Lab Inquiry Activity
Exploring Pendulums
Purpose: The students will be able to practice their problem solving skills using the
scientific method.
The students will be able to recognize the characteristics of a simple
pendulum.
The students will be able to manipulate the variables involved with a
pendulum to make it behave in a specific way.
Standards Covered:
5.1.2 Begin to evaluate the validity of claims based on the amount and quality of
the evidence cited.
6.1.2 Give examples of different ways scientists investigate natural phenomena and
identify processes all scientists use, such as collection of relevant evidence,
the use of logical reasoning, and the application of imagination in devising
hypotheses and explanations, in order to make sense of the evidence.
Materials Needed:
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(per group)
The pendulum definition handout.
6ft length of string
Heavy duty paper clips
Several washers
Stop Watches and Meter Sticks
Introduction:
The pendulum is a simple apparatus that is
found all around us in the world that we live in.
Pendulum motion can be seen with swings at a
playground, a grandfather clock, the wrecking ball
on a crane, or the motion of a watch used to
hypnotize a psychology patient. The definition of a
simple pendulum is a mass or weight suspended
from a fixed support so that it swings freely back
and forth under the influence of gravity. If a
pendulum is pulled back to position (a) from its
resting position (b) it will move periodically back
and forth, from (a) to (c) in an arced path. The fixed
point is in the center of the circular path, and the length (L) of the supporting string is
the length of the radius of the circle. The circular motion of the swinging mass is a
direct result of the “pull” of the rope on the mass that is always directed toward the
center of the circle.
The time it takes the pendulum to swing from point (a) to point (c) and back to
point (a) is called one period (T) of the pendulum. The period of any pendulum is only
dependent upon the length (L) of the pendulum arm, (L = the distance between the
mass and the fixed point of rotation) Changing the mass of the pendulum, has no
effect on the period of a pendulum. The relationship between the length of the
pendulum and the period of the pendulum is this: as the length increases the period
increases, and as the length decreases the period decreases.
Procedure:
1. Divide the students into groups and have them complete the “Pendulum
Definition Worksheet”.
2. Have each group briefly share their answers from the worksheet with the rest of
the class. (What are the similarities between the pictures? Based on you above
answer, what is the definition of a pendulum? And, what are other things that
act like pendulums?)
3. Give the students the definition of the period of a pendulum. (“The time for a
pendulum to complete one swing”) Note that the period is different for different
pendulums.
4. Give each group one long string, a paper clip, and 5 washers. (Tell the students
they have to construct a pendulum with a period of approximately one second.)
Instruct them to tie the paper clip on the end of the string so they can add paper
clips to it for weight. (Tell them, when measuring the period they should pull it
back the same amount each time.)
5. After each group has successfully constructed a pendulum with a period of
about 1 sec., have each group record the weight of the objects on the end of the
string or simply the number of washers and the length of the string, from the
point of rotation to the mass.
6. Draw a table on the chalkboard with two columns: one for mass (or # of
washers), and the second for the length of the string. In the table, have each
group record their mass and length values.
7. Discuss the following questions with the class:
 What are some of the things you could change about your pendulum?
What made them different from one another?
 What happened to the period if you added more washers to the
pendulum?
 What happened to the period if you increased the length of the string of
the pendulum?
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How did your group make the pendulum have a natural period of five
seconds?
Conclusion:
In the discussion the students should discover that only the length of the
pendulum has any affect on its period. (For a period of 1 sec. the length of the
pendulum should be approximately 0.25m or 0.814ft) If the students are having
problems getting to that conclusion, refer to the table written on the board. Note that all
of the lengths are approximately the same, yet the weights or numbers of washers are
different. So it is evident that the length of the pendulum is what is determining its
period. If they still have difficulties, have them go back to their pendulums. Tell them
to leave the length the same and add weights to see if it affects the period.
Extension Topics:
Discussion about the forces on a pendulum during its swinging motion presents
opportunities to cover other areas addressed in the Indiana State Standards for science.
Newton’s Laws of Motion:
When the pendulum is at rest, hanging straight down, the forces on the
pendulum are balanced. Therefore, according to Newton’s 2nd Law, the object will
be at rest. Newton’s 3rd Law explains why the forces are balanced: every action will
have an equal and opposite reaction. The force the rope exerts on the object at the
bottom of the pendulum is equal and opposite the force that gravity exerts on the
object. So the forces are balanced. When the pendulum is pulled back and released,
the forces on the pendulum are no longer balanced, and the object at the end of the
pendulum begins to move. Every time the pendulum is in the bottom position, the
forces are momentarily balanced once again, but the pendulum keeps moving.
Newton’s 1st Law can offer an explanation: …an object in motion tends to stay in
motion. Therefore, since the object is already moving, it will want continue moving,
and it will swing past the bottom position, where the forces are momentarily once
again.
Circular Motion:
The path of the mass of a pendulum is that of the arc of a circle, with the
radius of the circular path equal to the length (L) of the pendulum arm. (See above
diagram) The “pull” of the rope or rigid arm, constantly toward the center of the
circular path, is what causes the mass or weight to travel in a circular path. The
definition of the force required for circular motion, is a constant force always
directed toward the center of the circular path.
Gravity:
The non-contact force of gravity is what causes the swinging motion of a
pendulum when it is drawn back from its normal resting position. The universal
acceleration experienced by all objects under the influence of gravity, can be
identified with a pendulum by noting that the mass of the pendulum has no effect
on the period. A pendulum can be thought of as a device that demonstrates a
“controlled falling” of its mass. If the acceleration of the object is independent of its
mass or weight for the pendulum, it is also true for any falling object, regardless of
mass or weight.
Energy Conservation:
The energy associated with the mass at the end of a pendulum during its motion,
is a great example of energy being converted from one form to anther. When the
pendulum is at rest in the bottom position, we could define the mass as having a
gravitational potential energy of zero. When the mass is pulled back, it is raised to a
vertical height that is above the resting position. This adds to the mass some energy
in the form of gravitational potential energy because it is now at a certain height
above the zero energy position. It now has the “potential” to move. Once the raised
mass/weight is released it swings downward back toward the original position. As
vertical height is lost, potential energy is lost. Energy cannot simply disappear or be
destroyed; it only is transformed into a different kind of energy. The gravitational
energy that is lost is transformed into kinetic energy. (Kinetic Energy is the energy
that is associated with moving objects) When you observe the motion of a
pendulum you are seeing a constant conversion between gravitational potential
energy and kinetic energy as the mass/weight slows down and speeds up.
Pendulum Definition Worksheet
Name: _____________________
Directions: All of these things act like a pendulum. Look at the similarities between all of the
pictures and write your definition of pendulum at the bottom of the worksheet. Include as many
similarities as you can!
1. What are the similarities between the pictures?
2. Based on your answer above, what is a pendulum?
3. What are other things that act like pendulums?