1. Interpreting Information – Graphing
Equipment needed in Investigation 1:
Wooden Disks, Ruler, and Tape Measure
Nuts, Bolts and Balance
The purpose of this lab is to develop skill in interpreting data through an alternative means –
creating an appropriate graph or graphs and examining them to determine what the data can tell us.
In laboratory investigations, you generally control one quantity (variable) and measure its
effect on a second quantity (variable) while holding all other factors constant. After the data is
collected, it is helpful to visualize the relationship between these variables by making a graph of
the second variable versus the first variable using proper graphing techniques. The resulting graph
will give you a better understanding of the relationship between the two variables and enable you
to interpret the physical system it represents.
When making a graph, use the following steps.
n Identify the independent variable (the quantity you control) and the dependent variable (the
quantity affected by the independent variable).
n Use the horizontal axis for the independent variable and the vertical axis for the dependent
variable. Label each axis with the name of the variable and the unit.
n Choose a scale. Make the graph as large as possible by spreading out the data on each of the
axes to cover more than half the available grid. Let each grid division stand for a convenient
amount. Choosing three grid divisions to equal 10 is not good because then each grid division is a
fraction. Choosing five grid divisions to equal 10 would be better. To avoid clutter, do not
number every grid division.
n Plot each data point as a dark dot with a small circle around it.
n If the data points appear to lie in a straight line, draw the best straight line through them with
a ruler and pencil. The line should go through as many points as possible with the same number of
points above or below the line. Do not just “connect the dots.” If the points do not form a straight
line, draw the best smooth curve possible keeping in mind the curve should go through as many
points as possible with the same number of points above or below the curve.
n Graphs do not necessarily go through the origin (0,0). Think about your experiment and
decide if the data would logically include a (0,0) point.
n Each graph should have a brief, yet meaningful, title. Examples are “Cart Moving with
Constant Velocity” or “Position versus Time” or “Variation of Pressure with Change in Volume.”
The following is an example of a graph prepared using good graphing techniques.
There are several relationships that occur frequently in physical processes, as illustrated by the
four graphs below. If the dependent variable varies linearly with (and is directly proportional to
the independent variable, the graph will be a straight line passing through the origin. If the
dependent variable varies linearly with (but is not directly proportional to) the independent
variable, the graph will be a straight line that does not pass through the origin. If the dependent
variable varies with the square of the independent variable, the graph will be a parabola. If the
dependent variable varies inversely with the independent variable, the graph will be a hyperbola.
Linear Graphs: A linear graph that passes through the origin is mathematically represented by
the equation where the constant m is called the slope of the straight line. In this case, the
dependent variable y is directly proportional to the independent variable x. A linear graph that
does not pass through the origin is mathematically represented by the equation where
the constant b is the intercept along the y-axis and the constant m is the slope.
Parabolic Graphs: An example of an object moving along a parabolic path is projectile motion
under the influence of gravity. The dependent variable y does not vary linearly with the
independent variable x. Parabolic curves are mathematically represented by the equation
where k is a constant.
Reciprocal, Inverse, or Hyperbolic Graphs: These names collectively refer to graphs
represented mathematically by the equation where k is a constant. As an example,
y = mx y = mx + b y = kx2 y = k/x
y mx b= +
y kx= 2
y k x=
0.1 0.2 0.3 0.4 0.5
Elongation versus Force
pressure is inversely proportional to volume for an ideal gas. The dependent variable y decreases
nonlinearly as the independent variable x increases. It is important to note that a straight-line
graph will result if the variable y is plotted versus the quantity (1/x).
Graphs are useful because they can give you information that you have not directly deter-
mined experimentally. This information can be obtained from the graph very easily. Reading the
pair of values for the dependent and independent variables at a point on the graph that lies between
two of the data points used to construct the graph is called interpolation. Reading a pair of values
from a point on the graph falling outside the range of data points used to construct the graph is
called extrapolation. In addition to the coordinates, the slope of a graph also contains information.
For many experiments the slope may be all that you really want to find from the graph. The slope
of a straight-line graph usually has an important interpretation in terms of the physical system
represented on the graph. As an example, the slope of a position-versus-time graph is Dx/Dt and is
the velocity, whereas the slope of a velocity-versus-time graph is Dv/Dt and is the acceleration.
Finding the slope of a graph.
A linear graph has a constant slope (or steepness) associated with it. In the general equation
for a straight line (y = mx + b), the slope is m and the y-intercept is b. The slope m can be either a
positive or negative number. If m is positive, the line slopes uphill because y increases as x
increases. If m is negative, the line slopes downhill because y decreases as x increases. The slope
can be calculated from the graph by choosing two points along the straight line (do not use actual
data points or (0,0)), as illustrated in the graph below. These points are separated on the x axis by
an amount Dx and on the y axis by an amount Dy. For good accuracy, always measure the slope
over as wide a range of Dx and
as possible. The slope is defined as the ratio of the change in the
vertical to the change in the horizontal, that is, slope = Dy/Dx = (y2 – y1)/(x2 – x1).
This is an example of finding the slope
of a graph. First, choose a point (mark-
ed x) near each end of the graph. Then
draw the legs of the triangle representing
Dx and Dy. In this case,
Dx = x2 – x1
= 0.45 dyne – 0.05 dyne
= 0.40 dyne
Dy = y2 – y1
= 4.5 cm – 0.5 cm
= 4.0 cm
Slope = Dy/Dx = 4.0 cm/0.40 dyne
Finally, the slope = 10 cm/dyne.
0 0.1 0.2 0.3 0.4
Elongation versus Force
During the course of the semester, you will often conclude your laboratory investigations
by comparing your results in either of two ways. In some of the investigations, the true value of
the quantity being measured is well known. In these labs, the accuracy of your experiment will be
determined by comparing the experimental value you obtained with the known or actual value.
This is done by calculating the percentage error.
Percentage Error = -Actual – Experimental
– × 100% (1)
In other investigations, a given quantity will be measured or calculated by two different
methods. There will then be two different experimental values E1 and E2, but there may not be a
true value to use for comparison. In these labs, the percentage difference between the two
experimental values will be calculated. Note that this does not tell you anything about the
accuracy of the experiment, but it is a measure of the precision of your experiment.
Percentage Difference = 5 1!21″(1!41″)
5 × 100% (2)
The average is chosen as the basis for comparison when there is no reason to think one of the
values is more reliable than the other. Note that both of these quantities are often referred to as the
Investigation 1-1. Circumference and Diameter
(1) You will determine the relationship between the diameter and circumference of circular
objects by graphing. There are four wooden disks at your lab station. Using the best available
method, measure (DO NOT CALCULATE) the diameter and circumference of each disk.
Record these values in Part B of your Lab Report.
(2) Make a graph of your data on the grid provided by plotting the diameter, D, on the horizontal
axis and the circumference, C, on the vertical axis. Choose the scales for the graph so that
your data cover a majority of the grid. Draw a best-fit straight line that passes through most
data points. If it is reasonable to extend the line through the origin, do so. Your graph
represents the relationship between the diameter and the circumference.
(3) In Part C of your Lab Report, determine the slope of the graph.
Investigation 1-2. Determining Relationships from Graphs
In Part D of your Lab Report, you will use graphing techniques to determine a physical property of
an object. The goal is to measure the mass of a bolt indirectly by making some measurements and
interpreting them using a graph.
(1) Place one nut on the bolt.
(2) Measure and record the mass of the bolt + nut combination.
(3) Add another nut on the bolt/nut combination.
(4) Measure and record the mass of the bolt + 2 nuts.
(5) Repeat with 3 and 4 nuts threaded on the bolt.
(6) Graph your data with the y-axis as the mass values and the x-axis as the number of nuts
threaded on the bolt.
(7) Calculate the mass of the bolt as the y-intercept.
(8) Calculate the mass of the nut as the slope.
(9) Check your calculation by measuring the mass of a nut by itself and the mass of the bolt by
itself. Calculate the uncertainty in your value.
Investigation 1-3. Area of a
Area and volume of regular-shaped objects can be calculated by using the appropriate
formula. But this is not possible for irregularly shaped objects and we must resort to an
operational definition. An operational definition of the area of any shaped figure, no matter the
shape, is to find the number of standard squares that fit inside the figure. You will use the
operational definition of area to find the areas of the four circles below. A grid of standard
squares, with each square 0.5 cm x 0.5 cm = 0.25 cm2, has been imposed over the four circles. By
counting the number of standard squares and estimating the partial squares (those squares which
are more than half inside the circle count as one, while those which are less than half inside the
circle are not counted), you can determine approximately the unknown area.
0 1 2 3 4 5 6 7 8 9 10
(1) Find the area of each circle below by counting the number of squares (according to the
operational definition above) and multiplying by the area of one square. Record the area in the
second column of the table provided in Part E of your Lab Report.
(2) Determine, to the nearest 0.1 cm, the radius of each of the four circles and record the values in
Part E of your Lab Report.
(3) Make a graph of your data on the grid provided in Part F of your Lab Report. Plot the radius
on the horizontal axis and the area on the vertical axis. If the points do not appear to lie on a
straight line, draw a smooth curve that passes as close to as many data points as possible.
(4) When a graph is not a straight line you need to identify the type of curve. Often it will be one
of the graphs discussed earlier. To verify which of these curves you have, the data are
systematically manipulated until a graph is found that yields a straight line. For example, you
can square one of the variables, or take the square root of a variable, or find the reciprocal of a
To help determine the type of curve, square the radius and record the values in the fourth column
of the table in Part E of your Lab Report.
(5) Make a second graph of your data on the grid provided. Plot the square of the radius on the
horizontal axis and the area on the vertical axis. Draw a line that best fits the data.
(6) In Part H of your Lab Report, determine the slope of this last graph.
LAB REPORT FOR INVESTIGATION 1
Name ___________________________________ Date ___________________________
Lab Period (Day & Time) ___________________ Station_________________________
All measurements must have units. Calculations not involving ratios of like quantities must have
units. Either put the unit with each number or, if the numbers are contained in columns,
label the column with the unit. Include all steps in your calculations. Follow the format and
general procedures described in the Grading Policy.
Part A: Measurements of Diameter and Circumference
Disk Diameter Cicumference
1 4.2 14.5
2 6.8 20.7
3 7.5 24.7
4 11.2 35.5
5 14.8 43.7
Part B: Using the graph paper provided, plot this information.
Part C: Determination of the slope of the circumference-versus-diameter graph (show your
C1 = ________ C2 = ________ DC = ________
D1 = ________ D2 = ________ DD = ________
slope (m) = _________
Q1: What is the equation you generally use to find the circumference of a circle from the
Q2: Find the percentage error between the slope you calculated for the circumference-versus-
diameter graph and the value of the constant in the above equation. Ask your instructor what
percentage error is within experimental error to decide if the values of the slope and the constant
you determined are the same. If you measured carefully, this value of the slope should be very
familiar to you. What is this constant called?
Q3. Considering the things you did to measure circumference and diameter, what might cause
uncertainty in your result? Do not forget to consider the objects themselves.
# of nuts Mass of bolt with nut (g)
Q4: Briefly describe the relationship between mass and number of nuts shown on your graph.
Q5: Determine the slope of the graph. Include units
m1 = ________ m2 = ________ Dm = ________
# of nuts 1 = ________ # of nuts 2 = ________ D# of nuts = ________
slope = _________
Q6. Write an equation (of the form of the straight line equation, y=mx+b), that describes the above
Q7. Considering the things you did to obtain your measurements, what might cause uncertainty in
your result? Do not forget to consider the objects themselves.
Part E: Measurements of Radius and Area.
Part F: Graph radius vs area
Part G: Graph radius2 vs. area
Part H: Determination of the slope of the area-versus-radius squared graph.
A1 = A2 = DA =
= = D(r2) =
Q9. What is the equation you generally use to find the area of a circle?
Q10. Find the percentage error between the slope you calculated for the area-versus-radius
squared graph and the value of the constant in the above equation. Ask your instructor what
percentage error is within experimental error to decide if the values of the slope and the constant
you determined are the same.