Heavy Newspaper
Demonstrate the incredible properties of air and air pressure using a
sheet of newspaper, a piece of wood, and some karate-chopping power.
Materials
- Several pieces of pine wood or wood paneling (1" wide x 36" long x 1/4"
thick)
- Several large sheets of newspaper
- Work gloves
- A table
Experiment
1. Place the piece of wood on a table and let one end hang over the edge
about 4 inches. Ask the spectators, "What will happen if I hit the piece of
wood that is hanging over the edge of the table? " Make sure everyone
is out of harms way as you karate-chop the stick. Of course, the stick
goes flying end over end just as expected.
2. Return the stick to the table allowing about 4 inches of the stick to
hang over the edge. "Let's use a piece of newspaper to help secure the
stick in place." Show a single sheet of newspaper and fold it in half 3 or
4 times. Place the folded newspaper over the end of the stick that is lying
on the table. Again, make sure everyone is standing away from the table
as you hit the end of the stick that is hanging over the edge of the table.
What happened? Did the newspaper help to hold the stick in place? Of
course the answer is "NO."
3. Finally, show the spectators a new sheet of newspaper and use it to
cover the portion of the stick that is lying on the table. Make sure that the
newspaper is flush with the edge of the table. "What do you think will
happen now if I hit the stick with the unfolded newspaper covering the
stick?" You might anticipate an answer like, "The newspaper will go
flying...or the sheet of newspaper will tear apart." Smooth down the
newspaper with your hands so that there are no pockets of air under the
sheet of paper. Put on your karate-chopping glove to protect your hand.
Strike the protruding edge of the stick with your hands with a sudden
sharp hit. To everyone's amazement, the stick breaks. Remind the
audience that the weight of flat newspaper is exactly the same as the
folded newspaper, yet the flat newspaper held the newspaper in place.
That's amazing... but how does it work?
How does it work?
The results of the experiment prove that the newspaper is more difficult
to lift when it is spread out over a large area, yet the weight of the folded
and flat newspaper remain the same. What other force is exerted on the
newspaper that could account for these differences? The answer is a
simple as the air we breathe. It is the pressure of the air pushing
downward on the newspaper that prevents the paper from rising. It might
be useful to picture a giant column of air resting on top of the newspaper.
However, this column of air is 250 miles (402 km) tall! This column of air
above the newspaper pushes down with a force of 14.7 pounds or
pressure per square inch (this is at sea level). In other words, each
square inch of the newspaper has 14.7 pounds pushing down on it. So,
if you know the area of the newspaper, you can calculate the total amount
of pressure pushing downward on the paper. Let's say that the
newspaper dimensions measure 20 inches by 30 inches. The total area
is 20 inches X 30 inches = 600 square inches. If each square inch has a
force of 14.7 pounds pushing on it, then 600 square inches X 14.7
pounds per square inch = 8,820 pounds! That's the equivalent weight of
two large automobiles. It's no wonder that the newspaper stayed in place
at the moment when you hit the stick. Smoothing down the newspaper
with your hands prior to hitting the stick is a crucially important step. You
want to make certain that there is no air under the newspaper that might
help it to lift up when you strike the stick.
Additional Info
As a follow-up activity, have the students calculate the force of the air
pressure exerted on the folded sheet of newspaper and compare this
number to the force pushing on the flat newspaper. The comparison is
startling.
Experiment From Steve Spangler Science
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