Mechanics - the Study of the Motion of Objects

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Transcript Mechanics - the Study of the Motion of Objects

Mechanics - the Study of the
Motion of Objects
Kinematics is the science of
describing the motion of objects
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using words, diagrams, numbers,
graphs, and equations.
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Warm-Up Quiz 08/11/09
• Two chains of length 1.0 m are used to
lamp,
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The distance between the two chains is
1.0 m along
the ceiling.
What is the
vertical distance
from the lamp to
the ceiling?
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Describing Motion
• Scalars are quantities which are fully described by a
magnitude (or numerical value) alone.
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• Vectors are quantities which are fully described by both
a magnitude and a direction.
• Categorize each quantity as being either a vector or a
scalar
a) 5 m
b) 30 m/sec, East
c) 5 mi., North
d) 20 degrees Celsius
e) 256 bytes
f) 4000 Calories
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Distance and Displacement
Distance and displacement are two quantities
which may seem to mean the same thing yet have
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distinctly different definitions and meanings.
• Distance is a scalar quantity which refers to
"how much ground an object has covered"
during its motion.
• Displacement is a vector quantity which
refers to "how far out of place an object is";
it is the object's overall change in position.
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Consider the motion depicted in the diagram
below. A physics teacher walks 4 meters
East, 2 meters South, 4 meters West, and
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finally 2 meters North.
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The diagram below shows the position of a crosscountry skier at various times. At each of the indicated
times, the skier turns around and reverses the direction
of travel.
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other
words,
the skier moves from A to B to
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C to D. Use the diagram to determine the resulting
displacement and the distance traveled by the skier
during these three minutes.
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To summarize…
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• a vector quantity such as displacement is
direction-aware
• a scalar quantity such as distance is
ignorant of direction
• What is the displacement of the crosscountry team if they begin at the school, run
10 miles and finish back at the school?
• What is the distance and the displacement
of the race car drivers in the Indy 500
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Speed and Velocity
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• Speed is a scalar quantity which refers to
"how fast an object is moving." It is the rate
at which an object covers distance.
• Velocity is a vector quantity which refers to
"the rate at which an object changes its
position." When evaluating the velocity of
an object, one must keep track of direction.
• Describe your trip to school today in terms
of velocity!
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Warm-Up 08/13/10
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Please sketch a graph of your
distance (y-axis) vs time (x-axis) on your
trip to school
Include a title, labels, units, and a dot-to-dot
line (NOT a best-fit line this time).
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Thanks!
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Calculating Average Speed
and Average Velocity
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• While on vacation, Lisa Carr traveled a
total distance of 440 miles. Her trip took
8 hours. What was her average speed?
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Average Speed versus Instantaneous Speed
• Instantaneous Speed - the speed at any given instant
in time.
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• Average Speed - the average of all instantaneous
speeds; found simply by a distance/time ratio
• Now let's consider the motion of that physics teacher
again. The physics teacher walks 4 meters East, 2
meters South, 4 meters West, and finally 2 meters
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North. The entire motion lasted for 24
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seconds. Determine the average speed and
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the average velocity
• Now do the same for the skier using the
times provided in the diagram.
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Vector Addition and PhET
Simulations Lab
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Please log on to the following website:
http://phet.colorado.edu/
Click Play with sims
On the left side, scroll down to find “Math
Tools” and click it. New icons appear on the
right side of the screen. Scroll down on the right
side until you see “Vector Addition” and click
on the icon. The program may take a few
minutes to load.
• Answer the questions on GRAPH PAPER
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Tonight’s Homework
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• p. 27-30, #49, 58, 61, 63, 72, 73, 80, 95, 96,
102
• Finish Vector Lab, if necessary
• Study for Ch. 1 Quiz
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