Lecture Outlines
Chapter 4
College Physics, 6th Edition
Wilson / Buffa / Lou
© 2007 Pearson Prentice Hall
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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
4.1 The Concepts of Force and 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.
4.1 The Concepts of Force and Net Force
This figure illustrates
what happens in the
presence of zero and
nonzero net force.
4.1 The Concepts of Force and Net Force
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.
2. A force that acts at a distance, such as
gravity, the magnetic force, or the electric
force.
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.
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.
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).
Later, Newton realized that mass is a
measure of inertia.
4.2 Inertia and Newton’s First Law
of Motion
Newton’s first law is sometimes called the
law of inertia:
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).
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.
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.
4.3 Newton’s Second Law of Motion
The units of force are called newtons.
1 N = 1 kg . m/s2.
4.3 Newton’s Second Law of Motion
An object’s weight is the force exerted on it by
gravity.
Here, g is the acceleration of gravity:
g = 9.81 m/s2
Weight therefore has the same units as
force—newtons.
4.3 Newton’s Second Law of Motion
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!
4.3 Newton’s Second Law of Motion
Newton’s second law applies separately to
each component of the force.
4.4 Newton’s Third Law of Motion
For every force (action), there is an equal and opposite
force (reaction).
Note that the action and reaction forces act on
different objects.
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.
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?
4.5 More on Newton’s Laws: FreeBody Diagrams and Translational
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.
4.5 More on Newton’s Laws: FreeBody Diagrams and Translational
Equilibrium
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.
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).
Depending on the
circumstances, friction may
be desirable or undesirable.
4.6 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
4.6 Friction
We observe that the frictional force is
proportional to the normal force. For static
friction:
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.
4.6 Friction
For kinetic friction:
The constant μk is called the coefficient of
kinetic friction, and is usually smaller than μs.
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.
PROBLEM
• A cabinet initially at rest on a horizontal
surface requires a 115 N horizontal
force to set it in motion. If the
coefficient of static friction between
the cabinet and the floor is 0.38,what is
the normal force exerted on the
cabinet? What is the mass of the
cabinet?
SOLUTION
– Given: Fs,max = 115 N
us = 0.38
g = 9.81 m/s2
– Unknown: Fn = ? m = ?
• Use the equation for the coefficient of static friction
to find Fn.
• Use the definition for the normal force to find m.
• A ship launched from a dry-dock slides into the
water at a constant velocity. Suppose the force
of gravity that pulls the ship downward along
the dry-dock is 4.26 . 107 N. If the coefficient of
kinetic friction between the ship’s hull and the
dry-dock is 0.25, what is the magnitude of the
normal force that the dry-dock exerts on the
ship’s hull?
4.6 Friction
The coefficients of friction depend on both
materials involved.
4.6 Friction
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.
(Why?)
4.6 Friction
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.
4.6 Friction
This figure shows the velocity as a
function of time for a falling object
with air resistance.
Review of Chapter 4
A force is capable of changing an object’s
state of motion; that state of motion will
change if and only if there is a net force on the
object.
Newton’s first law: In the absence of
unbalanced external forces, an object’s
velocity will not change.
Newton’s second law:
Review of Chapter 4
Weight is the force exerted on an object by
gravity:
Newton’s second law holds separately for
each component of the force and
acceleration.
Review of Chapter 4
Newton’s third law: For every force, there is an
equal and opposite force (reaction force) acting
on the other object.
Translational equilibrium: an object having no
net force on it in any direction. Its velocity is
constant or zero.
Review of Chapter 4
Friction is the resistance to motion that
occurs when different surfaces are in
contact.
Static friction:
Kinetic friction:
An object falling in air experiences air
resistance; this resistance increases until the
object reaches its terminal velocity.