Transcript Types of Forces

Chapter Six Notes:
Newton’s Second Law of Motion – Force And
Acceleration
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Unbalanced forces acting on an object causes
the object to accelerate.
 acceleration ~ net force
 ~ stands for “is directly proportional to”
For a constant force, an increase in the mass will
result in a decrease in the acceleration.
 acceleration ~ 1/mass
 Inversely – means that the two values change in
opposite directions.
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Newton’s second law states that the
acceleration produced by a net force on an
object is directly proportional to the
magnitude of the net force, is in the same
direction as the net force, and is inversely
proportional to the mass of the object.
 acceleration ~ net force
mass
 a = F
m
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The force of friction between the surfaces
depends on the kinds of material in contact and
how much the surfaces are pressed together.
The friction force is the force exerted by a
surface as an object moves across it or makes an
effort to move across it. The friction force
opposes the motion of the object. For example, if
a book moves across the surface of a desk, the
desk exerts a friction force in the direction
opposite to the motion of the book.
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Friction results when two surfaces are pressed
together
closely,
causing
attractive
intermolecular forces between the molecules of
the two different surfaces. As such, friction
depends upon the nature of the two surfaces and
upon the degree to which they are pressed
together. The friction force can be calculated
using the equation:
 Ffriction = µ x Fnorm
 where µ = coefficient of friction
Friction is not restricted to solids sliding on one
another.
Friction also occurs in liquids and
gases. They are both called fluids, because they
flow. Ea: Try running through waist deep water.
Air resistance is friction through air.
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A diagram showing all the forces acting on an
object is called a free-body diagram.
 Drawing Free-Body Diagrams
Free-body diagrams are diagrams used to
show the relative magnitude and direction of
all forces acting upon an object in a given
situation. A free-body diagram is a special
example of the vector diagrams discussed in
Chapter 2; these diagrams will be used
throughout your study of physics.
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The size of the arrow in a free-body diagram
is reflective of the magnitude of the force.
The direction of the arrow reveals the
direction in which the force acts. Each force
arrow in the diagram is labeled to indicate the
type of force. It is customary in a free-body
diagram to represent the object by a box
and to draw the force arrow from the center
of the box outward in the direction in which
the force is acting. One example of a freebody diagram is shown right.
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The free-body diagram above depicts four forces
acting upon the object. Objects do not always have
four forces acting upon them. There will be cases in
which the number of forces depicted by a free-body
diagram will be one, two, or three. There is no hard
and fast rule about the number of forces which must
be drawn in a free-body diagram. The only rule for
drawing free-body diagrams is to depict all the forces
which exist for that object in the given situation.
Thus, to construct free-body diagrams, it is
extremely important to know the types of forces. If
given a description of a physical situation, begin by
using your understanding of the force types to
identify which forces are present. Then determine the
direction in which each force is acting. Finally, draw a
box and add arrows for each existing force in the
appropriate direction; label each force arrow
according to its type. If necessary, refer to the "Net
Force Help Sheet" for descriptions of the force types
and their symbols. (Next three slides.)
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Types of Forces
A force is a push or pull acting upon an object as a result
of its interaction with another object. Forces may be
placed into two broad categories, based on whether the
force resulted from the contact or non-contact of the two
interacting objects.
Contact Forces
Frictional Force
Tensional Force
Normal Force
Air Resistance
Force Applied
Force Spring Force
Action-at-a-Distance Forces
Gravitational Force
Electrical Force
Magnetic Force
These types of forces will now be discussed in detail. To
read about each force listed above, continue scrolling
through the table on the next three pages.
Type of Force and
its Symbol
Description of Force
Applied Force Fapp
An applied force is a force which is applied to an object by
another object or by a person. If a person is pushing a
desk across the room, then there is an applied force
acting upon the desk. The applied force is the force
exerted on the desk by the person.
Gravity Force (also
The force of gravity is the force with which the earth,
known as Weight) Fgrav moon, or other massive body attracts an object towards
itself. By definition, this is the weight of the object. All
objects upon earth experience a force of gravity which is
directed "downward" towards the center of the earth. The
force of gravity on an object on earth is always equal to
the weight of the object as given by the equation:
Fgrav = m * g where: g = acceleration of gravity = 9.8 m/s2
(on Earth)
m = mass (in kg)
(Caution: do not confuse weight with mass. See slide 13.)
Type of Force and
its Symbol
Description of Force
Normal Force Fnorm
The normal force is the support force exerted upon an
object which is in contact with another stable object. For
example, if a book is resting upon a surface, then the
surface is exerting an upward force upon the book in
order to support the weight of the book. On occasion, a
normal force is exerted horizontally between two
objects which are in contact with each other.
Friction Force Ffrict
The friction force is the force exerted by a surface as an
object moves across it or makes an effort to move
across it. The friction force opposes the motion of the
object. For example, if a book moves across the surface
of a desk, the desk exerts a friction force in the
direction opposite to the motion of the book.
Friction results when two surfaces are pressed together
closely, causing attractive intermolecular forces between
the molecules of the two different surfaces. As such,
friction depends upon the nature of the two surfaces
and upon the degree to which they are pressed
together. The friction force can be calculated using the
equation on a previous slide.
Type of Force
and its Symbol
Description of Force
Air Resistance
Force Fair
Air resistance is a special type of frictional force which acts
upon objects as they travel through the air. Like all
frictional forces, the force of air resistance always opposes
the motion of the object. This force will frequently be
ignored due to its negligible magnitude. It is most
noticeable for objects which travel at high speeds (e.g., a
skydiver or a downhill skier) or for objects with large
surface areas.
Tensional Force
Ftens
Tension is the force which is transmitted through a string,
rope, or wire when it is pulled tight by forces acting at each
end. The tensional force is directed along the wire and
pulls equally on the objects on either end of the wire.
Spring Force Fspring
The spring force is the force exerted by a compressed or
stretched spring upon any object which is attached to it.
This force acts to restores the object, which compresses or
stretches a spring, to its rest or equilibrium position. For
most springs (specifically, for those said to obey "Hooke's
Law"), the magnitude of the force is directly proportional to
the amount of stretch or compression.
Mass vs. Weight
The force of gravity is a source of much confusion to
many students of physics. The mass of an object
refers to the amount of matter that is contained by
the object; the weight of an object is the force of
gravity acting upon that object. Mass is related to
"how much stuff is there" and weight is related to the
pull of the Earth (or any other planet) upon that stuff.
The mass of an object (measured in kg) will be the
same no matter where in the universe that object is
located. Mass is never altered by location, the pull of
gravity, speed or even the existence of other forces.
For example, a 2-kg object will have a mass of 2 kg
whether it is located on Earth, on the moon, or on
Jupiter; its mass will be 2 kg whether it is moving or
not (at least for purposes of this study); and its mass
will be 2 kg whether it is being pushed or not.
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On the other hand, the weight of an object (measured
in Newtons) will vary according to where in the
universe the object is. Weight depends upon which
planet is exerting the force and the distance the
object is from the planet. Weight, being equivalent to
the force of gravity, is dependent upon the value of g
(acceleration of gravity). On Earth's surface, g is 9.8
m/s2 (often approximated to 10 m/s2). On the
moon's surface, g is 1.7 m/s2. Go to another planet,
and there will be another g value. In addition, the g
value is inversely proportional to the distance from
the center of the planet. So if g were measured at a
distance of 400 km above the earth's surface, you
would find the value of g to be less than 9.8 m/s2.
(The nature of the force of gravity will be discussed in
detail in in a later chapter of Conceptual Physics.)
Always be cautious of the distinction between mass
and weight. It is the source of much confusion for
many students of physics.
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You must thoroughly understand the
meaning of each of these forces if you are to
successfully proceed through this unit.
Ultimately, you must be capable of reading
the description of a physical situation and
knowing enough about these forces to
recognize their presence (or absence) and to
construct a free-body diagram which
illustrates their relative magnitudes and
directions.
Free-body diagrams for four situations are shown below. For each
situation, determine the net force acting upon the object. Depress the
mouse to view the answers.
Situation A: Net Force =
0 Newtons
Situation B: Net Force =
5 Newtons left
Situation C: Net Force =
0 Newtons
Situation D: Net Force =
15 Newtons upward
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The amount of force per unit area is called
pressure.
For a constant force, an increase in the area
of contact will result in a decrease in the
pressure.
 Pressure = force/area of application
 P = F/A
 Pressure = Newtons/Square Meter
 = Pascals (Pa)
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Recall that mass (a quantity of matter) and
weight (the force due to gravity) are
proportional.
In symbolic notation:
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F/m
=
F/m =
g
Where F stands for the force (weight)
and m stands for the mass
The ratio is the same for all objects
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All freely falling objects fall with the same
acceleration because the net force on an
object is only its weight and ratio of weight to
mass is the same for all objects.
 Since the cannon ball has both
 a greater weight and inertia (m)
 one offsets the other. Now the
 accelerations of both are equal.
.
Acceleration = g
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When the forces of gravity and air resistance
act on a falling object, it is not in free fall.
 The air resistance force an object experiences
depends on the object’s speed and area.
 Air resistance force ~ speed x frontal area
 Terminal Speed:
 Terminal speed is the speed at which the
acceleration (g) of an object is zero because
air friction balances the weight.
 Terminal Velocity is Terminal Speed &
Direction
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