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

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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

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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

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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

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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

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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

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0 1 2 3 4 5 6

Time (s)

Dis

tanc

e (m

)