Download drburtsphysicsnotes2 - hardingscienceinstitute

PHS 116
Conceptual Physical Science
Notecards
Name
E-mail (preferred)
Phone number (preferred)
(schedule on back)
Who are you?

Tell everyone
your name
 Year and major
 Hometown
 A word that
starts with the
same letter as
your first name
Who are you?

Burt Hollandsworth
(Dr. Burt)
 Grew up in VA
 B.S. Chem. from
Roanoke College
(where I met my
wife)
 Ph.D. from Ohio
State
June, 2000
Who are you?
Taught @ Capital U. and OSU-Mansfield
 1 YR Postdoctoral Research @ USc.
 Here @ Harding since late May
 Brutus

Who are you?
Tell everyone your name
 Year and major
 Hometown
 A word that starts with the same letter as
your first name

Syllabus
[We’ll go over the syllabus]
Prologue
Science is a body of knowledge that
goes back to the beginning of
humankind.
 As soon as man began to observe the
surroundings, science began.
 Rational thought became popular in
Greece in the 3rd and 4th centuries B.C.

The Scientific Method
1. Ask a question
2. Make an educated guess (a hypothesis)
3. Predict consequences that can be
observed (not inferred)
4. Make observations (do experiments)
5. Formulate a Rule
Hypotheses
An educated guess
 In science, a hypothesis must be
testable:

The Moon is made of green cheese
 Matter is filled with undetectable particles
 There are parts of the universe that will
never be found by man

Speculation

A “speculation” is a statement that
cannot be tested
imaginary people are nice
 that rock formation is pretty
 when we die we go to Heaven or Hell

Limitations
Science cannot tell you how to live your
life
 Science does not decide what is art
 Science does not help with matters of
faith
 Science asks “how”, art asks “who”, and
religion asks “why”
 They can coexist

Technology
Practical uses of science
 Fundamentally different from science
itself

Rules of Science

A fact is simply something that anyone can
observe and agree to be true.
 In science, facts may change depending on
outlook.
 When a scientific fact has been tested over
and over again without disproof, it is called a
law
 A theory is the synthesis of facts and welltested hypotheses
Observations
Observations are a fundamental part of
science. Today, we’ll do an activity
where we use our senses to make some
observations of the world around us.
 Observation does not equal inference

[observation activity]
Chapter 1
Classical Motion
1.1 Aristotle on Motion
(384 – 322 BC)
Recognized falling as a natural motion.
 Recognized collisions as violent motion.

1.1 Aristotle on Motion

Aristotle thought all motions were due to
nature or to a push or a pull
1.2 Galileo (1546-1642)
Whereas Aristotle relied
on logic, Galileo relied on
experiment
 Dropped both heavy and
light objects from a tower

Galileo
The result was that, if we
disregard the effects of air
friction, then all objects fall
with the same “pull”
 http://video.google.com/vid
eoplay?docid=6926891572
259784994&q=astronaut+fe
ather&hl=en

Inertia
Galileo also discovered that force is
needed to start an object moving
 Force is needed to stop an object
 No force is required to keep an object in
motion


INERTIA – the tendency of moving
objects to remain in motion
or?
The experiment
Roll balls down hill. Gravity speeds it up.
 Roll balls up hill. Gravity slows it down.
 Roll balls flat, on a very smooth surface.

Galileo’s Experiment
1.3 Mass
Mass is a measure of inertia
 The more massive an object, the less it
will tend to change its motion
 Measured in kilograms

1L of water = 1 kg
Weight
Weight is the force on an
object due to gravity
 Measured in Newtons
(1 kg m2 s2)
 On Earth, 1 kilogram
weighs 10 Newtons

Sir Isaac Newton
Newton

1 Fig Newton approx. 0.01 kg = 0.1 N
Question?

Does a 2 kilogram block has twice as much
inertia as a 1 kg block?
YES
 Does a 2 kilogram block have twice as much
inertia as a 1 kg bunch of bananas?
YES
 How does the mass of a bar of gold vary on
the moon?
NONE
1.4 Net Forces
A force is a push or a
pull
 Forces have a
direction and a
magnitude
 We call them vectors
 The net force on an
object is the sum of all
forces on that object

Net Forces
1.5 The Equilibrium Rule

When an object is not moving, we say
that it is in equilibrium, and the net force
on the object is zero
F=0
1.5 The Equilibrium Rule
Worksheet
[pages 1 and 2]
Question
What is the sum of the forces on you
right now
 Assume you are not moving relative to
other objects on earth
 (even though we are moving relative to
the rest of the solar system)

1.6 Support Force
There is a force that opposes gravity
 Called the normal force
 Force of the floor or ground pushing up!

1.6 Support Force
Q: If you straddle equally, two scales,
what will the reading be on each scale?
 A: Half your weight (try it!)

Equilibrium and Moving?
An object might be in motion, but if not
accelerating, then it’s in equilibrium!
 Acceleration – change in velocity over
time

change in speed
 or change in direction

Example: moving a desk
Equilibrium and Movement?
F=0
1.8 Friction
A force that opposes motion
 Microscopic surface effect

1.8 Friction
Depends on the material
 Which has more friction? Sandpaper or
marble?

how can we explore the friction
of a variety of materials?
“Speed” and “Velocity”
Speed is distance covered in a particular
amount of time
 Velocity is speed in a particular direction

Equation for Speed
Average Speed = total distance covered
time
 Ex. If I go 100 miles in 2 hours then my
average speed is 100 miles / 2 hours =

50 miles/hour
Constant speed/velocity

Constant speed means
neither slowing down or
speeding up
 Constant velocity is
constant speed and
constant direction
 If you are turning, then
you are accelerating!
http://www.youtube.com/watch?v=uUurALr_Ckk
1.10 Acceleration
Variations in speed or direction are
acceleration
 Note a ball rolling down a hill

Acceleration

Distance between points gets bigger
each second!
Equation
Acceleration = change in velocity
time
Example: If we increase our velocity by 5
meters per second in 3 seconds, what is
the acceleration?
5 m/s
=
1.67 m/s per s = 1.67 m/s2
3s
Deceleration
Negative
Acceleration
 Like a ball rolling
up a hill

Free Fall

When an object is free to fall to the Earth
due to the acceleration of gravity, it
accelerates at about 10 m/s2
10 m/s2 = g
Free Fall
Throwin’ Up

When you throw an object up in the air, it
decelerates (-10 m/s2) until it reaches
zero, and then accelerates at 10 m/s2
Worksheet
[pages 3 and 4]
Hang Time
The time that an athlete can “hang” in
the air depends on how high they can
jump
 Gravity pulls on all athletes the same,
regardless of weight


(ignoring air resistance)
Hang Time Formula
Hang time = Square root (2 h / g)
Example: If you can jump up 1 meter, what is
your hang time?
Hang time = square root (2 * 1 / 10 )
= square root (0.2)
= 0.44 s
Hang Time

Even the best athletes could only hang
for about 0.5 seconds!
Ch. 1 Homework Problems
Due Next Week
 4th Ed. Exercises 1, 6, 9, 10, 17, 22, 25,
28, 29
 Problems 1, 8, 9, 11

Chapter 2
Newton’s Laws
http://www.youtube.com/watch?v=cWOv7NyOnhY
2.1 Newton’s First Law

Every object continues in a state of rest, or
in a state of motion in a straight line at
constant speed, unless it is compelled to
change that state by forces exerted upon it.
http://www.youtube.com/watch?v=iC1zmLgUjco
2.1 Newton’s First Law

Objects at rest stay at rest. Objects in
motion stay in motion.
The Earth Moves!

Copernicus, 1543
The bird problem
The bird problem
How can a bird drop down and catch a
worm if the Earth is moving?
 Earth moves at 30 km/s?
 The bird and the worm are
already moving in the same
direction

Worksheet
[pages 5 and 6]
2.2 Newton’s Second Law
Acceleration is proportional to the force
on an object …
 … and in the same direction …
 … and is inversely proportional to mass.

Newton’s 2nd Law

Acceleration = Force / mass
Higher force means
Higher acceleration
Higher mass means
Less acceleration
Activity
[Newton’s 2nd Law]
When Acceleration = G
free Fall
 gravity pulls twice as hard
on an object twice as big
 But it resists twice as hard
 Acceleration = g

Non-Free-Fall
Free Fall

http://video.google.com/videoplay?docid
=4826509460083131357&q=free+fall&hl
=en
Terminal Velocity
When you are slowed to a point of no
acceleration.
 Constant velocity.
 150-200 km/h without parachute
 15 to 25 km/h with parachute

conclusion:
Force = mass times acceleration
2.3 Forces and Interactions
Forces are pushes and pulls
 Also require interaction
 It takes two to tango …

2.4 Newton’s Third Law
When an object exerts a force on
another object, the second object exerts
an equal and opposite force.
 “To every action, there is an equal and
opposite reaction.”

The Rule
Action: A exerts a force on B
Reaction: B exerts a force on A
The Rule
Example:
Hot gases push on the bullet
The bullet pushes on the hot gases
The foot steps on the curb
The curb pushes back on the foot
Worksheet
[page 13]
The Rule
Example:
Hot gases push on the bullet
The bullet pushes on the hot gases
The foot steps on the curb
The curb pushes back on the foot
Very Different Masses

The force between two objects is always
the same. Sometimes the acceleration
of the heavy object is too small to
observe.
Example
M1A1 = M2A2
Example
Defining your system
If all forces have an equal and opposite
force …
 Then why don’t all forces always cancel?
 Well, depends on how you define your
system.

Earth
Earth
Solar System
Galaxy
Homework for Chapter 2
Exercise 1, 8, 11, 20, 23, 28, 34, 35, 39
 Problems 1, 2, 6, 10, 13, 15

Chapter 3
Momentum and Energy
3.1 Momentum
momentum = mass x velocity
3.2 Impulse
Force times time is called impulse.
3.3 Impulse-Momentum
Impulse is also change in momentum:
Force x time = change in (mass x velocity)
Increasing momentum

If we want to increase momentum
Apply a large force or
 Extend the contact time

Force x time = change in (mass x velocity)
Decreasing momentum

If we want to decrease momentum

Increase time of contact
Force x time = change in (mass x velocity)
Decreasing momentum

If we want to decrease momentum

Increase time of contact
Force x time = change in (mass x velocity)
If someone punches you …
Decrease momentum (short
time)

At short times of contact, forces are
large.
Activity
[egg toss activity]
3.4 Momentum is Conserved
In the absence of an external force, the
net momentum of a system remains
unchanged.
 Law of Conservation of Momentum

Collisions
Momentum before = Momentum after
This is usually only in ideal cases.
We call these types of collisions “elastic”
collisions.
Collisions
Momentum before = Momentum after
This is usually only in ideal cases.
We call these types of collisions “elastic”
collisions.
Collisions

“Inelastic” collisions occur when an
object deforms or if heat (from friction) is
generated.
3.5 Energy
Essential for life
 The ability to do
work (for
instance, to move
an object)
 Much of it comes
from our sun

Work
Work = force x distance
-
Force is exerted
Something moves
The unit is Newtons x meters = N m
= 1 Joule
Problem

How much work is needed to lift an
object that weighs 500 N to a height of
2m?
W = force x distance = 500N x 2m
= 1000 Nm or J
Power
This includes the time
component.
Power = work done /
time interval
Unit is Joules per
second (a.k.a Watt)
Power
Problem

How much power is expended when
lifting a 1000-N load a vertical distance
of 4 m in a time of 2 s?
Power = Work Done/Time Interval
Work = Force x Distance
Power = 1000 N x 4 m / 2 s = 2000 W
Potential Energy

The stored energy that a body
possesses because of its position.
PE = mgh
Kinetic Energy

Energy of motion
KE = ½ mass x speed2 or KE = ½ mv2
Work-Energy Theorem

The work done on an object equals the
change in kinetic energy of the object
Work = KE
or
F d = KE
Problem

Part 1- Calculate the change in kinetic
energy when a 50-kg shopping cart
moving at 2 m/s is pushed to a speed of
6 m/s
KE= ½ m (vf2-vo2)
= ½ 50kg[(6m/s)2-(2m/s)2]
= 800 J
Problem Continued

Part 2- How much work is required to
make this change in kinetic energy?
W=  KE
W = 800 J
Comparing Kinetic Energy &
Momentum

Kinetic Energy
Nonvector (scalar) quantity
 Depends on the square of velocity
KE= ½ mv2


Momentum
Vector quantity (directional)
 Depends on velocity (momentum = mv)

Conservation of Energy

In the absence of external work input or
output, the energy of a system remains
unchanged. Energy cannot be created or
destroyed.
Potential Energy

Only depends on height.
Same potential energy! PE = mass x g x height
Kinetic Energy
A pendulum
PE becomes KE!
Worksheet
[page 25]
3.10 Machines
Multiply forces or change direction of a
force.
 Like a lever
 Levers trade distance for added force.
 Never adds energy!
 Work in = Work out

Lever
Pulleys

Also change the direction of a force
Pulleys
The tradeoff – you lose distance
Efficiency = work done / energy used
Example
Efficiency is ruined by heat loss
Energy Sources
Two main sources, the sun, and nuclear
power.
 The sun relies on nuclear fusion or the
ability to combine atoms
 Nuclear power relies on nuclear fission
or a strong force found within atoms

More sources
Wind (sun heats earth)
 Solar cells (direct electricity from sun)
 Fossil fuels (sun – plants – fossil fuels)
 Hydroelectric (sun – evaporate water –
rain – river – dam)
 Geothermal (caused by nuclear
reactions in Earth’s core)

Geothermal – very clean!
Chapter 3 Homework
Exercise: 1, 2, 7, 18, 30, 36, 42, 48
 Problem: 1, 2, 3, 10, 14
[Finish worksheets]
