PH 211 Winter 2014
Friday January 31
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ac5
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Which topic would you like to discuss in class on
today?
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37f
Number of homework questions in
MP. What do you think?
A. I would like to see
more questions
assigned.
B. I am happy with the
current number.
C. I think there are too
many questions.
71%
22%
...
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it h
nk
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hi
ha
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Ia
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pp
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o
et
lik
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to
o
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se
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or
cu
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..
re
..
7%
Cotton Ball Experiment
1
2
𝐷 = 𝑎𝑒𝑓𝑓 𝑡
2
Measure fall 2 m takes 1.2 s, a-eff = 2.7 m/s2
Measure fall 1 m takes 0.7 s, a-eff = 4.0 m/s2
Cotton Ball Experiment
1
2
𝐷 = 𝑎𝑒𝑓𝑓 𝑡
2
A car goes around an upward curve (like a large hill) while
maintaining a constant speed. Which of the following is
the correct direction of the acceleration of the car
1.
2.
3.
4.
Upward
Zero
Forward
Downward
A car goes around an upward curve (like a large hill) while
maintaining a constant speed. Which of the following is the
correct direction of the acceleration of the car
84%
Do
w
nw
ar
d
ar
d
w
Fo
r
ro
1%
Ze
d
ar
Upward
Zero
Forward
Downward
Up
w
A.
B.
C.
D.
9%
6%
This is the angular velocity graph of a wheel. How
many revolutions does the wheel make in the
first 4 s?
f
Slide 4-85
This is the angular velocity graph of a wheel. How many
revolutions does the wheel make in the
first 4 s?
90%
4%
10
8
2%
1%
6
2%
4
2
4
6
8
10
2
A.
B.
C.
D.
E.
circular motion:
Uniform Circular motion
• KNOW: acceleration is toward center of circle
• Observation: hitting a bowling ball
– http://paer.rutgers.edu/pt3/experiment.php?topici
d=5&exptid=56
• Observation: rollerblading
– http://paer.rutgers.edu/pt3/experiment.php?topici
d=5&exptid=39
• What can we hypothesize from these observations?
A ball is allowed to rotate in the circular ring. What
will be the motion of the ball once the board is
removed? USE your hypothesis to make your
prediction!!
1.
2.
3.
4.
It will continue to move in a circle
It will exit the ring moving straight with no more curve to the
motion
It will exit the ring and move outwards
It will exit the ring in an arc, making a curved path that isn’t as
‘tight’ as the original circle
http://paer.rutgers.edu/pt3/experiment.php?topicid=5&exptid=57
A ball is allowed to rotate in the circular ring. What will
be the motion of the ball once the board is removed?
USE your hypothesis to make your prediction!!
It will continue to move in a
circle
B. It will exit the ring moving
straight with no more curve
to the motion
C. It will exit the ring and move
outwards
D. It will exit the ring in an arc,
making a curved path that
isn’t as ‘tight’ as the original
circle
86%
A.
1%
e
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co
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to
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..
n.
m
w
It
9%
4%
ill
it
ex
th
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g
r in
w
It
d
an
ill
.. .
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t
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g
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a.
Conclusions and representations
• In order to accelerate, we need a force IN THE DIRECTION of the
acceleration:
• Force is proportional to acceleration
• Force and acceleration are both represented by vectors
• We know how to find acceleration from motion diagrams –
there must be a (total) force in that direction
• Draw a diagram WITH COORDINATE AXIS (necessary to
represent a vector!)
• Show the direction of the (total) force
• Check if it is in the direction of acceleration
ForceAcceleration
Newton’s second law:


F  ma
All mechanics follows from this!
Copernicus 1473-1543
Brahe 1546-1601
Kepler 1571-1630
Galileo 1564-1642
Andreas Cellarius:
Celestial Atlas
1660
Newton 1642-1727
Woolsthorpe
Quotations
• If I have been able to see further, it was only
because I stood on the shoulders of giants.
Letter to Robert Hooke
Quotations
• I know not what I appear to the world, but to
myself I seem to have been only like a boy
playing on the sea-shore, and diverting myself
in now and then finding a smoother pebble or
a prettier shell, whilest the great ocean of
truth lay all undiscovered before me.
Quoted in D Brewster, Memoirs of Newton
Quotations
• I will not define time, space, place and
motion, as being well known to all.
Principia Mathematica
• Truth is ever to be found in the simplicity, and
not in the multiplicity and confusion of things.
Units
Unit of force: Newton
A force of 1 N gives a mass of 1 kg an
acceleration of 1 m/s2
Drawing Motion Diagrams AND Free Body
Diagrams for simple situations:
• a hockey puck being pushed NW along a frictionless
surface
• a baseball mid flight (with and without drag)
A car goes around an upward curve (like a large speed
bump) while maintaining a constant speed. Which of the
following is an accurate representation of the VERTICAL
forces acting on the car? g = ground, e = earth, c = car
Fgc
1
1. One
2. Two
3. Three
Fgc
2
Fec
Fec
Fgc
3
Fec
A car goes around an upward curve (like a large speed bump)
while maintaining a constant speed. Which of the following is an
accurate representation of the VERTICAL forces acting on the
car? g = ground, e = earth, c = car
55%
39%
re
e
Th
Tw
o
6%
On
e
A. One
B. Two
C. Three