Transcript PHYS111_12 - MrLaFazia.com

Conceptual Physics
THURSDAY September 30th, 2010
LESSON GOALS:
• Discuss nature of Forces
• Look at history of Galileo and his contributions to Force concepts
• Run Interactive Physics simulation 6.3
• Complete real-life experience discussion prompt
• Review important definitions (with corresponding simulations)
• Read Poem Newton Stole the Limelight
• Take a quick quiz
http://mrlafazia.com/PHYS111/lessons/201011Fall/PHYS111_12.ppt
1/19
Discussion
• “What is a force”?
– Is it a physical thing?
– Are there many types?
– Is there a single equation that describes forces?
• “How do forces relate to our Energy
concepts”?
2/19
Galileo Galilei on Inertia
a MrLaFazia.com original snapshot
NOTE:
Galileo lived
from 1564 to
1642… NOT
1489!
SOURCE:
http://www.cartoonstock.com/newscartoons/cartoonists/bro/lowres/bron1636l.jpg
3/19
Introduction & Overview
• Galileo’s life teaches us many things
– (not the least of which is—DON’T STARE AT THE
SUN THROUGH A TELESCOPE!)
• For today, however, we will discuss his ideas
on MOTION
1) the Leaning Tower of Pisa
2) defining Force and the role of friction
3) Inertia (ramp experiments)
4/19
The Leaning Tower of Pisa
• This tower portion is considered “legend,” of course…but...
• Wherever he dropped the objects, Galileo showed that less
massive objects fall at the same rate as more massive ones
– air resistance must either be equal or eliminated
SOURCE: http://improbable.com/airchives/paperair/volume15/v15i3/v15i3-webimages/KIM-Galileo_of_the_lem_opt.jpeg
5/19
Defining “Force”
• Forces are most simply defined as
SOURCE:
http://goozydumps
.files.wordpress.c
om/2008/09/galile
os_experiment.jpg
– A PUSH
– Or A PULL
• It was once believed that forces were needed to KEEP
objects in motion.
– People believed this because life taught them that objects slow
down and eventually stop (like your car not moving) unless a
force keeps them going.
• Galileo showed that objects need no force to maintain their
motion.
– What people originally had not realized was the fact that friction
is a FORCE as well!
• The fact is: Forces that aren’t balanced out will ALWAYS
cause a change in any object’s motion (whether speeding
up or slowing down or changing direction)!
6/19
Inertia
• Galileo did a number of experiments to test
his theories of forces and motion:
7/19
SOURCE:
http://www.cartoonstock.com/cartoonview.a
sp?catref=mhen235
8/19
Inertia
• Imagine if a ball were allowed to roll down a
ramp and roll right onto a level ramp.
– Without a force to slow it down or speed it up,
what would happen to its motion?
• THIS is the concept of “inertia.”
– What would be needed to speed up the ball?
– What would be needed to slow down the ball?
– ______________
Frictional Forcesslow down the ball in the real
world. This does NOT violate the concept of
Inertia, but rather justifies it!
9/19
The Law of Inertia
10/19
Forces…Motion…& Energy?
• Do the results of the ramp experiments (see page
32, Figure 3.4 of Conceptual Physics, if applicable)
match up with our understanding of the
Conservation of Energy (“Total Energy is Boring”)
Principle?
• Perhaps you have heard the formation of these
concepts attributed to Sir Isaac Newton or
another scientist, before. There is a somedayfamous poem which describes this misconception
quite well. I will read it to you at the end of the
class.
• STAY TUNED!
11/19
A Final Bit of Humor
SOURCE:
http://emdashes.com/assets_c/2
010/04/Galileoroll2-thumb182x224.png
12/19
Interactive Physics
• Open and view Simulation 6.3….
13/19
Discuss With A Partner
• “Describe an experience you have had in life
where you have had the following things
happen (or have caused these things to
happen)” :
• Had to keep an object moving (how did you do this?)
• Had to keep an object FROM moving (how did you do
this?)
• Had to stop an object while it was already IN motion
(again, how did you do this?)
• Try to provide a name (of your own choosing) for each
force involved!
14/19
Definitions (w/ Simulations)
– Mass vs. Weight
• PLEASE learn the difference.
– European scales?
– American vs. European Scales on the Moon?
• Simulation: Interactive Physics 4.1; 4.2.
– Inertia
• Note that this is NOT an actual “quantity.”
• Simulation: Interactive Physics 6.4.
– Net-Force
• Simulation: Interactive Physics 6.5; 6.6.
– the Equilibrium Rule
• possible both while moving AND while not moving
– terminal speed (or “terminal velocity”)
• Simulation: Interactive Physics 5.1; 5.2.
– support force
• also known as the “normal” or “perpendicular” force
– I primarily call it the “Normal” force…just so you know.
15/19
Poem:
Newton Stole the Limelight, by D. LaFazia
The English stole the limelight
While history is clear
That an Italian earned the copyright
To the Physics law presented here.
A generation before Newton
--knighted though he was-Galileo Galilei
Had already killed Newton's buzz.
Take Newton's Law of Inertia
(you can steal it like he did).
This "1st Law" as they call it
Was discovered before Newton was a kid.
Or even of his 3rd,
But when you hear of his blatant copying
You'll mark him as a lying, stealing nerd.
Now my aim is not to disrespect
Or slander Newton's name.
His contributions rightly earn him
A considerable amount of fame.
All that I am saying
(and on this I must be right)
Is that the English are plainly guilty
Of stealing Italy's limelight.
Forza Italia!
Galileo knew that objects
Simply do not change their speed
Unless something else acts on them
To precipitate this need.
So objects that are moving
Will keep moving as they were
And objects standing still at first
Need a force to make them stir.
You may not know of Newton's 2nd
16/19
Quick Quiz
“A parachutist jumps out of an airplane from the usual height (under 3 miles
above the Earth’s surface). She is of average size and mass. Describe her
descent in the real world (with air resistance) at the following segments
of her fall”:
• What happens during the time from when she has just jumped out of the
plane to just before she pulls the ripcord to deploy her ‘chute?
• In the first few moments during and after she pulls the ripcord to deploy
her ‘chute, does she (choose the appropriate response)
– Fall faster
– Rise upward suddenly
– Fall slower
– Experience no change in her motion?
• Would her eventually-reached terminal velocity be more, less, or remain
unchanged if she had never opened the ‘chute?
• BONUS) Describe how the parachute would respond and affect her
descent if there were no air resistance.
17/19
Recommended Reading:
Sections 3.2-3.7
Sections 4.1-4.4
These sections of the text prepare you for the Force concepts
from Unit 2. We will look more at the motion concepts in later
lessons within Unit 2.
18/19
Looking Ahead:
• Next class we will take a closer look at Force concepts by exploring
Newton’s Laws of Motion.
• You will also be assigned HMWK #5 next class.
19/19