Chapter 2a Physics71
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Transcript of Chapter 2a Physics71
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Announcements
Syllabus is still in press.
Form 5s please
Consultation hours:Mondays9AM to 12NN and 1PM to 4PM;
Tuesdays/Thursdays3PM to 5PM;
Fridays10AM to 12NN and 1PM to 3PM EMAIL: [email protected]
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Chapter 2: Motion along a
Straight Line
Second Semester AY 2012-2013
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On unit vectors (cross products)
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Coordinate system
These are by
convention!
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Vector quantities
Mathematical entities that has both
magnitude and direction.
They can be added, subtracted, multiplied,
and transformed according to well-defined
mathematical rules.
A position vector can be
represented by anarrow pointing from the
origin of a chosen
coordinate system to
the location of the
object.
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Scalar quantities
Values that are NOT dependent on the
orientation of any coordinate system and
has no direction associated with it.
Mass, distance, length, area, temperature,
and volume are some examples.
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Introduction to Vectors
Vectors have magnitudeand direction
Consider a vector Magnitude is
Components:
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Definitions
Speedhow fast a particle is moving;
scalar quantity
Velocityhow fast AND in what
direction its moving; vector quantity
Accelerationrate of change of velocity
with time; vector quantity
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Displacement, Time, and Average
Velocity
x
0x
1x
2x
Displacementchange in the coordinate
x
0x
2 1x x x
t1 t2
Direction is to the right
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Displacement, Time, and Average
Velocity
Average Velocitythe x-component of thedisplacement divided by the time interval during
which the displacement occurs.
2 1
2 1
av x
x x xv
t t t
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Displacement, Time, and Average
Velocity
x
0x
1x
2x
2 1x x x
t1 t2
2 1
2 1
av x
x x xv
t t t
Average velocity
is ALWAYS
POSITIVE
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Displacement, Time, and Average
Velocity
x
0x
2x
1x
2 1x x x
t2 t1
Direction is to the left
Average velocity
is ALWAYS
NEGATIVE
2 1
2 1
av x
x x xv
t t t
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Displacement, Time, and Average
Velocity
Whenever x is positive and increasingor
is negative and becomes less negative,
the particle is moving in the +x-direction
and vav xis positive
Whenever x is positive and decreasing
or is negative and becomes more
negative, the particle is moving in thex-directionand vav xis negative
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Graphical Picture
x-t Graph of the
Dragster
Average velocity depends ONLY on the totaldisplacementx that occurs during the timeintervalt and not on the details of whathappens during the time interval
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Finding Velocity on an x-t Graph
The velocity of a particle can be foundfrom the graph of the particles positionas a function of time.
Instantaneous
velocity
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Average velocity of a particle during a
time interval cant tell us how fast, or in
what direction, the particle was moving
at any given time during the interval
Finding Velocity on an x-t Graph
Instantaneous Velocityvelocity at any specific
instant of time or specific point along the path.
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Finding Velocity on an x-t Graph
Motion diagram showing the position and velocity of the particle
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Average and Instantaneous
Acceleration
x
0x
1x
2x
2 1x x x
t1 t2
2 1x x xv v v
2 1
2 1
x x x
av x
v v va
t t t
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Graphical Picture
Note:
Instantaneous Acceleration
same procedure that was used in
defining instantaneous velocity
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Finding Velocity a vx-t Graph or
an x-t Graph
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Finding Velocity a vx-t Graph or
an x-t Graph
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Test you understanding
At which points P, Q, R, and S isthe velocity vxpositive? At which
points is the velocity negative? At
which points is it zero? At which
point is the speed greatest?
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Thank you very much!See you all next meeting.