Acceleration

Measuring motion

By Vinesh VoraAndrew Jaisingh

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)

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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

TimeSpeedInitialspeedFinalonAccelerati __

Units of acceleration:m/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

2/44

/0/16

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sms

smsmTime

SpeedInitialSpeedFinalonAccelerati

Question

• A skydiver accelerates from 20 m/s to 40 m/s in 2 seconds. What is the skydiver’s average acceleration?

2/102/20

2/20/40

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smssm

ssmsm

TimespeedInitialspeedFinalAccel

2/102/20

2/20/40

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smssm

ssmsm

TimespeedInitialspeedFinalAccel

Graphing Acceleration

• Can use 2 kinds of graphs– Speed vs. time– Distance vs. time

Graphing Acceleration:Speed vs. Time Graphs

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1) Speed is increasing with time = accelerating2) Line is straight = acceleration is constant

Graphing Acceleration:Speed vs. Time Graphs

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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:Distance vs. Time Graphs

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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.

Area under a graph

• The area under a velocity-time graph shows the distance an object has traveled.

• In order to find the distance (m), multiply the velocity (m/s) by the time (s).• Area=(base)(height)

=(time)(velocity)=distance

• When the area under the velocity-time graph is a right triangle, the area is found using the equation.

• Area=1/2base X height

Question:

• What is the total distance traveled by the bus during the first 10 seconds?

Answer:• First, calculate the first 5 seconds, which is a right-triangle.

Area=1/2base X height=1/2(5m)(10m/s)

=25m• Next, calculate the last 5 seconds, which is a rectangle.

• Area=(base)(height)=(time)(velocity)

=(5s)(10m/s)=50m

• Last, add the two totals to get the entire area under the graph, which is your total distance.

• 25m+50m=75m

• Answer=75m

Question

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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:

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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 distance vs. time graphs: curved line = accelerating, flat line = constant speed

Question: Hard one

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Above is a graph showing the speed of a car over time.1)What would a distance vs. time graph for thislook like?

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Slopes of graphs reveal how objects move

Because the plane is moving at aconstant velocity, its acceleration is zero.

It is called non-uniform motion because the object’s speed or directionis changing.

Form to use when accelerating object has an initial velocity

Form to use when accelerating object starts from rest

http://www.youtube.com/watch?v=hD0ineZXpcw

Video