Transcript Force

Chapter 5
Newton’s Laws of Motion
Dr. Jie Zou PHY 1151
Department of Physics
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Outline
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Force and Mass
Newton’s First, Second, and Third Laws of
Motion
Free-Body Diagrams
Examples of applying Newton’s 2nd Law of
Motion
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One-dimension
Inclined Plane
Dr. Jie Zou PHY 1151
Department of Physics
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Force and Mass
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Force: A force, simply put, is a push or a pull.
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Force is a vector quantity, specified by
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the magnitude (or strength) of the force, and
the direction in which the force acts
Total force (or the resultant force, or the net
force): The total force exerted on an object is the
vector sum of the individual forces acting on it.
Units for forces: newtons (N). 1N  (1/4) of a pound.
Mass: Mass is a measure of the quantity of
matter in an object. It is a scalar. It is an
inherent property of an object. Units = kg
Dr. Jie Zou PHY 1151
Department of Physics
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Typical Examples of Forces
N
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Book
Force of gravity Fg: The attractive force
exerted by the earth on an object.
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Fg
N
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Direction: Directed toward the center of the earth.
Magnitude: The magnitude of the force of gravity
on an object is called the weight of the object W.
Weight: W = mg
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Fg
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m: the mass of the object.
g: the acceleration due to gravity (or free fall
acceleration)
Normal force N: It is a contact force that is
acting perpendicular to the surface.
Dr. Jie Zou PHY 1151
Department of Physics
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Newton’s First Law of Motion
(The Law of Inertia)
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Newton’s First Law of Motion: If the net
force on an object is zero, its velocity is
constant.
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“Net force is zero” means: 1) no force acts on the
object or 2) forces act on the object, but they sum
to zero.
Inertia: The property of objects to resist changes
in motion is called inertia. (The literal meaning of
the word inertia is “laziness”.)
Mass is that property of an object that specifies
how much inertia the object has.
Dr. Jie Zou PHY 1151
Department of Physics
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An Example of Inertia
The bus suddenly slows down. Do you lurch
forward or backward? And why? How about
when the bus suddenly speed up from rest?
STOP
Standing in a moving bus
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Department of Physics
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Newton’s Second Law of
Motion
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Newton’s Second Law of Motion: The
acceleration of an object is directly
proportional to the net force acting on it
and inversely proportional to its mass.
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a = ( F)/m, or  F = ma
The above equation is equivalent to three
component equations:
 Fx = max,  Fy = may,  Fz = maz.
Dr. Jie Zou PHY 1151
Department of Physics
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Active Example 5-1
The Force Exerted by Foamcrete: Foamcrete is a
solid paving material soft enough to collapse under
the weight of an airliner. It is designed to stop an
airplane that has run off the end of a runway.
Suppose a 747 jetliner with a mass of 1.75  105 kg
and an initial speed of 26.8 m/s is slowed to a stop
in 122 m. What is the magnitude of the retarding
force exerted by the Foamcrete on the plane?
Dr. Jie Zou PHY 1151
Department of Physics
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Free-Body Diagrams
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Free-body diagram: A sketch that
indicates each and every external force
acting on a given object.
(a) The physical
situation
(b) The free-body
diagram for the
book
Dr. Jie Zou PHY 1151
Department of Physics
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Active Example 5-2
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Find the Speed of the Sled:
A 4.60-kg sled is pulled
across a smooth ice
surface. The force acting on
the sled is of magnitude
6.20 N and points in a
direction 35.0 above the
horizontal. If the sled starts
at rest, how fast is it going
after being pulled for 1.15
s?
Dr. Jie Zou PHY 1151
Department of Physics
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Example 5-9: Object on an
Inclined Plane
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A child of mass m rides on
a toboggan down a slick,
ice-covered hill inclined at
an angle  with respect to
the horizontal.
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(a) What is the acceleration
of the child?
(b) What is the normal force
exerted on the child by the
toboggan?
Dr. Jie Zou PHY 1151
Department of Physics
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Newton’s Third Law of Motion
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Newton’s Third Law of Motion: If two
objects interact, the force F12 exerted by
object 1 on object 2 is equal in magnitude to
and opposite in direction to the force F21
exerted by object 2 on object 1.
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For every action there is an equal and opposite
reaction.
The action-reaction forces always act on different
objects. As a result, the two forces do not cancel.
Forces always come in pairs.
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Department of Physics
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Examples of Action-Reaction
Pairs
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The action-reaction
forces always act on
different objects. As a
result, the two forces do
not cancel.
Since the actionreaction forces act on
different objects, they
generally produce
different
accelerations.
Dr. Jie Zou PHY 1151
Department of Physics
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Homework
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Chapter 5, Page 131, Conceptual
Questions: #12.
Chapter 5, Page 132, Problems: #3,
10, 15, 22, 25.
Dr. Jie Zou PHY 1151
Department of Physics
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