Center of Gravity--On the Move!
PURPOSE
To understand the concept of
the center of gravity; to be able to determine where the center of gravity
is located on a given object; and to predict how it will change when weight
is added somewhere on an object.
PERFORMING THE EXPERIMENT
-
Take an index card and number
each corner 1, 3, 5, and 7 and number the center of each side 2, 4, 6,
and 8.
-
Predict where the card's center
of gravity is located. (This is the spot where the card will balance
flat on your finger tip.) Mark that spot with a pencil.
-
Place the dot on the tip of
your index finger to see if it balances. If it does, you have found
the center of gravity. If the card does not balance, observe how
much it tilts. Also observe in what direction it tilts. Use
this information to move the card on your finger tip. When the card balances,
your finger is on the center of gravity. Have your partner help you
mark that spot. Have your partner repeat steps 2 and 3 on the same
card.
-
Attach a paper clip to weight
spot 1 on the card. You and your partner should each put a dot on
the card where you think the new center of gravity will be. Put your
initials next to your dots. Place your dot on your finger tip and
find the center of gravity as you did before. Now have your partner
test his prediction of where the center of gravity is. Mark the new
center of gravity. Use your ruler to see if you or your partner was
closer.
-
Add a paper clip to any one
or more weight spots in any combination. You and your partner should
predict where you think the new center of gravity is. Mark the card
with a dot and your initials and then find the center of gravity to see
who was closer.
-
Assuming the United States
is totally flat - for example, no mountains or valleys - in which state
would you expect to find the geographic center of the 48 contiguous states?
Determine how accurate your prediction is by finding the center of gravity
on the map of the United States on your table. (HINT: Balance the map on
a pencil tip in order to define more narrowly the center of gravity.
Place the map upside down on the pencil tip to pinpoint the center more
accurately. It will be easier to do this if you work with a partner.)
The locations of the actual geographical center of gravity is given at
the end of these instructions.
-
Predict and then find the center
of gravity of the irregularly shaped pieces of wood and other objects on
your table. To find the center of gravity, place your 2 pointer fingers
(palms facing each other) under each end of an object, so that you can
hold it up in front of you. Gradually move your hands together, keeping
the object balanced so that it does not fall. When your hands meet,
you have found the center of gravity.
QUESTIONS TO THINK ABOUT
1. How would the Rocky Mountains
affect the geographical center of gravity if you allowed for their presence
on the USA map? Use paper clips to test your ideas.
2. How would the Great Lakes
change the geographical center of gravity? Use paper clips to test
your ideas.
EXPLANATION
Center of gravity is the exact
spot on an object where there is the same amount of weight on one side
of the spot as there is on the opposite side. Once you change the
weight anywhere on the object, the center of gravity changes too.
The geographic center of the United States (the contiguous states) is located
in north central Kansas. The geodetic center, which allows
for the curvature of the earth instead of the flat plane of a map, is about
50 miles south of the geographic center
EQUIPMENT NEEDED
3 x5 cards, paper clips, pencil,
ruler, small maps of the United States; variety of irregularly shaped pieces
of wood, metal, etc.
REFERENCE
Modified from WonderScience,
Hands-on Science Activities for Elementary School Teachers and Students,Volume
10, Number 7, April 1996, American Chemical Society/American Institute
of Physics, page 6.
MAP OF THE 48 UNITED STATES
(The geographic center is
just outside of Lebanon, Kansas.)
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