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7/29/2019 Chapter 2 Mini-lecture
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Chapter 2 mini-lecture:
Motion along a Straight LineTopics:
Displacement
Velocity Acceleration
Key Idea:
Motion is described in terms of a
coordinate system that defines
position x
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PHSX 213, Chapter 2 mini-lecture
Kinematics
The subject of describing motion is called
kinematics
Starting with Chapter 5 well explore the
causes of motion. This subject is called
dynamics
2
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Terminology
This chapter presents important terminology
that well use all semester
Physics uses words from everyday Englishin precise technical ways
Ill refer to these technical terms as
physics-speak Its important to be able to use these terms
in their technical sense
PHSX 213, Chapter 2 mini-lecture 3
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Coordinate axis
In this chapter the motion will be limited to a
straight line (like I-70)
Call the positionx (units of meters)
Define an origin and a direction that is positiverelative to the origin
PHSX 213, Chapter 2 mini-lecture 4
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Stationary object
An object is sitting
still 2 m to the left
of the origin
Thenx=-2 m is theposition in our
coordinate system
The graph of
position vs. time is
just a horizontal
line
PHSX 213, Chapter 2 mini-lecture 5
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Displacement
Its more interesting when an object changes
position
Assume an object starts at positionx1and at a later
time is at position x2
The displacement is the change in position, x
PHSX 213, Chapter 2 mini-lecture 6
x>0, if the object moves to the right
x
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Distance traveled
Displacement can differ from distance
travelled
Initial and final position is all that matters
for displacement, not what happens in
between
PHSX 213, Chapter 2 mini-lecture 7
x path
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Average velocity
Displacement divided by the time interval is the
average velocity
tis always taken to be positive
The direction of the average velocity is
determined by the direction of the displacement
PHSX 213, Chapter 2 mini-lecture 8
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Instantaneous velocity
PHSX 213, Chapter 2 mini-lecture 9
Given the position as
a function of time,
x(t), take thederivative to get v(t)
The magnitude of the
instantaneous velocity
is the speed.
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Example
PHSX 213, Chapter 2 mini-lecture 10
35)( 2 ttx
ttv 10)(
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Acceleration
Acceleration is the rate of change of the velocity
The sign of the acceleration comes from the sign
ofv a>0does not necessarily mean your speed is
increasing (see checkpoint 3 in the chapter)
PHSX 213, Chapter 2 mini-lecture 11
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Constant acceleration
Important special case
Ex: an object falling under gravity near the
Earths surface
PHSX 213, Chapter 2 mini-lecture 12
constatdtatv )(
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Position for constant acceleration
PHSX 213, Chapter 2 mini-lecture 13
constattvdtatvdttvtx2
21
00 )()()(
Other equations can be derived from these two.
Ex: eliminate tand get )(2 02
0
2xxavv
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Constant a graphs
PHSX 213, Chapter 2 mini-lecture 14