Download class slides for Chapter 5

Chapter 5 Lecture
Chapter 5:
Applications of
Newton's Laws
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Goals for Chapter 5
• To draw free-body diagrams, showing forces on
an individual object.
• To solve for unknown quantities using Newton's
2nd law on an object or objects connected to one
another.
• To relate the force of friction acting on an object to
the normal force exerted on an object in 2nd law
problems.
• To use Hooke's law to relate the magnitude of the
spring force exerted by a spring to the distance
from the equilibrium position the spring has been
stretched or compressed.
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The Conditions for a Particle to be in
Equilibrium
• Necessary conditions for an object to settle into
equilibrium:
Or in component form:
• Note: an object in equilibrium may be at rest or
moving with a constant velocity.
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Equilibrium in One Dimension – Figure 5.1
• Follow Example
5.1 on page
123.
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Two Dimensional Equilibrium – Example 5.2
• Both x- and y-forces must be considered
separately.
• Follow Example 5.2 on page 124.
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An Example Involving Two Systems –
Example 5.4
• See the worked
example on page 126.
• This example brings
nearly every topic we
have covered so far in
the course.
• This is an equilibrium
problem because
system moves with
constant speed!!
• Note: x-axis for the cart does not have to align with the
horizontal direction and is different from the bucket.
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Let's Examine Applications of Newton's Second Law.
 Non-equilibrium or Dynamic Problems
• Although this
container is on a level
surface, the liquid
surface is on a slant
because the
apparatus is being
accelerated to the left.
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Application I – Example 5.5
• This experiment works in your car, a bus, or
even an amusement park ride!
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Application II – Example 5.6
• This sled ride is
worked out for you on
page 129.
• Similar to Example
5.4, but now velocity
is not constant
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Application III – Example 5.7
• This problem involves two interactive systems in
a common lab experiment.
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Contact Force and Friction
• We need to re-examine
problems we formerly did
as "ideal."
• We need to be able to find
frictional forces given the
mass of the object and the
nature of the surfaces in
contact with each other.
• There are two regions of friction:
1) when an object is sliding with respect to a surface 
kinetic-friction force
2) when there is no relative motion  static-friction force
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The Microscopic View of Friction –
Figure 5.12
• A surface will always
have imperfections,
your perception of
them depends on the
magnification.
• The coefficient of
friction (μ) will reveal
how much force is
involved.
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No Dependence on Surface Area
• The normal force determines friction.
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Friction Changes as Forces Change –
Figure 5.13
• Forces from static friction increase as force
increases while forces from kinetic friction are
relatively constant.
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How Much Effort to Move the Crate?
• Dynamics as in the last chapter with a new
force.
• See the worked solution on page 135.
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Forces Applied at an Angle
• The previous example has one new step if the
force is applied at an angle.
• Please refer to the worked example on page
136.
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A Toboggan on a Steep Hill with Friction –
Example 5.12
• Similar to Example 5.6, but now at constant
speed.
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Forces in Fluids – Figure 5.20
• This topic is fully developed in advanced
courses.
• Conceptually, observe the drag as objects fall
through "thicker" liquids.
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Elastic Forces
• Springs or other elastic
material will exert force
when stretched or
compressed.
• The magnitude of the spring
force Fspring is given by
Hooke's Law:
Where k is spring constant
[N/m] and ΔL [m] is distance
the spring is stretched or
compressed from its
equilibrium length.
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Stretch a Spring to Weigh Objects –
Example 5.14
• The force settings on the spring are calibrated
with mass standards at normal earth gravity.
• The spring scales are often calibrated in force
(N) and mass (kg).
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There are a Variety of Force Laws in Nature
•
•
•
•
•
Gravitational interactions
Electromagnetic interactions
Strong interaction
Weak interactions
A "holy grail" of physics is the unified field
theory. The goal will be to find the overriding
principles that give rise to each of these very
similar phenomena.
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