Transcript Lecture 9

Physics 218
Lecture 9
Dr. David Toback
Physics 218, Lecture IX
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Overview
• Today’s lecture is about problem
solving for Chapters 4 & 5
• Learn how to use everything we’ve
learned so far to solve problems
with:
– Rope
– Friction
– Friction and Uniform Circular Motion
Physics 218, Lecture IX
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Physics 218, Lecture IX
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Rope Problems
A PHYS218 rope is a perfect rope. It is
massless and it doesn’t stretch. This
means:
• The acceleration of any part of a rope is
the same as any other part
• The magnitude of the acceleration of the
two things it attaches are equal
• The tension (Force) exerted by each end
is the same
Physics 218, Lecture IX
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Two Boxes and a Pulley
You hold two boxes, m1 and
Ignore the mass of the pulley
m2, connected by a rope
and rope and any friction
running over a pulley at
associated with the pulley
rest. The coefficient of
kinetic friction between
the table and box I is m.
You then let go and the
mass m2 is so large that
the system accelerates
Q: What is the magnitude
of the acceleration of the
system?
Physics 218, Lecture IX
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Banking Angle
You are a driver on
the NASCAR circuit.
Your car has m and
is traveling with a
speed V around a
curve with Radius R
What angle, Q, should
the road be banked
so that no friction is
required?
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Skiing
You are the ski designer
for the Olympic ski
team. Your best skier
has mass m. She plans to
go down a mountain of
angle Q and needs an
acceleration a in order
to win the race
What coefficient of
friction, m, do her skis
need to have?
Q
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Skidding on a Curve
A car of mass m rounds a curve on
a flat road of radius R at a speed V.
What coefficient of friction is
required so there is no skidding?
Kinetic or static friction?
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2 boxes connected with a string
Two boxes with masses m1 and m2 are placed on a
frictionless horizontal surface and pulled with a
Force FP. Assume the string between doesn’t
stretch and is massless.
a)What is the acceleration of the boxes?
b)What is the tension of the strings between the
boxes?
M2
M1
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An Incline, a Pulley and two Boxes
In the diagram
given, m1 and m2
remain at rest
and the angle Q
is known. The
coefficient of
m2
static friction is
m and m1 is
known. What is
the mass m2?
Ignore the mass of the pulley
and cord and any friction
associated with the pulley
Physics 218, Lecture IX
Q
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Is it better to push or pull a sled?
You can pull or push a sled with the same force
magnitude, FP, but different angles Q, as shown in the
figures.
Assuming the sled doesn’t leave the ground and has a
constant coefficient of friction, m, which is better?
FP
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Next Week
• HW4 Due Monday
• Labs & Recitation as usual: Chapter 5
problems
• For Lecture next Tuesday
– Reading
• Chapter 6
• Chapter 7, Section 1 & 2
– Reading Questions Q7.1 and Q7.2
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Physics 218, Lecture IX
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Before we begin…
• Exam this Thursday
– I will give you a formula sheet
• A copy is already on web
– Here at 8:00AM
– Only on topics through Chapter 3 on
Syllabus
– 3 multi-part problems: Some short answer,
dominated by chapter 3 topics and full
length problems using only variables
• Today’s lecture not on the exam
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Yet more before we begin
• HW1, HW2, HW3 (+ associated quizzes)
and math quizzes are now past due
– If you need an extension, I need an email
• Mini-practice exam becomes available when
you have gotten perfect scores on all of the
above
• 5 Bonus points on the exam if you get a
perfect score on the Mini-Practice BEFORE
the exam on Thursday
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Next time
Thursday
– I will give you a formula sheet
• A copy is already on web
– Here at 8:00AM
– Only on topics through Chapter 3 on
Syllabus
– 3 multi-part problems: Some short answer,
dominated by chapter 3 topics and full
length problems using only variables
• Today’s lecture not on the exam
Physics 218, Lecture IX
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Thursday and Next Week
• Thursday: Exam
– I will give you a formula sheet (new version on the web)
– Here at 8:00AM
– Only on topics through Chapter 3 on Syllabus
– Make sure you know your section number and your UIN
• HW4 Due Monday
• Labs & Rec as usual, work on Chapter 5
• For Lecture next Tuesday
– Chapter 6
– Chapter 7, Section 1 & 2
– Reading Questions Q7.1 and Q7.2
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Exam 1 Results from Last Semester
• Overall:
Mean=52/75 or ~70%
• Green: People who took
the mini-practice exam:
Mean = 56(+5)/75
• Red: People who didn’t:
Mean = 38/75
My advice is to study for the exam by finishing all the HW
and testing whether you really understand by turning them
all in and getting a 100 on the mini-practice exam by
yourself in as few tries as possible
Extra problems atPhysics
the end
of my
218, Lecture
IX lecture notes
18
Need help?
• Supplemental Instruction is free and meets 3
times a week.
• Helpdesk has hours everyday and is free.
– Hours are posted on my website
• Private tutoring available by appointment
For more information about all of these see:
http://physics218.physics.tamu.edu/help/
Physics 218, Lecture IX
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• Overall: Mean=52/75
or ~70%
• Green: People who
took the mini-practice
exam. Mean =
58(+5)/75 (not shown)
• Red: People who
didn’t. Mean = 44/75
• Bonus points will be
added later.
Students
Exam 1
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The Skier in slush
A skier is going
down a mountain
which is at an
angle Q. She has
mass m but the
snow is so slushy
that she has
constant velocity.
Q
What is the
coefficient of
Physics 218, Lecture IX
friction m?
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Conical Pendulum
A small ball of mass m
is suspended by a
cord of length L and
revolves in a circle
with a radius given
by r = LsinQ.
1. What is the velocity
of the ball?
2. Calculate the period
of the ball.
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Circular Motion Example
A ball of mass m is at the end of a
string and is revolving uniformly in
a horizontal circle (ignore gravity)
of radius R. The ball makes N
revolutions in a time t.
a)What is the centripetal
acceleration?
b)What is the centripetal force?
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