Transcript class slides for Chapter 4

4
Newton’s Laws of Motion
Lectures by James L. Pazun
Goals for Chapter 4
• To understand force – either directly or as the
net force of multiple components.
• To study and apply Newton’s First Law.
• To study and apply the concept of mass and
acceleration as components of Newton’s
Second Law.
• To differentiate between mass and weight.
• To study and apply Newton’s Third Law.
• To open a new presentation of problem data in
a free body diagram.
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Dynamics, a new frontier
• Stated previously, the onset of physics
separates into two distinct parts:
– statics and
– dynamics.
• So, if something is going to be dynamic, what
causes it to be so?
– A force is the cause, it is either
• pushing or
• pulling.
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Types of Force Illustrated I – Figure 4.1
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Types of Force II – Figure 4.2
• Single or net
– Contact force
– Normal force
– Frictional force
– Tension
– Weight
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A force may be resolved into components – Figure 4.4
•Fx = F CosΘ
•Fy = F SinΘ
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Components and Resultants II – Figure 4.6
• An example of
component resolution.
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Newton’s First Law – Figure 4.7
•“Objects at rest tend to
stay at rest and objects in
motion tend to stay in
motion.”
•What that common
statement of the first law
often leaves out is the final
phrase “until acted upon
an external force”.
•In one word, we say
“inertia”.
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We determine effect with the net force. – Figure 4.8
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Forces are inertial and non-inertial. – Figure 4.9
•The label depends on the
position of the object and
it’s observer.
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Mass and Newton’s Second Law I – Figure 4.11
• F=ma
• We can
examine the
effects of
changes to
each
component.
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Mass and Newton’s Second Law II – Figure 4.12
•Let’s examine some
situations with more than
one mass.
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Forces and free body diagrams I – Example 4.1
•Observe the
worked
example on
page 108.
•The forces
are in
equilibrium so
there is no
motion.
•This is a
good example
of forces in
statics.
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Forces and free body diagrams II – Example 4.2
•Like the previous example, we account for the forces and
draw a free body diagram.
•In this case, the net force is unbalanced.
•This is a good example of forces in dynamics.
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Mass and Weight – Figure 4.17
•Perhaps saying “go
weigh the object” is at
fault.
•Mass is a measure of
“how much material
do I have?”
•Weight is “how hard
do I push down on the
floor?”
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We can solve for dynamic information – Example 4.4
•Knowing force and mass
we can sketch a free body
diagram and label it with
our information.
•We can solve for
acceleration.
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Measurement of mass – Figure 4.20
•Since gravity is
constant, we can
compare forces to
measure unknown
masses.
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Newton’s Third Law
•“For every action there is
an equal and opposite
reaction.”
•Rifle recoil is a wonderful
example.
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Same objects, different situations – Figure 4.22
•Making only subtle changes in the positions of the apple and the
table we can observe a number of different situations.
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Use free body diagrams in any situation – Figure 4.24
•Find the
object of the
focus of your
study and
collect all
forces acting
upon it
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Forces transmit themselves as tension – Example 4.9
•We can solve for several outcomes using the elevator as our
example.
•Follow the worked problem on page 120.
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