AP Physics

Special Focus:
Graphical Analysis
Graphical Analysis of Motion: Kinematics
Dolores Gende
Parish Episcopal School
Dallas
Graphical analysis is one of the most fundamental skills that introductory physics
students should acquire. This article presents a practical approach that stresses
conceptual understanding and interpretation of motion graphs in one dimension.
Randall Knight1 reports that even though nearly all students can graph a set of data
or can read a value from a graph, they experience difficulties with interpreting the
information presented graphically. Some student difficulties include:
• Many students don’t know the meaning of “graph a versus b.” They graph the first
quantity on the horizontal axis, ending up with the two axes reversed.
• Many students think that the slope of a straight-line graph is found from y/x
(using any point on the graph) rather than Δy/Δx.
• Students don’t recognize that a slope has units or don’t know how to determine
those units.
• Students don’t recognize that an “area under the curve” has units or don’t
understand how the units of an “area” can be something other than area units.
Describing Motion
The study of one-dimensional kinematics is concerned with the multiple means by
which the motion of objects can be represented. Such means include the use of words,
graphs, equations, and diagrams.
A suggested sequence for the introduction of one-dimensional kinematics includes:
• Constant velocity: Qualitative and quantitative analysis and interpreting graphs
• Accelerated motion: Qualitative and quantitative analysis and interpreting graphs
Analysis of motion, both qualitative and quantitative, requires the establishment of a
frame of reference. The exercises in this article assume a frame of reference with respect
to the Earth.
The direction of motion is determined by using a Cartesian coordinate system, where
1 R. Knight, Instructor’s Guide: Physics for Scientists and Engineers, Prentice Hall, 2004.
AP Physics: 2006–2007 Workshop Materials 9

Special Focus:
Graphical Analysis
the initial position is denoted as x0 = 0. If the object moves to the right, its direction is
positive; if it moves to the left, its direction is negative.
Arons2 suggests that an effective way of reaching students and improving their
conceptual understanding is to lead them through direct kinesthetic experiences,
giving them problems in which they must translate:
• From the graph to an actual motion
• From an actual motion to its representation on a graph
Constant Velocity: Position vs. Time Graphs
A simple analysis of constant velocity can be done using a bowling ball rolling on
a carpeted floor or using a battery-operated car. Video analysis is a great tool for
analyzing the motion in detail. The objective is for the students to be able to interpret
graphs of x vs. t in different directions. Here are some examples:
Give a qualitative description of the motion depicted in the following x vs. t graphs:
x
a. b.
t
t
x
Solution: Object starts at Solution: Object starts to the right
the origin and moves in of the origin and moves in the
the positive direction with negative direction with constant
constant velocity. velocity ending at the origin.
x x
c. d.
t t
Solution: Object starts at Solution: Object starts to the right
the origin and moves in of the origin and moves in the
the positive direction with negative direction with constant
constant velocity. velocity ending at the origin.
2 A. B. Arons, Teaching Introductory Physics, John Wiley & Sons, 1997.
AP Physics: 10 2006–2007 Workshop Materials

Special Focus:
Graphical Analysis
Constant Velocity: Velocity vs. Time Graphs
Give a qualitative description of the motion depicted in the following v vs. t graphs:
v
a. b.
t
t
v
Solution: Object moves to the Solution: Object moves to the
right at a fast constant speed. left at a slow constant speed.
Quantitative Approach
The next step is to have the students calculate the slope of an x vs. t graph and
understand that the value obtained is the average velocity. When the velocity is
constant, the average velocity over any time interval is equal to the instantaneous
velocity at any time.
The students should also be able to calculate the area under the curve of a v vs. t graph
and understand that the value obtained is the displacement.
Accelerated Motion
McDermott3 and her Physics Education Research group have suggested an excellent
approach that presents students with situations of a ball rolling along a series of level
and inclined tracks. This experiment can be performed in the classroom or lab using a
ball-track setup or a dynamics track and a cart.
The students should draw qualitative graphs of x vs. t, v vs. t, and a vs. t.
v0 = 0
x0 = 0
3 L. C. McDermott and P. S. Shaffer, Tutorials in Introductory Physics, Prentice Hall, 2002.

Special Focus:
Graphical Analysis
Solution:
x v a
t
t t
Solution:
x0 = 0
x v a
v0 = 0
t t
t

Special Focus:
Graphical Analysis
Solution:
v0 ≠ 0
x0 = 0
x v a
t t
t
v0 = 0
x0 = 0

Special Focus:
Graphical Analysis
Solution:
x v a
t t
t
x0 = 0
Solution:
x v a
v0 = 0
t t
t

Special Focus:
Graphical Analysis
After the students have made their predictions, they should conduct different
experiments to verify their graphs. The use of motion detectors and software programs
such as Logger ProTM or Graphical AnalysisTM is very effective in this analysis.
This qualitative approach will help the students understand that the signs of the velocity
and the acceleration are the same if the object is speeding up and that the signs of the
velocity and the acceleration are the opposite if the object is slowing down.
Interpreting Graphs
1. Give a qualitative description of the motion of an object at the different time
intervals depicted in the following v versus t graph:
A B C D E F
Answer:
A-B Positive acceleration, object is speeding up
B-C Object is moving with positive constant velocity
C-D Negative acceleration, object is slowing down
D-E Negative acceleration, object is speeding up
E-F Object is moving with negative constant velocity
F-G Positive acceleration, object is slowing down
t (s)
G

Special Focus:
Graphical Analysis
2. Give a qualitative description of the motion of an object at the different time
intervals depicted in the following x versus t graph:
Answer:
A-B Object is at rest
B-C Negative acceleration, object is slowing down
C-D Negative acceleration, object is speeding up
D-E Object is at rest
E-F Positive acceleration, object is speeding up
F-G Object is moving with positive constant velocity
Quantitative Approach
The quantitative approach should include calculations of:
• Slope of the tangent of an x vs. t graph and definition of instantaneous velocity
• Slope of the v vs. t graph and the understanding that the value obtained is the
average acceleration
• Area under the v vs. t graph and the understanding that it gives the displacement
• Area under the a vs. t graph and the understanding that it gives the change
in velocity

Special Focus:
Graphical Analysis
1. x
t
t
Solution:
Object starts at the right of the
origin, moves right at constant
v, stands still, and then moves
left at faster constant v.
2. v x
t
t
v
Solution:
Object moves left at constant v,
then moves to the right at
constant v but slower, then
stands still.
Graphical Analysis of Motion: Free-Response Questions from
Past AP Physics Exams
Answers to these questions can be found in College Board publications, on the AP
Central Web site, or at AP Summer Institutes and workshops.
1982 Physics B, Question 1
The first meters of a 100 meter dash are covered in 2 seconds by a sprinter who starts
from rest and accelerates with a constant acceleration. The remaining 90 meters are run
with the same velocity the sprinter had after 2 seconds.
a. Determine the sprinter’s constant acceleration during the first 2 seconds.
b. Determine the sprinter’s velocity after 2 seconds have elapsed.
c. Determine the total time needed to run the full 100 meters.
d. On the axes provided below, draw the displacement vs. time curve for the sprinter.

Special Focus:
Graphical Analysis
100
80
60
40
20
0
0 2 4 6 8 10 12
Time (seconds)
Displacement (meters)
800
700
600
500
400
300
200
100
0
0 5 10 15 20
Time (s)
Scale Reading
(newtons)
A student whose normal weight is 500 newtons stands on a scale in an elevator and
records the scale reading as a function of time. The data are shown in the graph above.
At time t = 0, the elevator is at displacement x = 0 with velocity v = 0. Assume that the
positive directions for displacement, velocity, and acceleration are upward.
a. On the diagram below, draw and label all of the forces on the student at
t = 8 seconds.

Special Focus:
Graphical Analysis
b. Calculate the acceleration a of the elevator for each 5 second interval.
i. Indicate your results by completing the following table.
Time Interval (s) 0–5 5–10 10–15 15–20
a (m/s2) ____ ____ ___ ____
ii. Plot the acceleration as a function of time on the following graph.
0 5 10 15 20
10
5
0
–5
Acceleration (m/s2)
–10
Time (s)
c. Determine the velocity v of the elevator at the end of each 5 second interval.
Time Interval (s) 0–5 5–10 10–15 15–20
v (m/s) ____ ____ ___ ____
ii. Plot the velocity as a function of time on the following graph.
25
20
15
10
5
0
–5
–10
–15
–20
–25
0 5 10 15 20
Time (s)
Velocity (m/s)

Special Focus:
Graphical Analysis
d. Determine the displacement x of the elevator above the starting point at the end of
each 5 second interval.
Time Interval (s) 0–5 5–10 10–15 15–20
x (m) ____ ____ ___ ____
ii. Plot the displacement as a function of time on the following graph.
0 Time (s)
0 5 10 15 20
250
200
150
100
50
Displacement (m)

Special Focus:
Graphical Analysis
A 0.50 kg cart moves on a straight horizontal track. The graph of velocity v versus time t
for the cart is given below.
a. Indicate every time t for which the cart is at rest.
b. Indicate every time interval for which the speed (magnitude of velocity) of the cart
is increasing.
c. Determine the horizontal position x of the cart at t = 9.0 s if the cart is located at
x = 2.0 m at t = 0.

Special Focus:
Graphical Analysis
d. On the axes below, sketch the acceleration a versus time t graph for the motion of the
cart from t = 0 to t = 25 s.
e. From t = 25 s until the cart reaches the end of the track, the cart continues with
constant horizontal velocity. The cart leaves the end of the track and hits the floor,
which is 0.40 m below the track. Neglecting air resistance, determine each of
the following:
i. The time from when the cart leaves the track until it first hits the floor
ii. The horizontal distance from the end of the track to the point at which the cart
first hits the floor
iii. The kinetic energy of the cart immediately before it hits the floor

Special Focus:
Graphical Analysis
The vertical position of an elevator as a function of time is shown above.
a. On the grid below, graph the velocity of the elevator as a function of time.
b. i. Calculate the average acceleration for the time period t = 8 s to t = 10 s.
ii. On the box below that represents the elevator, draw a vector to represent the
direction of this average acceleration.
c. Suppose that there is a passenger of mass 70 kg in the elevator. Calculate the apparent
weight of the passenger at time t = 4 s.

Special Focus:
Graphical Analysis
2005 Physics B, Form B, Question 1
A student of mass m stands on a platform scale in an elevator in a tall building. The
positive direction for all vector quantities is upward.
a. Draw a free body diagram showing and labeling all the forces acting on the student,
who is represented by the dot below.
b. Derive an expression for the reading on the scale in terms of the acceleration a of the
elevator, the mass m of the student, and fundamental constants. An inspector
provides the student with the following graph showing the acceleration a of the
elevator as a function of time t.
c. i. During what time interval(s) is the force exerted by the platform scale on the
student a maximum value?
ii. Calculate the magnitude of that maximum force for a 45 kg student.
d. During what time interval(s) is the speed of the elevator constant?

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