**Document Id**: 01_13_03_1

**Date Received**: 2003-01-13 Date Revised:2003-05-23 Date Accepted:
2003-05-25

**Curriculum Topic Benchmarks**: S12.2.4; S12.2.5; S12.3.6; S12.3.7

**Grade Level**: Upper Elementary \[3-5\]

**Subject Keywords**: Force, Inertia, Friction

**Rating**: Moderate

# The Rubber Band Problem

By: Glenn Simonelli, Lakeview Elementary School, 6405 S. Main St.,
Bloomington, IN 47401

e-mail: gsimonel@indiana.edu

From: The PUMAS Collection http://pumas.jpl.nasa.gov

© 2003, California Institute of Technology. ALL RIGHTS RESERVED. Based
on U.S. Gov\'t sponsored research.

**Description**: Elementary Science Activity - Force, Friction and
Inertia

**Time:** Approximately 30 - 40 minutes, depending on discussion

**Grade:** 4^th^ through 8th

**Materials:** For each group of 3-4 students:

Small table with hard surface

2 text books

1 rubber band, cut once so that it can open out flat

**Procedure:** Group the students into groups of 3 or 4. Each group
should be at separate tables. Each table should be around 4 to 6 feet
long. In front of the class, show the two textbooks and the cut rubber
band. The textbooks should both be at least 3/4\" thick; 1\" thick or
more is even better. Lay one textbook down at one end of the table, put
part of the rubber on top of the book so that about half of the rubber
band is hanging over the edge, and then cover them both with the second
book, leaving part of the rubber band sticking out.

Tell the class to watch what happens when you pull on the rubber band.
Pull on the rubber band; the rubber band will pull out from between the
books and snap your figures, inflicting minor pain that you can
exaggerate for your class's amusement. (Practice this beforehand to make
sure that the rubber band pulls out without pulling the books forward.
You may need to use a smaller rubber band or books with a larger surface
area if the books can be moved across the table just by pulling on the
rubber band.) Explain to the class that their assignment is to figure
out a way to pull both books across the table by pulling on the rubber
band. They may not push the books; they may only pull on the rubber
band. The books must be kept together.

**Discussion:** Ideally, each group should have the same size table and
the same textbooks, but if this is not possible the activity is still
worth trying. 15 to 20 minutes is usually enough time for most groups to
discover a solution. Tell groups that solve the problem quickly to see
if they can solve it another way. I normally do not allow the students
to use tape or staples, and I insist that the rubber band be between the
two books rather than inside one, but you can be flexible about the
rules. The more flexibility you allow the students, the more different
solutions you will get, which will make for a more interesting
discussion afterwards.

There are many different solution to the problem. I have had students
stretch the rubber band across the top of the bottom book, with part of
it wrapped around under the book's cover. (Technically, the rubber band
is still going between the books.) I've seen students put a series of
round pencils under the books so that the books roll across the table,
and I've had groups tilt the table so that the books are being pulled
downhill.

This challenge may be used to introduce the concepts of friction, force,
and/or inertia, or it can be used to reinforce them. The key is to have
a discussion after the groups have solved the problem. A good way to
proceed is to have each group share their solution with the class. If
you have already introduced terms like force, inertia and friction, ask
the students to use them as they explain their method for getting the
books to move across the table. If you have not used these terms in
class yet, then you might try using questions to guide the students to
the concepts and then ask if anyone knows what that phenomenon is
called.

In general, to overcome the inertia of the books you need to reduce the
friction between the books and the table, (which is what placing the
books on pencils achieves), or increase the force exerted on the books
by the rubber band, or both. Ironically, increasing the friction between
the rubber band and the books allows you to increase the force that the
rubber band exerts on the books, so this problem can be solved by both
increasing and decreasing friction. (A good series of questions: How can
we solve this problem by decreasing friction? How can we solve it by
*increasing* friction?) Titling the table also increases the force by
adding the force of gravity to the force exerted by the rubber band.
There may be other solutions in your class, but chances are they rely on
one or both or these strategies.

This activity can then be extended to more practical applications.
Teachers might suggest that their students look for and identify real
world examples of the properties illustrated here. For example, students
might be asked how these properties might affect the design of car
brakes or how it might change one's approach to walking on a wet or icy
surface. Most students, with a little reflection, should be able to come
up with examples that illustrate these properties.