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ACCELERATION
A. What is Acceleration?
Acceleration is defined as the rate at which velocity changes. Remember that velocity has two
parts: speed and direction. Therefore, acceleration occurs when there is a change in either of two parts.
Acceleration occurs when speed increases, speed decreases, or there is a change in direction. This
means that an object that is changing direction is accelerating even if it is moving at a constant speed!
Acceleration can be positive, negative, or zero.
Speeding Up
Slowing Down
Constant Speed or Not Moving
(+) Acceleration
(-) Acceleration or “deceleration”
No (0) Acceleration
B. Calculating Acceleration
To calculate the acceleration of an object moving in a straight line, divide the change in velocity by the
time over which the change occurred.
To find the change in velocity, subtract the initial velocity (Vi), which is the velocity at the start of the time
interval, from the final velocity (Vf), which is the velocity at the end of the time interval. This is summarized
in the formula below.
𝒂𝒄𝒄𝒆𝒍𝒆𝒓𝒂𝒕𝒊𝒐𝒏 =
•
•
•
(𝒇𝒊𝒏𝒂𝒍 𝒗𝒆𝒍𝒐𝒄𝒊𝒕𝒚 – 𝒊𝒏𝒊𝒕𝒊𝒂𝒍 𝒗𝒆𝒍𝒐𝒄𝒊𝒕𝒚)
𝒕𝒊𝒎𝒆
Velocity…
Time…
Acceleration…
𝒂=
(𝑽𝒇 − 𝑽𝒊)
𝒕
measured in meters per second (m/s)
measured in seconds (s)
measured in meters per second squared (m/s2)
C. Acceleration on DT Graphs and ST Graphs
Time
Line Curves Upwards:
(+) Acceleration
Distance
Distance
Distance
The distance an object travels can be plotted on a distance-time (DT) graph. Time is plotted on the Xaxis and distance is plotted on the Y-axis. For a DT graph, acceleration is shown by a curved line:
Time
Time
Line Curves Downwards:
(-) Acceleration
Straight Line (no curve)
No Acceleration (constant speed)
Speed
Speed
Speed
The speed of an object can be plotted on a speed-time (ST) graph. Time is plotted on the X-axis and
speed on the Y- axis. For an ST graph, acceleration is the slope of the line.
Time
Positive Slope =
(+) Acceleration
Time
Time
Negative Slope =
(-) Acceleration
No Slope (Horizontal)
No Acceleration (constant speed)
REVIEW QUESTIONS - ACCELERATION
1. Use the text to define the words below.
Vocabulary
Definition
a. acceleration
b. (+) acceleration
c.
(-) acceleration
d. (0) acceleration
2. Another name for negative acceleration is ____________________________________________.
3. True or false: an object can technically be accelerating even when its speed is constant. Explain.
4. Circle the letter of each sentence that describes an example of acceleration.
a) A car follows a gentle curve in the road.
b) A batter swings a bat to hit a baseball.
c) A truck parked on a hill doesn’t move all day.
d) A runner slows down after completing a race.
5. Write down the formula used to calculate the acceleration of an object moving in a straight line.
6. A roller coaster’s velocity at the top of the hill is 10.0 m/s. Two seconds later it reaches the bottom of
the hill with a velocity of 26.02 m/s. What is the acceleration? Be sure to include proper SI units.
7. Complete the tale below on how to interpret the slope of a line on a distance time (DT) graph.
Slope
a. Line is curving upwards
b. Line is curving downwards
c.
Line is flat
What does it mean about the acceleration?