Transcript PPT

Physics 218
Lecture 7
Dr. David Toback
Physics 218, Lecture VII
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Before we begin
• Exam 1 Next Tuesday
– Only on topics through Chapter 4
• Homework and the Mini-practice Exam
– When you have 100% on all of them you
will have access to a mini-practice exam
– Get a 100% on the mini-practice exam
BEFORE the in-class exam, get 5 bonus
points
– Mini-practice exam is the right way to
study!
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Checklist for Today
•Things that were due last Thursday:
– Chapter 4 reading
•Things that are due yesterday (Monday):
– WebCT Prelim, Math Quizzes, Ch.2 HW
•Things that are due today:
– Reading for Chapter 5
•For this week and/or due next Monday:
– Read Chapter 6 for Thursday
– Recitation: Probs from Chap 3&4
– Lab 3
– All Ch3&4 problems on WebCT due Monday
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Chapters 5 & 6
• 3 Lectures for Chapters 5 & 6:
– One lecture on Chapter 5
– One lecture on Chapter 6
– One lecture on problem solving
• Lecture Reading Assignments
– Chap 5 due today
– Chap 6 due Thursday
Today’s lecture not on the exam
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Overview of Chapter 5
Where we’re going and why
•
•
•
•
– Dynamics vs. Kinematics
Force
Newton’s Laws of Motion
Normal Force
Example problems
– Note: It’s important to be good at
2-Dimensional motion at this point
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Where we’re going and why
Moving from: “How things move” Kinematics
To: “Why things move that way” Dynamics
• Why do you care? Different questions:
– Old: What acceleration do you need to go
from 0 to 60mi/hr in 6 sec?
– New: How much force does your car
engine need to exert to do it?
• Use all the kinematics, vectors and calculus
from Chapters 1-4
• Plan: Do the concepts, then do the
problems
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Newton’s Laws
You shouldn’t memorize them, rather you
need to be able to understand and use
them
• Don’t write them down from the overheads,
they’re in your book.
• We’re going to translate them into English
• Big picture:
Force
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First Concept
What is a Force?
• Examples:
– Push
– Pull
– Slap
– Gravity
– Others?
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Newton’s First Law
“Every body continues in it’s
state of rest or of uniform
speed in a straight line
unless acted on by a nonzero net force”
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Translate that into English: Force
To cause an acceleration
(change the velocity) requires
a Force
or
If there is an acceleration,
there must be a net Force
Force is a Vector
Add up all the forces (vectors) to
find the Net (or total) force
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Newton’s First Law
• Example of non-zero net forces:
– Friction: Makes a moving block a slow down
– Gravity: Makes a ball fall toward the earth
• Example of zero net force
– Car just sitting on the pavement
• No velocity, no acceleration→no net force
– Rocket ship in outer space
• Nothing to slow it down → constant
velocity → no net force
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Newton’s Second Law
“The acceleration of an object
is directly proportional to the
net force acting on it and is
inversely proportional to its
mass. The direction of the
acceleration is in the direction
of the net force action on the
object”
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Translate: Newton’s Second Law
The acceleration is in
the SAME direction
as the NET FORCE Vector Equation :


 This is a VECTOR
F  ma
equation
 If I have a force,
Fx  ma x , Fy  ma y
what is my


acceleration?
Weight  W  mg
 More force → more
acceleration
 More mass → less
acceleration
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Force to stop a car
You are a car designer. You must
develop a new braking system that
provides a constant deceleration.
What constant net force is required to
bring a car of mass m to rest from a
speed of V within a distance of D?
V0 = V
VF = 0
X0 = 0
XF = D
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Getting to Newton’s Third Law
How does one apply a
force?
• Applying a force
requires another object!
–A hammer exerts a force
on a nail
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Newton’s Third Law
“Whenever one object exerts a
force on a second object, the
second exerts an equal and
opposite force”
OR
“To every action there is an
equal and opposite reaction”
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Skater
•Skater pushes on a
wall
•The wall pushes back
– Equal and opposite
force
•The push from the wall
is a force
– Force provides an
acceleration
– She flies off with
some non-zero
speed
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Walking


FGround on the Person  - FPerson
When you walk you
push on the ground
and the ground
PUSHES you
forward
• Equal and opposite
force
on the Ground
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Normal Force
Consider a pen sitting on a table:
– Is the force of gravity acting on it?
– Is the pen accelerating?
– What is the Force? What is the
difference between a force and the
Net Force
– What keeps the pen from accelerating?
Clearly, there is a second force that
keeps it from accelerating
Call this the “normal” force!
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Moving towards Problems
Done with the concepts for
this chapter
• Overheard…
– “I understand the concepts,
but I can’t do the problems”
• Sigh… No exam points for
“concepts you can’t apply”
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Free Body Diagrams
Same tricks as in Chapters 1-4:
1.Draw a diagram: Draw each force on
an object separately! Force diagram!
2.Break each force into its X and Ycomponents, THEN sum!!!
– Show your TA that you know the
difference between a force, and a
component of force
– GREAT way to pick up partial credit
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Pulling a box
A box with mass m is pulled along a frictionless
horizontal surface with a force FP at angle Q as
given in the figure. Assume it does not leave the
surface.
a)What is the acceleration of the box?
FP
b)What is the normal force?
Q
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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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Rest of This Week
• Recitation:
– Problems from Chapters 3 and 4
• Lab: Tensile Strength
• Reading for Thursday:
– Read Chapter 6: More on force and friction
• HW3&4: Due Monday
• Are you ready for Exam 1 next Tuesday?
– Use WebCT to study!
– Get perfect scores on all Math and HW1-HW4
WebCT mini-practice exam becomes available
– Get a 100 on it BEFORE Tuesday’s exam and I’ll
give you 5 bonus points on the exam
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The weight of a box
A box with mass m is resting on
a smooth (frictionless)
horizontal table.
a) What is the normal force on
the box?
b) Push down on it with a force
of FP. Now, what is the
normal force?
c) Pull up on it with a force of
FP such that it is still sitting
on the table. What is the
normal force?
d) Pull up on it with a force such
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that it leaves the
table and
FP
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Atwood Machine
Two boxes with masses m1
and m2 are placed
around a pulley with m2
>m1
a) What is the
acceleration of the
boxes?
b) What is the tension of
the strings between the
boxes?
Ignore the mass of the
pulley, rope and any
friction. Assume
the
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rope doesn’t stretch.
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How to study for the exam
• Do all the HW problems
• 4 Problems. Dominated by Chapter 3,
almost nothing from Chapter 1.
• Make sure you can do all the HW
problems easily and by yourself with
no numbers
• Concentrate on the “hardest”
problems. Those will be the most like
the ones on the exam.
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Example 1
Force to accelerate a fast car
Estimate the net force needed to
accelerate a 1000kg car at ½g
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Hockey Puck
• Which of these three
best represents a hockey
puck in the real world?
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Example 4-16
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Before we begin
• Exam Thursday
– Formula sheet to be passed out is
already on web
– Here at 8:00AM
– Only on topics through Chapter 3 on
Syllabus
• Not circular motion!
• Today’s lecture not on the exam
• However, I’ll try to make it a
good review
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Mass vs. Weight
• Mass is a property of the body
– Aside: What is mass is a fundamental question
at the forefront of physics today. Particle
physics! String theory!
• Weight is a Force and depends on the mass
Definition of weight
on
the
earth

Weight  W  FG  mg  - mg ĵ
What is the difference between:
– Your mass on the moon and the earth?
– Your weight on the moon and the earth?
– Your weight in a space ship and the earth?
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Next Time
• Exam Thursday: Here during regular
time
– Bring a calculator and ruler
– Formula sheet will be provided (same as
on the web)
– Will cover:
•
•
•
•
Chapter
Calculus
Chapter
Chapter
1 (1-6)
1
2 (1-7)
3 (1-8, 10)
• Finish HW 3 to study for Exam
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• Labs/Rec/Web
quizzes as usual
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