Transcript Review Random Topics Right Hand Rule Gyroscopes

FUN SIDE OF MECHANICS
DAY 7:
FUN AND REVIEW
Jonathan Abbott
A TASTE OF QUANTUM: IT’S A WAVE
In quantum, you learn that everything (light and
mass) is a wave. This is pretty hard to believe
because we don’t see it, but it is evident when you
look at things that are very small (like little
particles).
 http://youtu.be/UANVMIajqlA

SPEED SKATING

Basic Inline Skating


http://youtu.be/OXWGB3aQTzI
Double Push

http://www.youtube.com/watch?v=4ZovJ1FPmBg
Think where the friction is and how it is used to
propel the skaters.
 Also notice what the arms do. What physics
concept might the arms help with.

FIGURE SKATING
http://youtu.be/_-xwx-Z3ijc
 Look for countersteering and conservation of
angular momentum

CAR ENGINE

We generally have an internal combustion
engine.

This means that energy from gasoline is released (as
it catches on fire) to kinetic energy using a motor.
http://auto.howstuffworks.com/engine1.htm
 We will look at a piston and learn about some of
the other parts of a car engine.

JOBS (DO THEY USE MECHANICS)

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

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
Check out 
bls.gov for

their
occupation

outlook
handbook 
for tons of
job
information. 
Chemist
Accountant 
Actor
Actuary
Agricultural 
and food
scientist

Architect

Artist

Automotive
mechanic

Bookkeeping 
clerk
Carpenter 
Childcare

worker

Civil

engineer

Coach
Computer

hardware

engineer
Computer

software
engineer

Computer
support

specialist
Cost

estimator

Court reporter
Dancer

Database
administrator 
Designer

Desktop
publisher

Disc jockey

Doctor

Drafter

Economist 
Electrical

engineer

Electrician 
Engineering 
technician

Environmen
tal scientist

Farmer

Financial
analyst

Firefighter

Human
resources

assistant
Judge
Landscape 
architect
Lawyer

Librarian

Loan officer 
Musician

Nurse

Paralegal

Pharmacist
Photographer 

Physicist
Pilot

Police

officer

Politician
Professional 
athlete
Psychologist
Real estate
agent
Recreation
and fitness
worker
Recreational
therapist
Reporter
Secretary
Social worker
Statistician
Surveyor
Systems
analyst
Teacher
Urban
planner
Veterinarian
Webmaster
Writer
Zookeeper
ECONOMICS MODEL LIKE PHYSICS
PHYSICS MODEL OF DAMPED HARMONIC
MOTION APPLIES TO US RECESSION
Ideal growth
GDP
Actual Economy
Second Dip
Recession
Time
REVIEW: WHAT WAS MECHANICS?
Mechanics:
Geometry
motion or
study of
motion
in
Physics
Mechanics
REVIEW: WHAT WAS FORCE, WHAT WAS
NEWTON’S SECOND LAW
Force:
A push
or a pull
Newton’s
Second law:
F=ma.
Force = mass *
acceleration
Mechanics
Force
Newton’s
Second Law
REVIEW: WHAT WAS NET FORCE, WHAT
WAS NEWTON’S FIRST LAW
Mechanics
Force
Newton’s
Second Law
Just keep pedaling,
just keep pedaling
Net Force
Newton’s
First Law
REVIEW: WHAT WAS NET FORCE, WHAT
WAS NEWTON’S FIRST LAW
Mechanics
Force
Newton’s
Second Law
Just keep pedaling,
just keep pedaling
Net Force
Newton’s
First Law
REVIEW: FREE BODY DIAGRAMS
Mechanics
Force
Newton’s 1st
and 2nd Law
Free Body
Diagram
REVIEW: FRICTION – KINETIC VS.
STATIC
Mechanics
Force
Newton’s 1st
and 2nd Law
Free Body
Diagram
Friction
REVIEW: NEWTON’S THIRD LAW &
ACTION REACTION PAIRS
Force
Newton’s 1st
and 2nd Law
Free Body
Diagram
1
2
3
Friction
Newton’s
Third Law
Action
Reaction Pair
REVIEW: COUNTER STEERING
Mechanics
Application:
Balance
Force
Newton’s 3
Laws
Normal
Free Body
Diagram
Friction
Turn right to lean
Countersteering
Turn left
REVIEW: MOMENTUM/COLLISIONS
Mechanics
Application:
Balance
Momentum
Force
Newton’s 3
Laws
Friction
REVIEW: ENERGY, MECHANICAL ENERGY
Mechanics
Application:
Balance
Momentum
Energy
Force
Newton’s 3
Laws
Friction
Kinetic
Potential
translation
height
rotation
elastic
Other
REVIEW: TRANSLATION VS. ROTATION
Mechanics
Translation
Force
Momentum
Newton’s 3
Laws
Free Body
Diagram
Friction
Rotation
Energy
REVIEW: TORQUE, MOMENT OF INERTIA,
ANGULAR MOMENTUM
Translation
Rotation
Force
Torque
Mass
Moment of
Inertia
Momentum
Angular
Momentum
REVIEW: TWO METHODS OF BALANCE
Countersteering
Twist body
Continuum
Change Point of
Contact
Change Shape of
Body
Translation
Rotation
Use both
techniques to
balance best
REVIEW IMPULSE
Mechanics
Translation
Force
Rotation
Energy
I should
have
worn a
helmet…
Momentum
Newton’s 3
Laws
Free Body
Diagram
Friction
Impulse
Impulse = Change in Momentum = Force * Time
CENTRIPETAL FORCE REVIEW



We call this force that causes circular motion the
centripetal force.
‘Centripetal’ means center seeking
Why might the rollercoaster passengers not fall out
during the loop shown below? [Hint: draw a free body
diagram]
CENTRIPETAL FORCES EXAMPLE

Conceptual Question: Why are roads banked?
RIGHT HAND RULE

Remember angular momentum (w)?
We used an arrow to describe rotation.
 A bigger arrow = spin faster, more rotational inertia

We can use an arrow to describe torque (and it
relates to angular momentum).
 Torque = r x F
 Torque = radial vector cross the force

RIGHT HAND RULE
Torque = r x F
 Torque = radial vector cross the force
 Curl your fingers from the radial vector to the
force (your palm).
 Your thumb here should point ‘out of the page’

RIGHT HAND RULE CONTINUED

The cool thing is that the torque vector points in
the direction the angular momentum is
increasing.
PRECESSION/GYROSCOPES
A spinning object can
precess or rotate when
a torque is applied
perpendicular to the
axis of rotation
 This is due to
conservation of
angular momentum
and can be shown
with the right hand
rule.
 http://youtu.be/8H98B
gRzpOM

http://www.flickr.com/photos/loopzilla/94042474/sizes/m/in/photostrea
m/
ADVANCED TURNING AND GRAVITATIONAL
POTENTIAL ENERGY
By leaning your unicycle, you automatically want to
turn.
 You can do this: Have you ever ridden a bike with no
hands? How can you turn? You turn by leaning the
bicycle underneath you. Gravity pulls on your body,
putting pressure on the tires making you want to turn.
 It’s hard to see without some pictures…

28
LEANING
29
RIDING A BIKE WITH NO HANDS
I don’t recommend it, but it is possible.
 Let’s go back to the unicycle. I said that when the
unicycle wheel was leaning to one side, the
unique would naturally “turn.” This was due to
gravitational potential energy. Let’s explore this
further.

GRAVITATIONAL POTENTIAL WELL
The unicycle wants to be the closest to the
ground that it can (too bad for me)…
 So if the unicycle is tilted front or back, the
center of mass of the unique actually is higher
than if the unicycle is square to the ground.

Notice how the
unicycle wheel is
actually shifted up
in the right picture.
Gravity pull the
unicycle to it is
closest to the
ground like the left
picture.
WHAT DOES THIS MEAN?
This means that if you can ‘tilt’ the wheel,
gravity will do the work to turn the wheel for
you. YOU CAN DO THIS!!!
 Bikes are designed so that the front wheel is
actually “unstable” and the front wheel will
naturally want to turn so the bike is closer to the
ground

UNSTABLE: TILT = TURN
As you tilt the bike (by twisting your
body) you make the front wheel
naturally try to turn in the direction
you are leaning
 Then you have to do some impressive
countersteering in order to fully stay
upright.
 Note: why doesn’t the front wheel
easily turn?
“Axis of
Rotation”

The faster you go, the greater a
“restoring force” (friction) keeps wheel
straight.
 Hence, you have to be going fairly fast to
ride with no hands.
Gravity
Normal
force

Friction
FRONT TIRE: A CLOSER LOOK
Looking down from “axis of rotation”
“Axis of
Rotation”
Gravity
Friction
The net torque is
Normal small; the
force
restoring force of
friction depends on
your speed as you
turn
The net torque
about the axis of
rotation is small
because friction is
a great restoring
force.
Normal
force
Friction