physics 20: unit 1 kinematics - linville kinematics.pdf · physics 20: unit 1 kinematics or...
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Unit 1: Kinematics
Physics 20: Unit 1 Kinematics
Or Describing Motion
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Introduction
• a scalar is a quantity that has no direction,
such as distance (ex. 56 m) and time
(magnitude only)
• a vector has magnitude and direction, i.e.
velocity 56 m east
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Units and directions
• Units and direction of vectors are always
important!
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• Vectors in opposite directions must be
given opposite signs in equations
• vectors that are parallel or point in the
opposite direction are collinear
• Vectors are indicated by arrows on
diagrams
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Example:
A person walks 100 m east and then 210 m east. The
vector diagram would look like this
A person walks 200 m east and then 400 m west
Angle between these vectors is 180o
100 m east 210 m east
200 m east 400 m west
Angle between these vectors is 0o
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• position is a distance measured from some
origin or starting point, plus the direction,
symbol is
d
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• distance and displacement are not the
same
• displacement is the change in position, the
change in any quantity is always the final
value minus initial value
d
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Start Point
(reference
point)
Finish point
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• the magnitude of vectors are indicated by
using the absolute value symbol ex.
|v|
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• speed is the rate at which an object moves
• velocity is a vector
• speed and velocity do not always have the
same magnitude
time
cetandisv
t
dv
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Example A golfer hits a ball as shown. Assume the golfer is
at the reference point. Determine the distance
traveled and the displacement of the ball
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if ddd
= 10 m - 0 m = 10 m right.
Distance = 15 m
Displacement = final position – initial position
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• If the ball was sunk in 6.2 s, determine
the average speed and average velocity
of the ball.
tan 152.4 /
6.2
dis ce mv m s
time s
101.6 / right
6.2
d mv m s
t s
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Uniform Motion
• uniform motion is motion in a straight line at a constant speed, the velocity is constant: no change in magnitude or direction
• the position-time graph for uniform motion is always a line
• the slope of the line is velocity
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position
position
Object is moving
away from the
reference position
Object is moving
towards the
reference position
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• velocity-time graph is horizontal
• slope = acceleration
• area = displacement
Horizontal line: slope = 0
Area= Lw
=velocity x time
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Average Velocity
• The velocity
for the interval
0 – 6.0 s is
not uniform
Dis
tan
ce
(m
)
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Average Velocity
• The average
velocity for the
interval (t)
0 – 6.0 s is
101.7 /
6.0
ave
ave
dv
t
mv m s
s
Dis
tan
ce
(m
)
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Example Chainsaw, the psychotic
physics feline, walks
12.3 m in 39.7 s, stops to
scratch the couch for
28.0 s, then runs at 11.4
m/s for 12.9 s. Calculate his
average speed.
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1
2
tan
12.3
11.4 / (12.9 ) 147.06
dis ceaverage speed
time
d m
d vt m s s m
147.06 12.31.98 /
39.7 28.0 12.9
speed
m mm s
s s s
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Example 1. Describe the
motion for the 8.0 s.
2. Determine the average velocity of the object between 2.0 s and 7.0 s
1.2 m/s south
3. What is the maximum distance travelled by the object?
Displacement north as a function
of time