Acceleration
Acceleration = speeding up
Acceleration – the rate at which velocity changes Can be an:
Increase in speed Decrease in speed Change in direction
Types of acceleration
Increasing speed Example: Car speeds up at green light
Decreasing speed Example: Car slows down at stop light
Changing Direction Example: Car takes turn (can be at
constant speed)
screeeeech
Question
How can a car be accelerating if its speed is a constant 65 km/h?
If it is changing directions it is accelerating
Calculating Acceleration
If an object is moving in a straight line
Time
SpeedInitialspeedFinalonAccelerati
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Units of acceleration:Distance/ time = distance/time2
timem/s2
Calculating Acceleration
0 s 1 s 2 s 3 s 4 s
0 m/s 4 m/s 8 m/s 12 m/s 16 m/s
2/4
4
/0/16
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sm
s
smsmTime
SpeedInitialSpeedFinalonAccelerati
Question
A skydiver accelerates from 20 m/s to 40 m/s in 2 seconds. What is the skydiver’s average acceleration?
2/10
2
/20
2
/20/40
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sm
s
sm
s
smsmTime
speedInitialspeedFinalAccel
Graphing Acceleration
Can use 2 kinds of graphs Speed vs. time Position vs. time
Graphing Acceleration:Speed vs. Time Graphs
1)Speed is increasing with time = accelerating2)Line is straight = acceleration is constant
Graphing Acceleration:Speed vs. Time Graphs
1)In Speed vs. Time graphs:Acceleration = Rise/Run = 4 m/s ÷ 2 s = 2 m/s2
Run = 2 s
Rise = 4 m/s
Graphing Acceleration:Position vs. Time Graphs
1)On Position vs. Time graphs a curved line means the object is accelerating.
2)Curved line also means your speed is increasing. Remember slope = speed.
Question
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
Question:
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
Remember: in position vs. time graphs: curved line = accelerating,
straight line = constant speed
Question: Hard one
Free fall
The constant acceleration of an object moving only under the force of gravity
If there was no air, all objects would fall at the same speed regardless of the masses
Acceleration due to gravitational force
d = vit + ½gt2
If vi = 0, then d = ½ gt2
g = acceleration due to the force of gravity
= 9.8 m/s2
= 10 m/s2
Free fall
After 1 second falling: v= 10 m/s
After 2 seconds: v = 20 m/s
After 3 seconds: v = 30 m/s
Falling Air resistance will
increase as it falls faster
An upward force on the object
Eventually gravity will balance with air resistance
Reaches terminal velocity - highest speed reached by a falling object.
Bozeman videos
http://www.bozemanscience.com/position-vs-time-graph-part-2
http://www.bozemanscience.com/speed-velocity-acceleration
Questions
Answer the following questions and email me your answers. ([email protected] )
1. How can a car accelerate without the speedometer changing?
2. What does the slope of a velocity vs time graph give you?
3. What is the value of the acceleration due to the force of gravity on Earth?
4. What is the color of the object that is rolled across the table by Mr. Andersen?