Chapter 9 Section 3: Acceleration
• Key concepts: What kind of motion does
acceleration refer to? How is acceleration
calculated? What graphs can be used to analyze
the motion of an accelerating object?
• Key term: acceleration
What is acceleration?
• Acceleration (OMG is that really how you
spell that? Why does that look so funny right
now…) is the rate at which velocity changes.
• Remember: velocity refers to speed AND
direction of an object.
Acceleration: a change in velocity
• In science, acceleration refers to increasing speed,
decreasing speed, or changing direction.
• Increasing speed = accelerates
• Decreasing speed = deceleration (or negative
acceleration)
• Changing direction: (even an object travelling at a
constant speed can be accelerating!!) a softball, for
example, accelerates when it changes direction as
it is hit; a runner as he/she rounds a curve
accelerates even if maintaining a constant speed
How can a car be accelerating if it is
maintaining constant speed?
Calculating acceleration
• To determine the acceleration of an object
moving in a straight line, you must calculate
the change in speed per unit of time.
• Acceleration =
final speed – initial speed
time
Acceleration calculations continued
• SI unit for acceleration is usually m/s2
• For example: imagine an adorable plane
The adorable plane
• The adorable plane accelerates as it takes off.
Of course it does, otherwise we would never
go anywhere and you would sit there on the
tarmac asking your flight attendant for more
tiny bags of peanuts for 8 hours and then you
would get off the plane (or “deplane) in
exactly the same place you started. Anyway…
It accelerates
• How do we calculate its acceleration?
• Please look at figure 7 on page 322 of your books
Do you remember the equation for acceleration?
• At first, the plane is not moving. So its initial
speed is 0 m/s
• Its final speed is 40m/s
• It takes 5 seconds
• So, to calculate the average acceleration of the
plane, you subtract the initial speed of 0 m/s (it
isn’t moving when it starts) from the final speed
of 40m/s, then divide the change in speed by
the time, 5 seconds. Your answer is 8 m/s2
In your notes
• Please attempt both math practice problems
on page 323. Remember acceleration’s
formula
• Acceleration =
final speed – initial speed
time
How to graph acceleration
• You can use both a speed vs time graph and a
distance vs time graph to analyze the motion
of an acceleration object
• Look at the graph in figure 8
• How can you calculate the acceleration from
this graph? Easy: calculate the slope of the line
• Rise/run = slope
What else can you tell from the graph?
• This is important: you will need to know how
to read graphs for the rest of your ever-lovin’
lives!
• B/c the line is straight: the acceleration was
constant
• b/c the line goes up: speed was increasing
Distance vs time graph figure 9 pg 325
• A curved line means that the object is
accelerating b/c you travelled a greater
distance each second than the second before
it
We’re done wid dis chaptahh
• Now we commits it to memoriess