring.
Important:
ENERGY IS A CONSERVED QUANTITY!!
We will talk about this a lot all semester.
What is work?
 You do work on an object by exerting a force on it, as it moves
in the direction of that force.
 As you lift a rock you do work on the rock.
In both cases you are transferring energy to the object.
Sometimes Work (transferring energy) makes an obvious change.
 Throw the ball
o Picks up speed, energy increases
 Kinetic Energy
o Pick up the rock
 It can do work on the objects beneath it if it drops
 Potential energy
 Energy stored in the forces between things
 Gravitational Potential Energy
18
DEMO: C8-04: HILL TRACK
DEMO: C8-11: INTERNAL VS EXTERNAL ENERGY - SPRINGCOUPLED SUPERBALLS
DEMO: C8-12: JUMPING MASSES WITH INTERNAL SPRINGS
19
The amount of work you do is determined by how hard you
push(force) and the how much distance you push it.
Work = Force x Distance
How much work do you do in lifting the piano? The work is
transferred energy into the piano in the form of Gravitational Potential
energy.
Gravitational Potential Energy of any object:
U  m g h
Net amount of work you do in lifting the piano is mgh, it doesn’t
matter how you get there!
If you lift 100kg to a height of 10m
U = m g h = 100kg x 9.8 m/s2 x 10m
~ 10,000 kg m/s2 x m = 10,000 Nm
= 10,000 Joules (J)
Now we can examine how a ramp allows to lift the piano with a little
force by giving us a mechanical advantage.
Force from
ramp
Net
force
weight
20
Small residual net force
 Ramp supplies most of force needed to keep it from
accelerating
If you have a 50m ramp go up 5m. for every 10m along ramp you go
up 1m. You can push a 2000N weight up this 10 to 1 grade with only
200N of force.
The total work is 50m x 200N = 10,000 J
It doesn’t matter if you go up the ladder
2000N x 5m = 10,000J
or slide it up the ramp with only 200N of force. Either way the final
energy (mgh) is the same.
Work = LARGE FORCE x small distance
= small force x LARGE DISTANCE
This ramp gives us a mechanical advantage.
SEESAWS:
Another form of mechanical advantage using torque.
We all now how a seesaw works. Need equal masses.
What if we have different weights?
 Balance it by changing distance from the pivot point.
21
DEMO: B2-32: EQUILIBRIUM OF TORQUES – LARGE
DEMO: B3-03: LEVER - WRECKING BAR
DEMO: B3-11: PULLEY - HUMAN LIFT
22
Wheels:
If we have a heavy object (book uses file cabinet) what happens
when we push on it?
We start pushing on the file cabinet, but it doesn’t budge.
 Why Not?
 Newton’s 2nd Law says if we push it should accelerate
Something must be pushing back harder as we push harder to cancel
the force.
 Net force = 0
 No acceleration
ANSWER:
FRICTION – a force that opposes the relative motion of two
surfaces in contact with one another.
Friction always opposes the relative motion.
Microscopic view of friction:
 Surfaces are not perfectly smooth.
 Microscopic hills and valleys.
o Show figure 1.4.3 from book.
Friction depends on
 Material
 Force pushing down
o F  downward force
DEMO: C6-03: INCLINED PLANE - FRICTION WITH THREE
BLOCKS
23
DEMO: C6-11: SLIDING FRICTION - LECTURE TABLE AND FELT
Really two kinds of friction:
 Static
 Sliding
To start the file cabinets moving you have to exert enough force to
overcome static friction. Once it is moving you have to exert enough
force to keep it from slowing down and stopping due to sliding friction.
Sliding friction is less is generally weaker because once the surfaces
are moving they no longer have time to “settle” into one another.
DEMO: C6-02: INCLINED PLANE - FRICTION BLOCK
WORK, ENEGY, & POWER:
Friction wastes energy.
 Not make it disappear
o Energy is conserved
 Energy can be transferred or converted from one type to
another
 Friction converts useful ordered energy into relatively useless
disordered energy
24
 Called thermal energy
o Energy associated with temperature
 Called internal energy or heat
DEMO I5-02: TRANSFORMATION OF MECHANICAL ENERGY
INTO HEAT
DEMO: I5-01: MECHANICAL EQUIVALENT OF HEAT - SHOT BAG
Remember before we talked about kinetic energy and potential
energy
Kinetic is associated with the energy of motion
 More kinetic energy = faster speed
 Losing energy it slows down
o Bowling ball hitting pins
25
o Running back hitting the defense
Potential energy is usually not visible
 Remember gravitational potential energy
o Ability to do work by falling down
Potential energy is the stored ability to do work
1.
2.
3.
4.
5.
Gravitational (ball at top of hill)
Elastic (a wound spring or a stretched rubber band)
electrostatic (cloud in a thunderstorm)
Chemical (a firecracker, gunpowder)
Nuclear (Uranium)
Energy has units of Joules (J) = Nm = kg m/s2
How quickly you do work or transfer energy is power
Power = work/time
= joule / s = watt
(light bulb!)
1 hp = 745.7 w
example:
100kg person up 3 floors(physics building) = 10m
U = mgh = 100 x 10 x 10 = 10,000J
1 hp =745.7 w = 745.7 j/s
how long to lift person?
T = work/power = 10,000J/ 745.7 J/s = 13 seconds
200 hp engine --> .067 seconds -----> 67/1000
26
DEMO: C8-34: POWER - INSTRUCTOR DRAGGING CONCRETE
BLOCK
Energy goes into friction which dissipates the energy into heat.
 Wasted energy!
 Minimize the friction
o Wheels!!
Move something without sliding using wheels!
 Once you get it moving there is no friction
o Will go on indefinitely with a constant velocity
 Newton’s Laws!
Still some sliding friction
 Motion between hub and fixed axle
 Small amount of distance per second
o Small radius
 Means force x dist = work is smaller
 Further reduce it by lubricationg
o Oil
 Can also use a bearing
o See figure 1.4.9
o No sliding friction at all
 Ball bearings all turn using static friction
27
Linear & Angular Momentum:
Measure of an object’s motion or tendency to continue moving in a
particular direction. (vectors)
Linear momentum (momentum)
P=mV
Angular Momentum (rotational momentum)
L=I 
Both are further examples of conserved quantities.
Kinetic energy
KE = ½ m V2
Conserved Quantity
Transfer mechanism
Energy
Linear Momentum
Angular Momentum
Work
Impulse
Angular Impulse
Conserved Quantities DEMOS
DEMO: C2-11: RACING BALLS
28
DEMO: D3-04: ROTATING STOOL AND WEIGHTS
DEMO: D3-05: ROTATING CHAIR AND BICYCLE WHEEL
DEMO: D4-04: BICYCLE WHEEL GYROSCOPE ON ROPE
29
DEMO: D3-32: KEYWHIP
DEMO: D4-22: MONORAIL CAR
END WHEELS & THE LAWS OF MOTION
30
Roller Coasters Section 2.3
The experience of acceleration
Show DEMO: D1-53: LOOP-THE-LOOP how does this work?
A car accelerates forward – what do you feel?
 you are pressed backward against seat
 as if gravity was pulling you back as well as down
o it is your own inertia resisting the acceleration forward
You are experiencing the feeling of acceleration.
Roller coasters are the ultimate experience in acceleration.
Close your eyes while traveling straight at constant speed
 can’t feel anything
Accelerations
 no problem feeling it.
Accelerating in car feels the same as when you are standing on the
floor.
 The floor supports the weight of your body.
 The car seat pushes on you to accelerate you forward.
When the ground is preventing you from falling you feel your weight
as the body senses the forces to support you so you don’t accelerate.
When you accelerate in car your body senses the internal forces
needed to accelerate you. Which you interpret as weight.
The apparent force you experience from acceleration is
indistinguishable from the force of gravity.
 Einstein’s general theory of relativity
 Draw elevator and digress on Einstein & bending of light
The experience during acceleration is called a fictitious force.
 Points in direction opposite the acceleration
31
Apparent weight is vector sum of gravity and fictitious forces.
 Figure 2.3.2 from book.
Circular motion
Merry-go-rounds & spin dryers …
Examples of centrifuge
A basket or some such thing spinning around a pivot point. Circular
motion about a center pivot.
What do we know from Newton about motion not in a straight line?
 There must be a force.
o Must be accelerating
DEMO: D1-33: ROTATING MASS ON STRING
How do we know there is acceleration (force)
 Can feel the tension on rope
What would happen if we let go of the rope?
 Ball would travel off in straight line
o Acceleration is towards the center making it turn and not
go off in a straight line.
32
DEMO: D1-31: TRAJECTORY FROM SPIRAL
Force keeps pulling in causing an acceleration towards the center.
 Uniform circular motion
Show figure 2.3.3 from book.
An acceleration of this type is called centripetal acceleration.
Caused by a centrally directed force called centripetal force.
The acceleration depends on the speed and radius of the circular
motion.
v2
a 
r
Now, since the person is accelerating inward towards the center, she
experiences a fictitious force outward. This is called
“centrifugal force”
is a fictitious force
 If centripetal force is removed the object will go in a straight line
Demos of v2/r dependence.
33
DEMO: D1-37: MUDSLINGER
Fictitious force is often measured relative to earth’s gravity.
 9 = 9.8 m/s2 or 9.8 N/kg
In an airplane you may experience a fictitious force >5 times that of
gravity or 5 g’s for short.
Example: close dryer
R = 0.25 m
V = 20 m/s
A= v2/r = 1,600 m/s2  163 g’s
If close had a mass of 5kg really weigh 49N (11 pounds) apparent
weight is 1800 pounds!!
The water doesn’t accelerate with the clothes which are spun dry.
DEMO: D1-52: FAIRGROUND ROTOR
34
Roller Coasters
Talk about roller coasters.
 Lots of accelerations!
 Discuss various motions and fictitious forces that result.
Your apparent weight (g + fictitious force) can be all over!
 Accelrate up
o Very heavy
 Accelerate down
o Apparent force can be less then real weight!
Consider accelerating down at just the right rate.
 Real weight and fictitious upward force just cancel
o You will feel perfectly weightless!!
What is the rate that causes this?
 G = 9.8 m/s2
How do get this acceleration?
 Free fall!!
An object not subject to any other forces other then gravity is in free
fall (no force on your feet holding you up).
Discuss free fall.
 No weight.
 Any object with you (sunglasses, pens, etc..)will hover in front
of you as you all accelerate together!
 Similarly, your internal organs don’t have to support each other.
DEMO: C4-51: WEIGHTLESSNESS IN FREE FALL - MASS IN
BEAKER
35
DEMO: C4-52: WEIGHTLESSNESS IN FREE FALL - MASS IN CUP
ON POLE
DEMO: C4-53: WEIGHTLESSNESS IN FREE FALL - MASS ON
SPRING
Now we can talk about roller coasters.
Draw figures of loop-the loop and indicate gravity, fictitious force and
apparent force.
Discuss first versus last car going over the top of a hill like in figure
2.3.9 in book.
DEMO: D1-53: LOOP-THE-LOOP
36
ROCKETS
(section 5.1)
Rockets work on one of the most basic of principles:
 Action and reaction (Newton’s 3rd law)
 Propelled forward by pushing material out its tail
A rocket gets its thrust by pushing gas out its tail.
Thrust is the force that accelerates it forward.
Newton’s 3rd law
 The rocket pushes on the gas so the gas pushes back on
rocket.
 The more gas and the faster the gas is going the more thrust
the rocket gets.
Recall before when we talked about 3rd Law.
DEMO: C5-19: ACTION AND REACTION - INSTRUCTOR AND
CART
Discuss speed, mass, conservation of momentum
Discuss throwing shoes off boat or on ice
37
In a rocket instead of throwing shoes it throws very fast gas
molecules.
 At room temp v=1,800 km/h
 At 2800 C (5000 F) V = 3 times that
 High velocity = high momentum
o Large thrust!
Conventional rocket engine. (figure 5.1.2)
 Chemical reaction to create VERY HOT exhaust gas.
 Potential energy stored in chemical bonds of molecules
becomes thermal energy.
o Thermal energy is mostly kinetic energy in random motion
of molecules
o Engine’s Nozzle converts random motion into directed
motion
 Permitting gas to escape from only one side
The nozzle exerts a net force on the gas.
 Starts out stationary and ends up going out exhaust
The gas pushes back on the nozzle.
Overall its own exhaust pushes the rocket forward!
It doesn’t need anything outside to push against.
DEMO: C5-14: ROCKET TRIKE
38
Will the rocket trike work just as well in the vacuum of space?
DEMO: C5-17: ROCKET BOTTLE
The space shuttle weighs about 20,000,000 N at launch. The trhust is
about 30,000,000 N So, it will accelerate upward.
Different kinds of rockets
 Solid fuel rockets
o V ~ 3000 m/s
 Liquid fuel
o V ~ 4500 m/s
Model rocket engines at hobby stores.
 Solid fuel
o “C” engine impulse ~ 10 Ns (10 kg m/s (p=mv))
if rocket weighs .1kg
V = impulse/mass
= 10/.1 = 100 m/s
> 200 mph
History of Rockets:
Date from the 13th century in China.
 Follow up from gun powder
39
 Burning gunpowder sent hot exhaust gas out
 Used a guide stick to keep it stable.
o Like a bottle rocket.
Guide sticks eventually replaced by banes attached to sides to give it
stability.
Liquid fuel rockets suggested later.
 Solid fuel was reliable & easy to make
 Liquid fuel offers more chemical potential energy per kg
o Liquid fuel can control the thrust
1926 – Robert Goddard launched first liquid fuel rocket.
Show figures 5.1.5 & 5.1.7 on solid & liquid fuel rockets.
Maximum speed of a rocket::
As long as a rocket can keep pushing material backwards it will
accelerate forward and go faster. The speed is limited by how much
of its weight is used as fuel.
If a rocket is comprised of 90% fuel by weight. The ultimate speed is
2.3 times the gas’ exhaust velocity.
To go faster you have to have even higher percentage of fuel.
Space bound rockets often accomplish this by using multiple stages.
Each stage smaller then the next. Once the 1st stage has used its
fuel, the whole stage is discarded and a new lighter rocket begins to
operate.
Orbiting Earth:
Show figure 5.1.8 from book.
If something was shot at high enough velocity it would not fall to earth
before the earth fell away because of its curvature.
40
At 7.9 km/s (17,800 mph) the cannonball would travel beyond the
horizon and would never actually hit the ground.
It would circle the earth, returning to the cannon in 84 minutes.
Spacecraft can do the same thing if it is going fast enough.
The spacecraft will Orbit the earth.
Orbit: is a path an object takes as it falls freely around a celectial
object.
Falling freely:
Which way is it accelerating? Center
Why? Gravity
Near surface g =9.8m/s2
Law of gravitation
F 
G m1 m2
r2
As the spacecraft moves further from earth the gravity is less and the
balancing force
m2 v 2
m2 ( 2r / t ) 2
m2 ac 

r
r
 4 2
41
m2 r
t2
G m1m2
2 m2r

4π
r2
t2
2
4π
t2 
r3
Gm1
The larger the radius of the spacecraft’s orbit the longer the orbital
period.
Space shuttle & most reconnaissance satellites orbit ~200km above
earth to avoid the atmosphere. The period here is about 90 minutes.
At 35,900 km (22,300 miles) the period is 24 hours.
 The satellite orbits at the same period as the earth’s rotation
 Stays directly above a single spot
 Called geosynchronous orbit
DEMO: E1-11: POTENTIAL WELL –MODEL
Show pictures of earth from spy satellites.
Discuss satellite TV etc….
42
Heat --- Chapter 6
What comes to mind when I talk about Heat?
 Hot, cold
 Temperature
We talked about thermal energy before
 Internal energy of an object
o Kinetic energy of atoms & molecules
 The more thermal energy (kinetic energy of atoms) the larger
the temperature
DEMO: I6-34: MOLECULAR MOTION DEMO - TEMPERATURE OF
A GAS
When you touch something hot what you feel is the thermal energy
flowing into your hand making it hotter
 The flow (or moving) of thermal energy is heat
DEMO: C8-04: HILL TRACK
Discuss motion of ball around potential minimum
 Kinetic(thermal) energy of molecules
 The more energy (temp) the larger the amplitude
 The temperature can effect the properties
o Size, pressure etc….
43
DEMO: I1-12: THERMAL EXPANSION - BALL AND RING
DEMO: I1-13: THERMAL EXPANSION - BIMETAL STRIP
DEMO: I4-17: AIR BALLOON ON LIQUID NITROGEN
DEMO: I1-52: TUNING FORK AT LIQUID NITROGEN
TEMPERATURE
44
Temperature tells you which way heat will flow between two objects.
 From hot to cold
DEMO: I1-42: THERMOELECTRIC FAN
 Until thermal equilibrium is reached
o No heat flow when in thermal equilibrium
DEMO: I2-27: THERMAL EQUILIBRIUM BETWEEN ALUMINUM
AND COPPER
Temperature scales:
Celsius
Fahrenheit
Kelvin
45
Freezing
0
32
273
Boiling
100
212
373
Abs zero
-273
-460
0
Wood Stoves
How do we produce heat?
Fuel:
 Chemical reactions to release energy?
DEMO: I1-63: HYDROGEN EXPLOSION
Burn wood
 Hydrocarbon molecules
o H,C,O
o Carbohydrate
 Hydrocarbons + O2  H20 +CO2
o Reaction products
 Have lower potential energy then the original
molecule
Activation Energy: Energy needed to break molecular bond. Then
the release of energy by going to the lower energy reaction products
is released. Also used to keep the reaction going for other molecules.
Light the wood  then it keeps burning
Use Hill track demo again
activation
energy
released
energy
46
Better Fuels are :
 Kerosene
 Natural gas
o Pure hydrocarbons
 Hydrogen
o 2H2 + O2  2 H2O
How does the Wood stove heat the room?
 How does heat move?
Remember the DEMO on thermal equilibrium?
 Conduction
o Contact
o Heat fows from hot to cold
How does it flow?
 Energetic molecules and atoms hit their neighbor
o So-on and so-on
 Bucke-Brigade
 Electrons are free to move in metals (mobile)
o Better conductors of heat
 And electricity
 Generally, good conductors of electricity are good
conductors of heat.
 Copper, silver, gold, aluminum etc…
 Bad conductors
 Air, plastic, wood, glass
 Exception
 Diamond
o Good substrate for electronics
Back to the wood stove:
 Heat flows from inside the stove to outside the stove walls by
conduction.  metal
o Outside surface gets very hot!
 What carries heat to room?
o Air is a very poor conductor!!!
47
Convection & Radiation:
Convection: Heat moving with air
 Warm air rises
o Causes a flow of air in room
o Show figure 6.1.5
 The air heated by contact with the hot stove
 Temperature lowers density of hot air
 Floats upward, pulling cold air behind it to replace it
 Eventually heated air cools and descends down to
floor
 Drawn back to hot stove to repeat cycle
Creates a Convection Current. Looping path is called a convection
cell.
Sometimes needs help:
 Adding a ceiling fan will bring the warm into the entire room
faster.
DEMO: I2-43: CONVECTION - HOT PLATE
48
Radiation:
How do you get warmed by the sun?
How does heat get here from the sun?
 No conduction
 No convection
Radiation!
Hot objects radiate energy!
 Electromagnetic waves
o Light, radio, tv, x-ray, microwave etc…
o Carry energy, travel through empty space at the speed of
light
o Can carry thermal energy
 Thermal energy will flow (heat) from a hotter object
to a colder object via radiation.
 Needs no medium between objects
Show electromagnetic spectrum (figure 6.3.2)
DEMO: I2-01: CROOKES' RADIOMETER
The type and amount of radiation depends on the temperature of the
object.  Thermal Radiation
49
Colder object may only emit radio or infrared
Hotter object can emit visible light
 The red glow of the hot coals in the stove!
 Hotter objects emit more thermal radiation
Eyes are only sensitive to visible light.
 We can’t see all the thermal radiation
Show blow up of em spectrum in visible (figure 6.3.3)
DEMO: I2-06: THERMOPILE WITH AUDIO OSCILLATOR
DEMO: N1-05: SPECTRA - VISIBLE AND INVISIBLE
50
Eyes see visible light from sun (5800 C)
Big Bang --> Microwaves (<3 K)
Body temperature --> Infrared
 Imaging for detecting people and night vision
Heat (Thermal radiation is like light)
 Can be focused
 Mirror
DEMO: L3-16: FOCUSING OF HEAT WAVES BY MIRRORS
Clothing & Insulation:
Why do we lose heat?
 Body temperature is higher then surrounding
 How do we maintain temperature instead of reaching
equilibrium?
o Warm blooded
o Produce heat to maintain higher temp
 T= 37C (98.6F)
T means heat flow
 80 cal/hr = 100 watts (light bulb)
Reduce or minimize heat loss
 Conduction
o QkA
T
X
 Thick
51
 Bad conductor
 Air, fat, skin
 Smaller T
 Lower temp in hands and feet
o Feels cold
 Convection
o Heat warms air around you
o How quickly?
 Specific heat capacity
 Air is 4 times water
 Why you cool off (lose heat) faster swimming
o Air is a poor conductor (remember wood stove)
 Skin warms only a small layer
 If air remains still wouldn’t lose much heat
o Convection removes the warm air keeps T higher
o The enhanced heat loss by moving air
 Wind chill
o Keep air from moving
 Insulation with air pockets
 Feathers
 Dual glass panes
DEMO: I2-09: DEWAR - TRANSPARENT WITH LN
 Radiation
o Amount depends on T and how well they absorb/emit
 L  T4
52
Tsun = 6000K
Tsun/Tme = 20
Tme = 300K
204 ~ 160,000 times more heat towards me
Night Sky --> T ~ 0
Lose about all of the 100w to the dark sky
Reduce heat loss by standing under a tree
Black is a good absorber of radiation
Mirror is a good reflector (remember match demo)
 Insulator to thermal radiation heat transfer
DEMO: I2-10: DEWARS - SILVERED AND UNSILVERED
Keeping cool – sweat
 Evaporative cooling
Buildings:
Air insulation
 Avoid convection
53
 Trap air in porous fibers
o Glass wool, foam, etc…
 Ceiling is important because the hot air rises
 Windows
o Air space
o Double pane windows
Gas- air or argon
Law of thermal radiation:
“Black Body” Radiation
Black Body
 Perfect radiator
 Perfect aborber
Need quantum mechanics to understand (led to quantum)
Light comes in indivisible packets (quanta) of energy
E = h
per “photon” – quantum of light
Intensity of a given color given by number of photons


54
8h
3
E ( )  3 h / kT
c e
1 
Higher Temperature
 Higher overall intensity
 Color is more blue then red
DEMO: I2-04: WIEN'S LAW OF THERMAL RADIATION
END Lecture
55
Incandescent Light Bulbs
Light is electromagnetic radiation
 Thermal radiation
Hot wire filament in a light bulb emits visible light as part of its thermal
radiation (show light bulb figure from 6.3)
DEMO: L1-02: TUNGSTEN-HALOGEN LAMP - PROJECTION OF
LAMP
Remember discussion of thermal radiation.
 Most is invisible to our eyes
 Distribution of wavelengths
 Eyes sensitive to a narrow window of wavelengths
o Show slide with figures 6.3.2, 6.3.3
Any object with T>400 C emits enough visible light to be seen in a
dark room
At higher temperatures it brightens and color shifts from red to orange
to yellow to white
 To reproduce sunlight the filament would have to be 5800C.
 Tungsten Filament Tmax = 2500 C
o Can’t really reproduce sunlight color
 Remember black body spectrum
56
Show figure 6.3.4 of wavelength distributions for 3 temps
Show table 6.3.1 discuss color temperature
Object
Heat Lamp
Candle Flame
Bulb Filament
Sun’s Surface
Blue Star
Temperature
500 C
1700 C
2500 C
5800 C
>6000 C
Color
Dull Red
Dim Orange
Bright yellow-White
Brilliant White
Dazzling Blue-White
Notice from figure 6.3.4 there are 2 principal shortcomings of
Incandescent light bulbs
 Poor efficiency at converting electrical energy (heat) into visible
light
 Low temperature color
The Filament:
The filament is made hot by giving it thermal energy (heat).
The energy comes by passing electrical current through it.
 Current consists of electrically charged particles
 Most of their energy is converted into electrical energy
o Kind of like thermal energy getting created by dissipating
the kinetic energy by friction  resistance
DEMO: K5-32: RESISTANCE VS DIAMETER AND LENGTH
57
Filaments
 First filaments were Carbon
o 1879 Thomas Edison
 bulb survived for several hundred hours
o carbon has highest melting temp (3550 C)
o However, it evaporates directly from solid to gas very
quickly
 Sublimation
DEMO: I4-51: SUBLIMATION OF DRY ICE – PROJECTION
o When enough sublimates away a gap occurs in filament.
It stops carrying current
 Bulb “burns out”
 Better choice id Tungsten
o Melts at 3410 C
o Sublimation very low
o Can be run at higher temps then carbon before
sublimation rate is too high
o Hotter = brighter, whiter light
How do you make a long and thin filament to get enough electrical
resistance?
A typical 60 Watt light bulb
 2 meters of 25 micron (.001 in) tungsten wire
o wound into a double spiral
 first wound into a thin spring-like coil
 then the coil is wound into a coil itself
58
 see Figure 6.3.5
 not accomplished until 1937
To keep the white hot filament from burning or subliming quickly the
surrounding gas cannot be air.
 If air (oxygen) is present it will burn immediately
DEMO: L1-03: LIGHT BULB WITHOUT VACUUM
Need to remove air or fill with inert gas
 Mostly argon
 No Oxygen
Gas wastes energy by allowing heat to escape from filament by
conduction and convection.
 Dark spot on top of bulb from convection currents carrying tiny
tungsten particles
Krypton is better then Argon
59
 Poor heat conductor
Persistence of light
 Current is 60 cycle per second AC
o Current goes on and off 60 times a second
 Why does the light appear continuous?
o Filament doesn’t cool off that fast
o 60 cycle is faster then eye can see
DEMO: L1-05: PERSISTENCE OF A FILAMENT
Extended Life and Halogen Bulbs
Extended life
 longer filament
o doesn’t get as hot
 won’t sublimate and burn out as quickly
 lower temps means less fraction of visible light
o can make it so long that it doesn’t get hot enough to give
off any visible light
Extended life bulbs are less energy efficient!
Halogen Bulbs
Run the filament hotter
60




brighter light
closer to sun’s color temp
more energy efficient
sublimation much higher
o burns out quickly
 Photoflood lamps for photography
Halogen bulb uses a trick to “rebuild” the filament as it sublimates to
allow it to last longer with the higher temps.
 Encased in a quartz tube
o Tolerate the higher temps and chemicals
 Contains molecules of a halogen (bromine, sometimes iodine)
 Halogen reacts with tungsten to make stable tung-halogen
molecules that don’t stick to glass
 These molecules drift around tube until they contact hot
filament
 Then torn apart and the tungsten sticks to the filament thus
rebuilding the filament
The rebuilding process is not completely uniform.
 Eventually the filament will get a break and burn out, but after a
much longer time
Halogen bulbs run at much hotter temperatures.
Brightness of bulb related to amount of heat energy per second to
heat up filament.
 Heat energy comes from electrical energy
 Energy/s = J/s = watts
o 60 W, 100W bulbs …
We have talked about incandescent bulbs
 bulbs that emit light by thermal radiation
61
There are other kinds of lights
 fluorescent
 laser
 they use quantum mechanics
o transitions between energy levels in atoms
o electron goes from a higher potential to a lower potential
o conservation of energy!
 Energy is converted to light energy (photon)
fall back down
emit photon (light)
redder
raise
energy
bluer
DEMO: L1-32: VISIBLE LASER
DEMO: N2-02: DIFFRACTION SPECTRA - THREE SOURCES EXPENDABLE GRATINGS
62
Thermodynamics -- Chapter 7
 heat flows from hot to cold
 rules governing the movement of heat
Study by examining 2 everyday machines
1. Air-conditioner
2. automobile engine
Talk about different kinds of energy. Not all are the same
 ordered
 disordered
AC  ordered electric energy to transfer heat from cold to hot
(against its normal flow)
Auto  thermal energy can be used to do work (ordered energy) but
at a cost
7.1 Air Conditioners
 want to remove thermal energy from something (e.g. room)
 transfers heat against its normal direction of flow
 requires ordered energy (electricity). Why?
How can we cool off our house on a hot day?
1. Let heat flow from yours to someone else’s house?
o 0th Law – 2 objects(houses) in thermal equil. With a
third(air) are in thermal equil with each other.
o Won’t work.
2. Destroy the thermal energy in the house
63
o Conservation of energy won’t allow you to do this.
o 1st Law –
o u = Q + W
 explain in words
Show some adiabatic Q=0 processes to illustrate
 U = W
DEMO: I5-11: ADIABATIC PROCESS - AIR PISTON WITH
THERMISTOR
DEMO: I5-13: ADIABATIC EXPANSION OF AIR - GRAPH OF TEMP
DEMO: I5-15: ADIABATIC EXPANSION OF CARBON DIOXIDE
64
Still doesn’t explain some things
 heat flowing from cold to hot
o pond freezing on a warm day
 weight cooling off and gaining kinetic or potential energy
also, number 3 in ways to cool off house
3. Convert thermal energy (heat) to electric energy and cool off
house
 Why won’t this work?
o Burning log example
 Won’t re-assemble very improbable (0)
o Once energy is scattered randomly among individual
particles (thermal energy) it won’t collect back again
spontaneously
o Ordered and thermal energy are not the same
o There is some other law we need to describe this
New Law of Thermodynamics (2nd Law):
 Disorder of an isolated system never decreases.
 System that starts out ordered ends up more disordered
Name: total disorder = Entropy
Heat carries entropy. The more heat the more disorder because of
the higher amount of random energy (thermal energy).
A weight cannot lower its temperature and raise itself in gravity
 Doesn’t violate energy conservation
 But, the disorder (entropy) would go down.
o Lower temp = lower heat = lower disorder
DEMO: I5-41: ENDOTHERMIC REACTION – ENTROPY
65
This law of entropy is an empirical law.
 Fact of nature
 Like the arrow of time
 No known physical reason why it is this way!
o Microscopic laws are all reversible
o Connected to time, quantum collapse?? Gravity??
This Law explains why you can’t make a perpetual motion machine.
AC lowers the entropy of the house (cools it off)  we must
somehow raise the entropy somewhere else even more then we
lowered it in the house.
Let see how this works mathematically
S = Q/T
Entropy in normal heat flow from Hot to cold

S = Q/T
HOT  COLD
TH
TC
SH = -Q/TH SC = Q/TC STOT = Q(1/TC – 1/TH) > 0
The heat entering the colder system creates more disorder in it then
order in the hotter system.
Example of tea party  two rooms with bunch of kids (lots of energy
= high temp = more disorder) and one with a tea party of adults (low
energy =low entropy). Let door open, flow of 1 kid into tea party. 
Creates more disorder in the tea party. The room with the kids
doesn’t get much more ordered!
66
AC does the seemingly impossible. How?
COLD  Heat Pump  Hot
^
|
ordered energy in converted to heat(disordered) to increase
entropy.
How do we put in ordered energy?
 Work or Power
o Electricity, fuel …
COLD  Heat Pump  Hot
^
|
WORK
S = Q/T
HOT  COLD
TH
TC
SH = +(Q+W)/TH SC = -Q/TC
STOT = Q(1/TH – 1/Tc) + W/TH > 0
<0
need work (ordered energy = electricity) to make entropy increase!
Nothing is for Free!!
How do we make this happen in an actual AC?
Working Fluid
 Transfer of heat
 Absorbs heat from inside
 Absorbs ordered energy (work) (pump)
 Releases heat and from inside heat taken + work converted to
additional disordered energy
67
3 main components
 Evaporator - inside – takes heat
 Condenser - outside – outputs heat
 Compressor - usually outdoors – does work on fluid
Show figure 7.1.2 from book and describe cycle
 High pressure liquid
 Constriction
 High pressure in  fast low pressure out
 Low pressure it evaporates and cools
 E.g. sweating evaporates
DEMO: I5-15: ADIABATIC EXPANSION OF CARBON DIOXIDE
 Cold liquid  cold gas in evaporator
 Temp drops
 Warm air in house is blown over evaporator coils by fan
 Takes heat away by conduction air cools
 Gas goes from cold to cool
 Cool gas leaves coil
 Compressor
 Takes cool high pressure gas in
 Compresses it - does work
 Adds heat  out comes high pressure hot gas
68
DEMO: I5-11: ADIABATIC PROCESS - AIR PISTON WITH
THERMISTOR
Draw diagram of how pump works on board.
DEMO: F4-52: FORCE PUMP – MODEL
Does work on gas W = P dV
U = W = P dV
Temperature goes up!
 Hot gas goes through condenser outside
o Cools off because it is now hotter then outside air
o Hot gas  goes to warm high pressure liquid
Start all over again in cycle
69
Pumps heat out of house. Adds heat from work done. Pumps heat +
heat from work to outside. Overall entropy goes up. You must add
ordered energy, or work, to make heat flow from cold to hot!
The Fluid itself.
 Gas at low pressure
 Liquid at high pressure
 Chloro fluorocarbons (Freons)
o Replaced ammonia
 Toxic
 Corrosive
Ozone layer is eaten by chlorine.
Hydro fluorocarbons  replaced by non-chlorine molecules
Not as energy efficient!
7.2 Automobiles
Engine:
Thermal Energy  Ordered Energy (work)
Avoids conflict with the entropy Law by being a heat engine
Get work out of disordered thermal energy as heat flows from hot to
cold. Some heat is wasted in flow of hot to cold thus increasing
entropy overall.
Can’t make a 100% efficient engine.
HOT  COLD
|
|- Work = QH - Qc
such that overll entropy increases.  some heat must flow into
COLD.
Internal Combustion Engine , see figure in book & gif on WEB.
70