Week 3
Video č You are expected to view this material in a venue where you can
concentrate. You may view the talk several times and you are allowed to take
notes.
When you have finished, you must
complete a short quiz. The quiz will ask a few questions about the material
presented in the video. If there are aspects of the talk that are confusing the
student should research the topic. However, the student is not expected to have
a detailed knowledge of the subject but should be able to explain the video to a
colleague.
Catherine Mohr: Surgery's past,
present and robotic future http://www.ted.com/talks/catherine_mohr_surgery_s_past_present_and_robotic_future.html
Announcements:
First HW set is being posted. I will place it in the HW1 folder and
students will have till the end of September to complete it.
You have 10 tries.
This should be enough to get the answer but not enough to simply get the
correct answer by guessing.
Yes, you get credit but HWs are also about mastering the
important material. I have held numerous
HW sessions where students delight in getting the correct answer despite not
understanding how they worked the problem.
This is a formula for a low grade.
With help from various sources most students should manage to receive a
decent grade for this portion of the course.
The best students will be able to duplicate this level of work on the
exams.
New video this week and I expect you to watch it and
complete the quiz by Friday. While I will often leave the quizzes open longer
so that students that encounter problems have time to resolve them, it is not
my intention to add extra time. In other
words I have added a grace period to the due date to allow for busy schedules,
unexpected assignments etc. so plan on completing assignments at least one day
ahead of the due date.
We need to understand how we describe motion. There are some basic notions that are
critical:
Average 
Traveling to DC you clock 120 miles on your odometer and
see that you arrive at your destination in 2 hours. 60 mph would be your average speed. You are well aware of the fact that this
doesn’t mean that your speedometer reads 60 mph at all points in the
trip. As a matter of fact an average
value may be a value that your speedometer never really reads. For example, I could travel
part of my trip (20 minutes) at a constant speed of 50 mph and then travel
another part (20 minutes) at 70 mph.
My average speed would be 60 mph even though no significant portion of
the trip was driven at that speed. Value which characterizes the
entire process. 
instantaneous 
During my trip I could monitor and plot or record the
value of my speedometer. This would be
my instantaneous velocity. A value that characterizes a very short element or period
of a process. 


Position 
Vector Establish reference (origincoordinate system x,y,z) – Find magnitude an
direction to describe a location wrt the
origin. Given two positions you can
find the change in position. This is the position of the second location but wrt the first position. 
distance 
Scalar – Number that describes how far irrelevant of
the direction 


Velocity 
Vector – change in position / change in time 
speed 
Scalar Magnitude of the velocity (speedometer) 


Acceleration 
Vector –change in velocity / change in time 
‘Amount” 
Scalar you can discuss simply the amount of acceleration
without reference to the direction 


Delta D 
D is short hand notation for a
difference or a change. You can find the change in temperature, change in height,
change in momentum, change in force, change in position. The result of the calculation preserves the
nature of the objects in that numbers remain numbers and vectors remain
vectors. So that a change in velocity is a vector. 
We need to know what these objects are and the relationship
among them. We need to be able to
calculate values based on other information (word problems). We need to be able
to represent these quantities on graphs (limit the problems to 1d).
Lec6Week3
Newton’s Laws
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natural state of motion is rest or
uniform motion
revisit inertia (discuss motion of
earthč
like sitting on a very fast moving train, 70,000 mph, speed of sound is 768
mph, plane rocket 5000 mph, Saturn rocket
25,000, train few hundred mph)
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Force is a push or a pull.
We know what a force is but we probably do not know how forces change
motion. The problem is that there are
usually several forces at play and we don’t break down motion so as to see the
connections. As a matter of fact, it took two of the
smartest people of all times to finally figure out motion, Galileo and Newton
Normally you relate the total force to the total
acceleration.
total balance
total accleration
Consider the throwing and catching of a ball. [Work only
with the horizontal motion and neglect falling motion and forces. If necessary we can simply roll the ball
along the foor so that the gravity force and the
normal force cancel out.] (One
interesting thing about vectors is that the results of forces, accelerations, velocities can be analyzed in perpendicular directions
separately. Break vectors into x,y,z
components. Analyze the individual x,y,z
motion. Combine these separate motions together for the total.)
Force of gravity on earth
We live in an environment where everything is being pulled
downward in proportion to their mass. This in combination with Newton’s 2^{nd} law.
Although now we know that there is a relationship between
space, time and gravity č General Relativity that explains the above equality
for this class it is a mystery why these two fundamentally different
definitions should have the same value.
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action interaction
key is defining the system
Recognize that different forces act on each system but some of these forces are related by Newton’s third law. The
action reaction rule gives a relationship where one finds two forces. The forces are the same magnitude and
directed oppositely and , HERE IS THE KEY, each force
acts on a DIFFERENT system.
Analysis of the motion of objects requires us to define what
are the objects of interest.
Example:
Cars on the highwayč We
don’t care what is happening to the objects that are considered part of the
car. So if two people are pushing each
other (or the seat is pushing up on the driver and the driver due to gravity is
pushing down) the forces are internal and though they may result in parts
changing position we can define the position of the car, center of mass, in a
way that is independent of the internal forces.
If we don’t allow anything in the car to move we can see that action and
reaction between two parts cancel. This may
be one reason why there is so much difficulty understanding Newton’s third law.
Horse and buggy.
Flat surface
Analyze as two separate systems
Horse pushes on the ground ground
pushes back: 1 force on the horse.
The horse pulls the buggy the buggy pulls back
: 2nd force on the horse
Gravity pulls the horse down 3^{rd}
force
Horse pushes down on the the earth
pushes up: 4^{th}
For horizontal motion F3=F4 sum is 0.
Buggy has the same forces 3,4 that will balance
Wheels (frictionless motion?
surface does allow for horizontal forces but
the wheel eliminates frictional losses. Wheels on ice are useless. They don’t
work because the smooth ice surface removes all horizontal surface forces. A
wheel requires large horizontal surface forces but these forces are no longer
able to retard the motion of the object so they may not be referred to as
frictional forces. For uniform motion the part of the wheel that touches the
surface comes to rest on the surface. For accelerated motion the wheel is at
rest but pushing on the ground. Consider your foot as you take a step. You push the ground back. If you are running you are trying to touch
the ground with a small relative speed.
If you don’t you slip or slide.)
when 1&2 balance and 3&4 balance No total force č constant motion
dealing with the cart and the horse separately we find that the horse pushes on the ground and the ground pushes on the horse. The horse will then pull on the cart with the exact amount of force so that the buggy moves with the horse. The buggy of course pulls back so that the total force on the horse is the ground force minus the buggy force and the total force on the buggy will be the force of the horse. The other forces cancel. If the buggy is very light then there is only a small force between the two. If the buggy is very heavy (mass is the critical factor) then the forces will be large.