Transcript Document

Chapter 4
Newton’s Laws of
Motion
PowerPoint® Lectures for
University Physics, Twelfth Edition
– Hugh D. Young and Roger A. Freedman
Lectures by James Pazun
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What are the properties of force(s)?
• Combinations of “push” and “pull”
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There are four common types of forces
• The normal force—
When an object rests or
pushes on a surface, the
surface pushes back.
• Frictional forces—In
addition to the normal
force, surfaces can
resist motion along the
surface.
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There are four common types of forces II
• Tension forces—When a
force is exerted through a
rope or cable, the force is
transmitted through that
rope or cable as a tension.
• Weight—Gravity’s pull on
an object. This force can act
from large distances.
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What are typical sizes for common forces?—Table 4.1
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How to denote a force—Figure 4.3
• Use a vector
arrow to indicate
magnitude and
direction of the
force.
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Use the net (overall) force—Figure 4.4
• Several forces acting on a point have the same
effect as their vector sum acting on the same point.
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Decomposing a force into components
• Fx and Fy are the parallel and perpendicular components
of a force to a sloping surface.
• Use F*Cosθ and F*Sinθ operations to find force
components.
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Notation and method for the vector sum—Figure 4.7
• We refer to the vector sum or resultant as the “sum of
forces” R = F1 + F2 + F3 … Fn = ΣF.
• Use Tanθ = Ry/Rx and R = (Rx2 + Ry2)1/2.
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Superposition of forces—Example 4.1
• Adding all x components and all y components allows
you to add many vectors. Example 4.1 has three.
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Newton’s First Law—Figure 4.9
• Simply stated—
“objects at rest tend to
stay at rest, objects in
motion stay in
motion.”
• More properly, “A
body acted on by no
net force moves with
constant velocity and
zero acceleration.”
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Newton’s First Law II—Figure 4.10
• Figure 4.10 shows
an unbalanced force
causing an
acceleration and
balanced forces
resulting in no
motion.
• Refer to Conceptual
Examples 4.2 and
4.3.
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Inertial frames of reference—Figure 4.11
• When a car turns and a rider continues to move,
the rider perceives a force.
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Newton’s Second Law—Figure 4.13
•
An unbalanced force (or sum of forces) will cause a mass to accelerate.
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An object undergoing uniform circular motion
• Refer to Figure 4.14. We have already seen the
centripetal acceleration. But, if we measure the mass
in motion, Newton’s Second Law allows us to
calculate the centripetal force.
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The relationship of F, m, and a
• Because a depends
linearly on m and F, an
acceleration will be
directly proportional to the
applied force.
• Solution of the units gives
a new combination of
(kg*m)/s2 for the force.
This is called… the
Newton.
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The relationship of F, m, and a redux
• Because a depends
linearly on m and F, an
acceleration will be
inversely proportional to
the object’s mass.
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Using the Second Law—Example 4.4
• Refer to Example 4.4, using Figure 4.18.
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Using the Second Law II—Example 4.5
• Refer to Example 4.5, using Figure 4.19.
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Newtons, kilograms, pounds, and slugs—Table 4.2
• Table 4.2 rightly points out that the pound is a
force. The popular culture refers to it as a weight
(which is actually a slug).
• The Dyne is actually a cgs version of the Newton
(sometimes used with fine work on tiny objects).
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Many have asked “how lethal is a coin dropped from atop a tall building”?
• Urban legends have said that
a penny dropped from the
top of the Empire State
Building can kill.
• Conceptual Question 4.6
ponders this enigma with a
euro.
• Cable TV has allowed those
two science guys who test
such “myths” to debunk this
one.
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g, and hence weight, is only constant on earth, at sea level
• On Earth, g depends
on your altitude.
• On other planets,
gravity will likely have
an entirely new value.
• Example 4.7 examines
“apparent weight” in a
rapidly stopping car.
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Newton’s Third Law
• Exerting a force on a body results in a force back
upon you.
• Figure 4.25 shows “an action–reaction pair.”
• See Example 4.8.
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Newton’s Third Law—Objects at rest
• An apple on a table or a person in a chair—there
will be the weight (mass pulled downward by
gravity) and the normal force (the table or chair’s
response).
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Newton’s Third Law—Objects in motion
• An apple falling or a refrigerator that needs to be
moved—the first law allows a net force and mass to
lead us to the object’s acceleration.
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Free-body diagrams—Figure 4.30
• A sketch then an accounting of forces
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