chapter 2
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Chapter 2Motion and Speed
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.
Frame of reference:
https://www.youtube.com/watch?v=Y75kEf8xLxI
PSCS 8 minutes
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.
Distance and Displacement graphic
Distance vs. Displacement http://
www.bing.com/videos/search?q=distance+vs+displacement+animation&FORM=VIRE1#view=detail&mid=F95E0B662B56B1874E89F95E0B662B56B1874E89
• Distance and Displacement can be the same.
• Distance can be longer than Displacement
• Displacement cannot be longer than Distance
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
Speed, Distance examples
https://www.youtube.com/watch?v=5e6QDSo_8W8
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?
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
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
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
Positive SpeedPositive Speed: Moving away
from the origin or reference point–Graph has a positive (upward)
slope
Constant, Positive Speed
•Straight Line•Speed isn’t changing
Fast vs. Slow Positive SpeedFast: has a steep slope Slow: has a shallow,
gradual slope
Changing, Positive Speed
Speeding Up
•Graph has a curve• The slope is changing, so speed is changing
Slowing Down
Negative SpeedNegative Speed: Moving
toward the origin or reference point–Graph has a negative
(downward) slope
Constant, Negative speed
Fast vs. Slow Negative SpeedFast: steep,
downward slopeSlow: shallow, gradual downward slope
Changing, Negative Speed
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)
Which one is the fastest? Which isn’t moving?
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
#1 How long will it take a bike rider to travel
450 meters at a constant speed of 2 m/s?
#2 If a car traveled 2500 m in 20 minutes,
what is the average speed of the car?
#3 If a bug travels for 50 seconds at 4 meters
per second, how far does it travel?
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
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.
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
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
Negative Acceleration(Slowing Down)
•Negative acceleration has a downward slope
- acceleration
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
An Object Changing Acceleration
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.
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.
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