Transcript Chapter 3

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N3 – Forces from Motion
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The Kinematic Chain
A particle moves along the x axis according to the
equation
x = 2.00 + 3.00 t - 1.00 t2
where x is in meters and t is in seconds.
At t=3.00 s, find
(a) the position of a particle,
(b) its velocity,
(c) its acceleration.
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Graphical Analysis of Motion
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v(t) is slope of r(t) graph
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a(t) is slope of v(t) graph
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Derivative is slope, so we can link r, v and a
graphically as well as mathematically
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Net Force Diagrams
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Construct a free-body diagram.
Copy the force arrows from the diagram
in sequence (tip-to-tail).
Draw a double-line arrow to show the
vector sum.
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Qualitative Example
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Skier going down slope (Fig N3.1)
FKF
FD
Fg
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Practice Problem
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N3B.1 Draw a free-body diagram and a netforce diagram for a moving motorboat whose
motor has just run out of gas.
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Third-Law Pairs
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When objects A and B interact, the force the
interaction exerts on A is equal in magnitude
and opposite in direction to the force it exerts
of B.
Force pairs connected by Newton’s 3rd Law:
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Each force in the pair must act on a different
object.
Each must reflect the same interaction.
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Second-Law Pairs
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Force pairs connected by Newton’s 2nd Law:
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Act on the same object
Arise from different interactions
Act along same axis
Are = b/c a=0 along that axis
Ex: Normal force and weight, when object is
at rest.
What if object is accelerating, like box in an
elevator??
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Coupling Forces
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Compare the forces exerted by the blocks on each other as they
move on a table with some friction. Draw free-body diagram for
each block. Determine if
A
B
C
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block A exerts a greater force
block B exerts a greater force
the forces are equal
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Coupling Forces
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Compare the forces exerted by the blocks on each other as they
move on a table with some friction. Draw free-body diagram for
each block. Determine if
A
B
C
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block A exerts a greater force
block B exerts a greater force
the forces are equal
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Third Law Pair Activity
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Equipment
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Open Logger Pro
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Wired force probe
Wireless force probe
Select Connect Interface, Wireless, and select the name of
your wireless probe
In the top right corner you will see two tabs, one for each
probe. Click on each tab and select zero sensor from the
calibrate menu. On one of the sensors, select change
direction.
Connect the hooks on the force probes together and
pull and push until the values are different. Record
your results.
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Group Problems
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N3B.4
N3B.6
N3B.9
N3S.2
N3S.5
Unit IV, Worksheet 4
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