Air Pressure Power
Time: 30-40 minutes
Materials (per class):
Plastic bag (sandwich size or larger)
Materials (per pair):
Paper
Pencil
Rubber band
12” ruler (wooden or plastic)
Jar or plastic container
Newspaper
Calculator
Grabber:
1. Put the plastic bag inside the jar so that it is pressed against the jar wall. Get as
much air out from between the bag and jar as possible.
2. Fold the top over the lip of the jar and use the rubber band to hold it in place.
3. Show students the jar and bag and ask them what will happen if you try to pull the
bag out of the jar.
4. After several replies, pinch the bag at the bottom of the jar and pull up. Make sure to
show the students what happens. The bag should resist being pulled out of the jar.
Let students guess why this is. The reason is air pressure between the bag and the
jar. Tell students they will be doing an experiment to experience air pressure.
Directions
1. Separate the students into pairs.
2. Have one student from each pair come up and get a ruler, sheet of newspaper, sheet
of paper, a pencil, and a calculator if using them. Have the other student move the
chairs away from the desk so there will be enough room to work.
3. Tell students to measure the long side of the newspaper, and record the
measurement in inches. If the measurement is not full inches have them round to
the nearest inch.
4. Now, do the same for the width of the newspaper.
5. Have students lay the ruler flat on the edge of the desk with 4 inches hanging over
the edge. The part that hangs over the edge will be the handle/lever for lifting the
newspaper.
6. Lay the fully opened newspaper over the ruler and make sure the edge is close to
the edge of the table.
7. Ask the students what they think will happen if they push down on part of the ruler
that is sticking over the edge of the table.
8. Challenge them to test their hypothesis by pushing hard on the ruler.
9. When they discover that they cannot easily get the newspaper to fly up, tell them to
give the ruler a harder hit downward, but be careful to not break the ruler.
10. Challenge them to make different approaches without tearing the newspaper. Ask
questions that may help lead them to ideas. For example, you might ask if the size or
shape of the newspaper makes any difference. Somethings they might try include
folding the newspaper or making the ruler stick out farther.
11. As each pair successfully gets their newspaper to fly up, have them measure the length
and width of the sheet that they were able to lift with the ruler. They should round
these measurements off to the nearest whole inch. If they folded their newspaper sheet
into, well, interesting shapes have them measure each of the dimensions that they will
need to know in order to calculate the surface area of the sheet. For example, if they fold
their shape into a triangle, they will need to measure the base and height of the triangle.
Have the groups write down all measurements.
12. Guide each group on how to calculate the surface area of the sheet. The general formula
of a square or rectangle length x width= surface area. The general formula for the
triangle base x height, divided by ½=area of the triangle.
13. Challenge students to guess how many pounds of force due to air pressure push on all
objects.
a. Answer: 15 pounds per square inch!
14. Ask them how they can figure out how much force was pushing on their sheet of
newspaper when the sheet was fully open flat. Have the students calculate and record
their calculations.
a. For example, if a newspaper sheet measures 28 in. by 22 in., its surface area is 28
in. x 22in. = 616 square in.
b. Then figure out the air pressure on the newspaper sheet by multiplying its area
by 15: 616 sq. in. x 15 lb. per sq. in. = 9,240 pounds of force!
15. Then, have them calculate how much force was pushing on the sheet when they
successfully flipped the newspaper into the air. Remind them to record their
calculations.
a. Suppose a group of students were able to flip the newspaper folded into a
rectangle that was 7in. by 11in. Its area would be 7in x 11in= 77 sq. in.
b. And the force on it would be 77 sq. in. x 15 lb. per sq. in.= 1,155 pounds. A
large force is still on the newspaper, but a lot less than was on it when it was
fully opened!
16. They should compare their calculations for the fully open sheet and the folded sheet of
newspaper. Ask them questions to help them explain what happened.
a. They should figure out that there was too much force due to air pressure on the
newspaper sheet when it was fully open to be flipped up with the ruler because
the sheet had lots of surface area for the air to push down on. They should
conclude that when the surface area is reduced by folding it there is less force on
the newspaper. Note, that the 15 pounds of air pressure per square inch remains
constant, and only the surface area changes.
Discussion
Q: Why does size affect the lift off of the paper?
The air above the newspaper pushes down on the whole surface of the newspaper in
the same way that your body weight would push down on the newspaper if you were to
sit on the newspaper. The average atmospheric pressure of the Earth is 14.7 pounds per
square inch, or about 15 pounds per square inch. That means that every square inch of
the surface of the newspaper, or any object, has 14.7 pounds of air pushing down on it.
If you figure out the total number of square inches on the surface of the sheet of
newspaper, by multiplying its length and its width, and then multiplying that product
by 14.7 pounds, you will find out that the weight of the air above the newspaper was
quite large! No wonder the newspaper could not be easily lifted up with the ruler.
However, when the surface area of the newspaper was made smaller, by folding it, there
was less total force pushing down on it. Eventually, when the newspaper gets small
enough, the ruler can push paper up into the air. That’s why it was easier to lift up the
folded newspaper.
Q: What was the hardest part of doing this activity? The easiest?
Answers will vary
Q: What you did to make the newspaper lift?
Folding the newspaper or repositioning the ruler are common answers.
Q: What do you think would happen to astronauts if they went outside the space shuttle
without a space suit? Why? *Neglect the freezing temperatures of space.
There is no air pressure in space to balance the pressure inside the astronaut’s body, so
the astronauts would explode!
Q: Where have you experienced air pressure in everyday life?
Stick your hand out of a window in a moving car, etc.