Download Chapter 4 Force and Motion

10/25/15
Chapter 4
Force and Motion
Units of Chapter 4
The Concepts of Force and Net Force
Inertia and Newton s First Law of Motion
Newton s Second Law of Motion
Newton s Third Law of Motion
More on Newton s Laws: Free-Body
Diagrams and Translational Equilibrium
Friction
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4.1 The Concepts of Force and Net Force
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4.1 The Concepts of Force and Net Force
This figure illustrates
what happens in the
presence of zero and
nonzero net force.
A force is something that is capable of changing an
object s state of motion, that is, changing its
velocity.
Any particular force may not actually change
an object s state of motion, as there may be
other forces that prevent it from doing so.
However, if the net force—the vector sum of
all forces acting on the object—is not zero,
the velocity will indeed change.
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4.1 The Concepts of Force and Net Force
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4.2 Inertia and Newton s First
Law of Motion
According to Aristotle, the natural state of
objects was to be at rest, and if you got them
moving, eventually they would come to rest
again.
We distinguish two types of forces:
1.  A contact force, such as a push or pull,
friction, tension from a rope or string, and so
on.
Galileo did experiments rolling balls down and
up inclined planes, and realized that, in the
absence of some kind of force, an object
would keep moving forever once it got started.
2.  A force that acts at a distance, such as
gravity, the magnetic force, or the electric
force.
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4.2 Inertia and Newton s First Law
of Motion
4.2 Inertia and Newton s First Law
of Motion
Galileo called this inertia:
Inertia is the natural tendency of an object to maintain
a state of rest or to remain in uniform motion in a
straight line (constant velocity).
In the absence of an unbalanced applied force
(Fnet = 0), a body at rest remains at rest, and a
body already in motion remains in motion with a
constant velocity (constant speed and direction).
Later, Newton realized that mass is a
measure of inertia.
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Question 4.1a Newton s First Law I
A book is lying at
rest on a table.
The book will
remain there at
rest because:
Newton s first law is sometimes called the
law of inertia:
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Question 4.1b Newton s First Law II
a) there is a net force but the book has too
much inertia
b) there are no forces acting on it at all
c) it does move, but too slowly to be seen
d) there is no net force on the book
A hockey puck slides
on ice at constant
velocity. What is the
net force acting on
the puck?
a) more than its weight
b) equal to its weight
c) less than its weight but more than zero
d) depends on the speed of the puck
e) zero
e) there is a net force, but the book is too
heavy to move
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Question 4.1c Newton s First Law III
You put your book on
the bus seat next to
you. When the bus
stops suddenly, the
book slides forward off
the seat. Why?
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Question 4.1d Newton s First Law IV
You kick a smooth flat
a) a net force acted on it
stone out on a frozen
b) no net force acted on it
pond. The stone slides,
a) the force pushing the stone forward
finally stopped pushing on it
b) no net force acted on the stone
c) a net force acted on it all along
c) it remained at rest
slows down, and
d) it did not move, but only seemed to
eventually stops. You
d) the stone simply ran out of steam
conclude that:
e) the stone has a natural tendency to be
at rest
e) gravity briefly stopped acting on it
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4.3 Newton s Second Law of Motion
Experiments show that the acceleration of an
object is proportional to the force exerted on it
and inversely proportional to its mass.
4.3 Newton s Second Law of Motion
The units of force are called newtons.
1 N = 1 kg . m/s2.
Force (newtons, N)
Acceleration
(m/sec2)
Mass (kg)
The acceleration of an object is directly proportional
to the net force acting on it and inversely
proportional to its mass. The direction of the
acceleration is in the direction of the applied net
force.
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5.2 Newton's Second Law
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5.2 Newton's Second Law
•  If you apply
more force
to an object,
it
accelerates
at a higher
rate.
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•  If an object
has more
mass it
accelerates
at a lower
rate because
mass has
inertia.
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4.3 Newton s Second Law of Motion
4.3 Newton s Second Law of Motion
An object s weight is the force exerted on it by
gravity.
Newton s second law may be applied to a
system as a whole, or to any part of a system.
It is important to be clear about what system
or part you are considering!
Here, g is the acceleration of gravity:
g = 9.81 m/s2
Weight therefore has the same units as
force—newtons.
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4.3 Newton s Second Law of Motion
4.3 Newton's Second Law
Newton s second law applies separately to
each component of the force.
Three forms of the second law:
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Question 4.2a Cart on Track I
Consider a cart on a
horizontal frictionless
table. Once the cart has
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Question 4.2b Cart on Track II
a) slowly come to a stop
b) continue with constant acceleration
been given a push and
c) continue with decreasing acceleration
released, what will
d) continue with constant velocity
happen to the cart?
e) immediately come to a stop
We just decided that the
cart continues with
constant velocity. What
would have to be done in
order to have the cart
continue with constant
acceleration?
a) push the cart harder before release
b) push the cart longer before release
c) push the cart continuously
d) change the mass of the cart
e) it is impossible to do that
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4.4 Newton s Third Law of Motion
For every force (action), there is an equal and
opposite force (reaction).
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4.4 Newton s Third Law of Motion
This figure illustrates the action–reaction
forces for a person carrying a briefcase. Is
there a reaction force in (b)? If so, what is it?
Note that the action and reaction forces act on
different objects.
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This image shows how a
block exerts a
downward force on a
table; the table exerts an
equal and opposite force
on the block, called the
normal force N.
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Question 4.9a Going Up I
A block of mass m rests on the
a) N > mg
floor of an elevator that is moving
b) N = mg
upward at constant speed. What is
c) N < mg (but not zero)
the relationship between the force
due to gravity and the normal force
on the block?
d) N = 0
e) depends on the size of the
elevator
v
m
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Question 4.9b Going Up II
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Question 4.12 Climbing the Rope
A block of mass m rests on the
a) N > mg
When you climb up a rope,
floor of an elevator that is
a) this slows your initial velocity, which
is already upward
b) N = mg
accelerating upward. What is
the first thing you do is pull
b) you don t go up, you re too heavy
c) N < mg (but not zero)
down on the rope. How do
c) you re not really pulling down—it
just seems that way
the relationship between the
d) N = 0
force due to gravity and the
e) depends on the size of the
elevator
normal force on the block?
you manage to go up the
rope by doing that??
d) the rope actually pulls you up
e) you are pulling the ceiling down
a
m
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4.5 More on Newton s Laws: FreeBody Diagrams and Translational
Equilibrium
Equilibrium
A free-body
diagram draws the
forces on an object
as though they all
act at a given point.
You should draw
such a diagram
whenever you are
solving second-law
problems.
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•  The condition of zero acceleration is
called equilibrium.
•  In equilibrium, all forces cancel out
leaving zero net force.
•  Objects that are standing still are in
equilibrium because their acceleration is
zero.
•  Objects that are moving at constant
speed and direction are also in
equilibrium.
•  A static problem usually means there is
no motion.
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4.5 More on Newton s Laws: FreeBody Diagrams and Translational
Equilibrium
4.6 Friction
The force of friction always
opposes the direction of
motion (or of the direction the
motion would be in the
absence of friction).
If an object is to be in translational
equilibrium, there must be no net force on it.
This translates into three separate
requirements—that there be no force in the
x-direction, the y-direction, or the z-direction.
Depending on the
circumstances, friction may
be desirable or undesirable.
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4.6 Friction
4.6 Friction
We observe that the frictional force is
proportional to the normal force. For static
friction:
Types of friction:
Static friction: when the frictional force is
large enough to prevent motion
Kinetic friction: when two surfaces are sliding
along each other
Rolling friction: when an object is rolling
without slipping
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The constant μs is called the coefficient of
static friction.
The static frictional force may not have its
maximum value; its value is such that the
object does not move, and depends on the
physical circumstances.
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4.6 Friction
4.6 Friction
This figure illustrates what happens as the applied
force increases: first, the static frictional force
increases; then the kinetic frictional force takes
over as the object begins to move.
For kinetic friction:
The constant μk is called the coefficient of
kinetic friction, and is usually smaller than
μs.
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4.6 Friction
4.6 Friction
The coefficients of friction depend on both
materials involved.
This form for the frictional force is an
approximation; the actual phenomenon is very
complicated. The coefficient of friction may
vary somewhat with speed; there may be some
dependence on the surface area of the objects.
Also, remember that these equations are for
the magnitude of the frictional force—it is
always perpendicular to the normal force.
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4.6 Friction
4.6 Friction
This figure shows the velocity as a
function of time for a falling object
with air resistance.
Air resistance is another form of friction. It
depends on an object s shape and size, as well
as its speed.
For an object in free fall, as the force of air
resistance increases with speed, it eventually
equals the downward force of gravity. At that
point, there is no net force on the object and it
falls with a constant velocity called the terminal
velocity.
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Question 4.19 Friction
on a frictionless truck bed.
a) the force from the rushing air
pushed it off
When the truck accelerates
b) the force of friction pushed it off
forward, the box slides off
c) no net force acted on the box
the back of the truck
d) truck went into reverse by accident
A box sits in a pickup truck
because:
e) none of the above
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