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Chapter 3Section 1: Acceleration
What is the first thing that enters your mind when you hear the
word acceleration?
Acceleration is the rate at which an object’s velocity
changes
MOTION DIAGRAM
MOTION DIAGRAMWhat do increasing and decreasing lengths of velocity vectors indicate on a motion diagram? - increasing length indicates the object is speeding up. Decreasing length indicates the objects is slowing down.
TYPES OF ACCELERATION
• Increasing speed–Example: Car speeds up at green light
• Decreasing speed–Example: Car slows down
at stop light
screeeeech
TYPES OF ACCELERATION
• Changing Direction–Example: Car takes turn (can
be at constant speed)
TYPES OF ACCELERATION
SUMMING UP
QUESTION
• How can a car be accelerating if its speed is a constant 65 km/h?–If it is changing its direction
QUESTION
• If it is changing directions is it accelerating?–Yes, because acceleration is
either a change in speed or direction (velocity)
CORRECTING MISCONCEPTIONS
Acceleration and deceleration is not solely on speeding up and slowing down. 1. What effect a positive acceleration will have on a object moving in a positive direction? - the object will speed up
CORRECTING MISCONCEPTIONSAcceleration and deceleration is not solely on speeding up and slowing down.2. What effect a negative acceleration will have on a object moving in a negative direction? - the object will speed up
CALCULATING ACCELERATION• If an object is moving in a straight line
TimeSpeedInitialspeedFinalonAccelerati __
Units of acceleration:m/s2
Question• A skydiver accelerates from 20 m/s to 40
m/s in 2 seconds. What is the skydiver’s average acceleration?
2/102/20
2/20/40
__
smssm
ssmsm
TimespeedInitialspeedFinalAccel
Solve # 5-10 on page 67
ANALYSIS
• Describe the motion of an object if its velocity and acceleration vectors have opposite signs.
- The object is slowing down
ANALYSIS
• Can you draw an acceleration vector for two successive velocity vectors that are the same length and direction? Explain.–No, acceleration is zero because
there is no change in velocity
GRAPHING ACCELERATION• Can use 2 kinds of graphs–Speed vs. time–Distance vs. time
Graphing Acceleration:Speed vs. Time Graphs
02468
101214
0 1 2 3 4 5 6Time (s)
Spee
d (m
/s)
1)In Speed vs. Time graphs:Acceleration = Rise/Run = 4 m/s ÷ 2 s = 2 m/s2
Run = 2 s
Rise = 4 m/s
QUESTION
02468
101214
0 1 2 3 4 5 6
Time (s)
Spee
d (m
/s)
Above is a graph showing the speed of a car over time.1) How is the speed of the car changing (speeding up,Slowing down, or staying the same)?2) What is this car’s acceleration?
1)The car is slowing down2)Acceleration = rise/run
= -6m/s ÷3s = -2 m/s2
Run = 3 s
Rise = -6 m/s
Graphing Acceleration:Distance vs. Time Graphs
05
101520253035
0 1 2 3 4 5
Time (s)
Dis
tanc
e (m
)
1)On Distance vs. Time graphs a curved line means the object is accelerating.
2)Curved line also means your speed is increasing. Remember slope = speed.
ANALYSIS
05
101520253035
0 1 2 3 4 5
Time (s)
Dis
tanc
e (m
)
1)Which line represents an object that is accelerating?
The black and red lines represent a objects that are accelerating. Black is going a greater distance each second, so it must be speeding up. Red is going less each second, so must be slowing down. The blue and green lines are at constant speed since they are flat lines.
ANALYSIS
02468
101214
0 1 2 3 4 5 6Time (s)
Spee
d (m
/s)
It is a negative acceleration since the line is curving downward, which means the object is slowing down
05
1015202530354045
0 1 2 3 4 5 6
Time (s)
Dis
tanc
e (m
)