Download - Chapter 2
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Chapter 2Motion and Speed
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Sec. 1 Describing MotionMotion occurs when an object
changes position.You don’t need to see it move to
know that motion has happened.–You just need a reference point (text p 38)–A reference point is a coordinate system
in which the position of the objects is measured.
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Frame of reference:
https://www.youtube.com/watch?v=Y75kEf8xLxI
PSCS 8 minutes
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Distance and Displacement Distance—how far an object moved
The SI unit for distance is the meter,m Displacement—the distance and
direction of an object’s change in position from the starting point.
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Distance and Displacement graphic
Distance vs. Displacement http://
www.bing.com/videos/search?q=distance+vs+displacement+animation&FORM=VIRE1#view=detail&mid=F95E0B662B56B1874E89F95E0B662B56B1874E89
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• Distance and Displacement can be the same.
• Distance can be longer than Displacement
• Displacement cannot be longer than Distance
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Speed Speed—the distance an object travels per
unit of time.Speed = distance s = d
time t
Ex. You ran 2 km in 10 min. What is your speed?s = 2 km s=0.2 km
10 min min
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Speed, Distance examples
https://www.youtube.com/watch?v=5e6QDSo_8W8
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Average & Instantaneous Speed
Average speed—total distance traveled divided by total travel time.–Used when speed is changing
Instantaneous speed—the speed at a given point in time.
Which one does a speedometer in a car measure?
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Velocity Speed describes only how fast
something is moving. Velocity—includes the speed of an
object AND the direction of its motion– Ex: 40 mi/hr North or 5 m/s up
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Change in Velocity Velocity can change in 2 ways
1. Change in speed
2. Change in direction
Or Both Ex: a race car has a constant speed
of 100km/hr around an oval track.– Speed is constant, but velocity
changes
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Graphing Motion On a distance vs. time graph, time
goes on the x-axis and distance goes on the y-axis.
The slope (steepness) of the line represents the speed of the object.
Distance
Time↓Speed
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Positive SpeedPositive Speed: Moving away
from the origin or reference point–Graph has a positive (upward)
slope
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Constant, Positive Speed
•Straight Line•Speed isn’t changing
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Fast vs. Slow Positive SpeedFast: has a steep slope Slow: has a shallow,
gradual slope
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Changing, Positive Speed
Speeding Up
•Graph has a curve• The slope is changing, so speed is changing
Slowing Down
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Negative SpeedNegative Speed: Moving
toward the origin or reference point–Graph has a negative
(downward) slope
Constant, Negative speed
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Fast vs. Slow Negative SpeedFast: steep,
downward slopeSlow: shallow, gradual downward slope
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Changing, Negative Speed
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Not Moving (0 speed)A horizontal line (zero slope) indicates no motion (or no change from the origin)
Time (seconds)
Dis
tan
ce
(m
ete
rs)
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Which one is the fastest? Which isn’t moving?
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Rearranging the speed equation The original speed equation is s=d/t This can be rearranged to solve for d or t. s = d
t d = s x t (how far?) t = d (how long?)
s
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#1 How long will it take a bike rider to travel
450 meters at a constant speed of 2 m/s?
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#2 If a car traveled 2500 m in 20 minutes,
what is the average speed of the car?
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#3 If a bug travels for 50 seconds at 4 meters
per second, how far does it travel?
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Acceleration Acceleration: Change in velocity over
time When something speeds up, it has a
positive acceleration When something slows down, it has a
negative acceleration Units for acceleration have one distance
unit and 2 time units–Example Units: m/s/s or m/s2, mi/hr/min
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Changing Direction A change in velocity
can be either a change in speed or a change in direction.
Any time an object changes direction, its velocity changes and it is accelerating.
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Calculating Acceleration Acceleration Equation:
acceleration = change in velocity
time
Change in velocity = final velocity (vf) – initial
velocity(vi) We can write the acceleration
equation as a = (vf – vi)
t
(vf -vi)a t
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Graphing Acceleration•On a Velocity vs Time Graph, the slope of the line is the acceleration
•Positive acceleration has an upward slope
Positive Acceleration (speeding up)
+ ac
cele
ratio
n
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Negative Acceleration(Slowing Down)
•Negative acceleration has a downward slope
- acceleration
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Zero Acceleration When an object has a constant velocity, it has 0 acceleration, so the graph is a horizontal line.
time
velo
city
0 acceleration
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An Object Changing Acceleration
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Acceleration Example #1 An airplane starts at rest and speeds up
to 80m/s at the end of the runway in 20 s. What is its acceleration?– a = (vf – vi) = (80 m/s – 0 m/s)
t 20 s
= 4 m/s2
The airplane is speeding up, so the acceleration is positive.
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Acceleration Example #2 A skateboarder is moving at 3 m/s
and comes to a stop in 2 s. What is the acceleration?a = (vf – vi) = (0 m/s – 3 m/s) = -1.5
m/s2
t 2 s
The skateboarder slowed down, so the acceleration is negative.
Note: words like at rest and stop mean velocity = 0.