PHSX213 class
• Class stuff
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Questionnaire (hand in at end of class)
Homework comments
Midterm Exam
Problem Solving Session tonight ?
More Newton III
Friction
Drag force
Circular motion
Wed. Feb. 9th
1
Questionnaire (Overall Goals)
Name:
(optional)
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1. What do you expect to learn from this course?
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2. What do you hope to do with this new knowledge?
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3. What do you expect the class time to do for you?
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4. What do you expect the book to do for you?
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5. How many hours per week do you think it will take to learn all you need to know from
this
course?
Wed.
Feb. 9thInclude class-time, lab, homework etc.
2
PTO
Questionnaire (Specifics)
Your opinions / suggestions please
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1. What do you think of the overall difficulty/pace?
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2. What do you think of the personal response system (clickers)?
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3. What do you think of the eGradePlus homework assignments?
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4. If you were teaching this class, what would you do differently ?
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5. Any other comments ?
Wed. Feb. 9th
3
HW Comments
Wed. Feb. 9th
4
Midterm Exam
• ( r, t ) = ( Budig 120, Wed. Feb. 16th 8:00 – 9:30 PM )
– We will have some small corner of this (huge) room.
• Chapters 1-6.
• If you cannot make this exam time because of some university
authorized activity – you need to let me know asap, and
preferably document it with an E-mail.
• The majority of the exam will consist of multiple choice
questions (5 options) similar to the ones from the practice exam
I’ll distribute on Friday.
– Questions will be a mix of conceptual and short calculations.
– Will contain a brief formula sheet (see practice exam).
– Only calculator and pencil allowed in exam.
• The remainder will probably consist of 1 or 2 questions a bit like
the homeworks which should be answered more in the style of
my written homework solutions.
Wed. Feb. 9th
5
Problem solving session tonight ?
• Malott 1089. 6:00 PM - > 7.15 PM.
• If I’m not there check my office.
Wed. Feb. 9th
6
Reading Quiz
• Which is the only one of the following that
is a force ?
• A) inertia
• B) weight
• C) mass
• D) pressure
• E) torque
Wed. Feb. 9th
7
Monday’s Check-Point 2
• A lead block is suspended
from your hand by a
string. The reaction to the
force of gravity on the
block is the force exerted
by:
• A. the string on the block
• B. the block on the string
• C. the string on the hand
• D. the hand on the string
• E. the block on the Earth
Wed. Feb. 9th
8
Check-again
• How many forces are involved,
if you were to draw free-body
diagrams for all the forces acting
on the hand, string, block and
Earth in the context of that
problem ?
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A) 5
B) 7
C) 9
D) 11 (by my counting)
E) 13
Wed. Feb. 9th
9
A worked example
Problem 5.42
Ah hah, a “rope problem”. Maybe
Newton III is useful here …
What is m2 ?
Wed. Feb. 9th
10
Problem notes
• Draw a “free-body diagram”, considering
ONLY the forces acting on one single body.
– i.e. only the forces that may cause this body to
accelerate.
• Sometimes, it may be advantageous to draw
a free-body diagram for a system of objects
Wed. Feb. 9th
11
Check-Point 2
In the figure is the
tension in the string
greater than,
less than, or equal to
the weight of block B?
A. Greater than
B. Less than
C. Equal to
Wed. Feb. 9th
12
Frictional Forces
• For now we’re mostly concerned with a
macroscopic description of friction.
• Discuss how things move
• Recognize :
– Different surfaces slide differently on other surfaces
• Characterize with coefficients of friction, ms, mk
– Frictional forces are involved in opposing objects
starting to move (static (ie. stationary) friction)
– Frictional forces are also involved in opposing the
motion of moving objects (kinetic (ie. moving) friction)
Wed. Feb. 9th
13
Frictional Forces
• Depend on the magnitude of the normal force, FN
• Static friction : Fsfr ≤ ms FN = Fsfrmax
• Kinetic friction : Fkfr = mk FN
• N.B. These are NOT vector equations (Ffr and FN
are in different directions !). The coefficients are
dimensionless numbers.
Wed. Feb. 9th
14
Friction : microscopic explanation
• Bonds or “cold-welds”
form at points of
contact between the
surfaces
• Force is required to
break the welds to
– Get the body moving
– Keep the body moving
as welds continue to
form
Wed. Feb. 9th
This is also qualitatively explains
why usually, ms > mk
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Static friction on incline
Show that ms = tanq
Wed. Feb. 9th
16
Another worked example
Problem 6.23
When the three
blocks are
released, they
accelerate at 0.5
M
2
m/s . What is the
coefficient of
kinetic friction
between block 2
and the table ?
Wed. Feb. 9th
2M
2M
M
M
17
Check-Point 3
• Why do raindrops fall with constant speed during
the later stages of their descent?
• A. The gravitational force is the same for all drops
• B. Air resistance just balances the force of gravity
• C. The drops all fall from the same height
• D. The force of gravity is negligible for objects as
small as raindrops
• E. Gravity cannot increase the speed of a falling
object to more than 9.8m/s
Wed. Feb. 9th
18
Check-Point 3
• Why do raindrops fall with constant speed during
the later stages of their descent?
• A. The gravitational force is the same for all drops
• B. Air resistance just balances the force of gravity
• C. The drops all fall from the same height
• D. The force of gravity is negligible for objects as
small as raindrops
• E. Gravity cannot increase the speed of a falling
object to more than 9.8m/s
Wed. Feb. 9th
19