Download What is a force?

PHY 2048C
General Physics I with lab
Spring 2011
CRNs 11154, 11161 & 11165
Dr. Derrick Boucher
Assoc. Prof. of Physics
Session 5-6, Chapters 5-6
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Chapters 5 & 6
Practice Problems
Chap 5: 19, 25, 27, 29, 33, 45, 47, 51
Chap 6: 5, 9, 19, 21, 29, 31, 33, 37, 45
Unless otherwise indicated, all practice
material is from the “Exercises and Problems”
section at the end of the chapter. (Not
“Questions.”)
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Chapter 5. Force and Motion
• This is a conceptual chapter. No math!
• Ah…but chapter 6 will introduce the math
AFTER we master these concepts.
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What is a force?
• A force is a push or a pull on an object.
• A force is a vector. It has both a
magnitude and a direction.
• A force requires an agent. Something
does the pushing or pulling.
• A force is either a contact force or a longrange force. Gravity is the only longrange force we will deal with until much
later in the book.
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Tactics: Drawing force vectors
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A Short Catalog of Forces

FB
The box also pulls
the Earth up toward
it!
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Force the Earth
exerts on the box.
A Short Catalog of Forces
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A Short Catalog of Forces
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
A Short Catalog of Forces
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
A Short Catalog of Forces
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
A Short Catalog of Forces
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
A Short Catalog of Forces
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
EXAMPLE 5.1 Forces on a bungee jumper

T

FG
This is a “Free Body” diagram.
It shows only the forces, as vectors.
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Newton’s first law is also known as the law of
inertia. If an object is at rest, it has a tendency to
stay at rest. If it is moving, it has a tendency to
continue moving with the same velocity.
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EXAMPLE 5.4 An elevator accelerates
upward
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Chapter 5. Summary Slides
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General Principles
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Aristotle vs. Galileo (and Newton)
Aristotle (c. 300 BCE)
Galileo (c. 1600)
• His ideas predominated
for ~2000 years
• Challenged Aristotle’s
ideas
• Stated that a
CONSTANT FORCE
was needed to keep an
object in motion
• Stated that constant motion
is natural UNLESS forces
intervene to slow it down
(e.g. friction) or speed it up

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Aristotle vs. Galileo (and Newton)
“It doesn't matter how beautiful your
theory is, it doesn't matter how smart you
are. If it doesn't agree with experiment, it's
wrong”
Richard Feynman,
American physicist
(1918-1988)
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General Principles


Fnet  ma
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Important Concepts
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Chapter 4
Clicker
QUIZ
Get your clickers ready…
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A ball is thrown upward at a 45° angle. In
the absence of air resistance, the ball
follows a
A. tangential curve.
B. sine curve.
C. parabolic curve.
D. linear curve.
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A hunter points his rifle directly at a
coconut that he wishes to shoot off a tree.
It so happens that the coconut falls from
the tree at the exact instant the hunter
pulls the trigger. Consequently,
A. the bullet passes above the coconut.
B. the bullet hits the coconut.
C. the bullet passes beneath the coconut.
D. This wasn’t discussed in Chapter 4.
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The quantity with the symbol ω is called
A. the circular weight.
B. the circular velocity.
C. the angular velocity.
D. the centripetal acceleration.
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For uniform circular motion, the
acceleration
A. points toward the center of the circle.
B. points away from the circle.
C. is tangent to the circle.
D. is zero.
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Chapter 6. Dynamics I: Motion Along a Line
This chapter focuses on objects
that move in a straight line, such
as runners, bicycles, cars, planes,
and rockets. Gravitational,
tension, thrust, friction, and drag
forces will be essential to our
understanding.
Chapter Goal: To learn how to
solve problems about motion in a
straight line.
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Chapter 6. Dynamics I: Motion Along a Line
Topics:
• Mass, Weight, and Gravity
• Equilibrium
• Using Newton’s Second Law
• Friction
• Drag
• More Examples of Newton’s Second Law
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Mass
Mass is a scalar quantity that describes an object’s inertia.
Loosely speaking, it also describes the amount of matter in
an object. Mass is an intrinsic property of an object. It tells
us something about the object, regardless of where the object
is, what it’s doing, or whatever forces may be acting on it.
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Gravity
It was Newton who first recognized that gravity is an attractive,
long-range force between any two objects. Somewhat more
loosely, gravity is a force that acts on mass. When two objects
with masses m1 and m2 are separated by distance r, each object
pulls on the other with a force given by Newton’s law of gravity,
as follows:
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Gravity
We can write the gravitational force even more simply as
where the quantity g is defined to be
R is the radius of the Earth. This approximation is only valid
when the distance from the Earth’s center to the object is nearly
R. (Within 1% if altitude is less than 32 km  20 miles.)
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Weight
If the scale is at rest relative to the earth, then the object
being weighed is in static equilibrium. The upward spring
force exactly balances the downward gravitational force, so
that Fsp = FG = mg.
Because we defined weight as the reading Fsp of a spring
scale, the weight of a stationary object is
If g = 9.8 m/s2 , then m must be in kg so that w is in N
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Equilibrium
An object on which the net force is zero is said to be in
equilibrium. The object might be at rest in static
equilibrium, or it might be moving along a straight line
with constant velocity in dynamic equilibrium. Both are
identical from a Newtonian perspective because the net
force and the acceleration are zero.
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Problem-Solving Strategy: Equilibrium
Problems
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Chap 6 EXAMPLE
Problem #30, page 179
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Using Newton’s Second Law
The essence of Newtonian mechanics can be expressed
in two steps.
• The forces on an object determine its acceleration a =
Fnet/m, and
• The object’s trajectory can be determined by using
the equations of kinematics.
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Problem-Solving Strategy: Dynamics
problems
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Chap 6 EXAMPLE
Problem #12, page 178
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Friction
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Simulations of Atomic-scale friction
• Friction is an atomicscale phenomenon
• Poorly understood until
very recently
• It can be understood via
computer modeling
(need to calculate
atomic forces…not
easy)
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Static Friction
The box is in static equilibrium, so
the static friction must exactly
balance the pushing force:
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Static Friction
There’s clearly a limit to how big fs can get. If you push hard
enough, the object slips and starts to move. In other words,
the static friction force has a maximum possible size fs max.
• An object remains at rest as long as fs < fs max.
• The object slips when fs = fs max.
• A static friction force fs > fs max is not physically possible.
where the proportionality constant μs is called the coefficient
of static friction.
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Kinetic Friction
The kinetic friction force is
proportional to the magnitude
of the normal force.
where the proportionality
constant μk is called the
coefficient of kinetic friction.
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Chap 6 EXAMPLE
Problem #18, page 178
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Chap 6 EXAMPLE
Problem #42, page 180
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Chapter 6. Summary Slides
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General Strategy
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General Strategy
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Important Concepts
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Important Concepts
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Applications
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Applications
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