Beacon Lesson Plan Library

Let's Weigh Air

Richard Angelini Sr.

Description

This lesson is a class experiment to measure the weight of air by measuring the weight of the mass it displaces. It incorporates simple equipment and procedures into a highly convincing demonstration of the weight and mass of air.

Objectives

The student understands the difference between weight and mass.

Materials

-1 Bathroom scale
-1 Toilet plunger
-1 Calculator
-1 Ruler
-1 Copy of the Pre-Post Assessment per student (See Associated File)
-Student notebooks or journals

Preparations

1. Buy an inexpensive bathroom toilet plunger. You will want it to be new (never used).
2. Borrow a bathroom scale from someone.
3. Practice the demonstration 2 times before doing it in class. You will need to use your muscles.
4. Duplicate the Pre-Post Assessment, one per student. (See Associated File)

Procedures

INTRODUCTION
1. It may be a good idea to review with the class air pressure before attempting this experiment.

2. Begin by administering the Pre-Assessment. (See Associated File) Use this assessment to create interest in the experiment.

HOOK--Tell the students, “This is not an ordinary plunger, it is magical. Watch.”
1. Press the toilet plunger onto the top of an unoccupied desk.

2. Lift the desk off the floor about 2 or 3 inches and then replace. Repeat the demonstration.

3. Ask the students, “What did you see?”

4. Tell them to write their observations in their notebooks.

5. Ask the students to form a hypothesis and enter it into their notebooks.

6. Repeat step 2 again and ask the students to observe and possibly modify their observations.

7. Ask the students if their hypothesis should be changed.

8. Pick a large student to run a repeat of the experiment. Again have the students observe and write what they see.

9. Ask the students, “Why did the plunger stick to the desk?” Get as many different answers as possible. Ask different students to write their theories on the board.

10. The reason is simple. Say to the class, “The air pressure pushing down on the top of the plunger weighs more than the desk. The extra weight of the air (weight of the air minus the weight of the desk) presses the plunger to the desk.”

11. Say to the students, “Let’s weigh air!”
a. Calculate the area of the plunger. Since the plunger imprints upon the desk in the form of a circle, use the formula for the area of a circle which is, A=3.14 x radius x radius.

b. Air weighs about 14 pounds per square inch at sea level on our planet, so multiply the area of the plunger imprint, (area), by 14 pounds. The result is the weight of air the plunger displaces. In other words, the plunger displaces a column of air a few inches in diameter up only one or two inches, but the column of air is several thousand feet tall! It displaces that air only an inch or two, but that is enough. The air keeps pushing, trying to get back to where it was. That pushing holds the plunger to the desktop.

c. Now that you have the weight of the air, let’s weigh the desk. Have a big student hold the desk off the ground while he/she stands on the bathroom scale. Record the weight.

d. Next weigh the student without the desk in his/her arms. Subtract the second weight from the first weight. The difference is the weight of the desk.

e. In order for the plunger to be able to hold the desk up, the air must weigh more than the desk.

f. Weigh one textbook using the same procedure as for the desk above. Weigh a student with the textbook and again without the textbook. When you have established a weight for an average textbook, repeat the experiment, beginning with step number one. Each time add one or two textbooks to the desk until the plunger can no longer hold up the desk. Add up the weight of the desk and the textbooks to get the total weight of the desk when the plunger fails to hold up the desk. At this point the desk (with textbooks) and the air the plunger displaces are roughly equal. This means that the column of air above the plunger weighs the same as the desk with the textbooks. Equilibrium, a balance of forces, has been established. You have weighed air by displacing air.

12. Explain that air has great weight on our planet. We do not notice it because the air weight, called air pressure, is everywhere; in our cells and in and around all things. To demonstrate this, place your thumb on top of the round part of the plunger and depress it to the surface of the desk. Ask the students, “Can you now imagine the energy needed to depress the plunger? Look at my finger, who would like to try to depress the plunger?” Let a few students try. They will soon realize the force needed is great.

13. For dramatization, draw a square one inch in length on each side on the back of your hand. Say to the students, “Each square inch of your skin has 14 pounds of pressure on it. You don’t feel it because you also have that air pressure in all of the cells of your body. To feel the pressure you must make the pressure outside of your body greater than the pressure inside. Next time you are in a car, hold your hand out of the window at a high speed. You will be able to feel the air pressure.”

14. This experiment graphically demonstrates the difference between mass and weight. (Mass is the amount of matter in an object. Mass always stays the same, unaffected by gravity. Weight varies with gravity. In space a massive object can weigh practically nothing. On a small planet an object will weigh little, while on a massive planet like Jupiter, an object can be very heavy. Weight is a measurement of the attractive force of the large object upon the small object. Mass is the measurement of the amount of matter in an object.) Definitely the desk has more mass. It takes many more thousands of feet of air to equal and surpass the weight of the desk because the air has much less mass. Explain this to the students.

15. Tell the students, “Think of mass and weight as two gallon jugs--one filled with chocolate milk and the other filled with chocolate pudding. Both have the same dimension, but the pudding weighs more and has more mass. The pudding is thicker. It has more protons and neutrons in the same space as the milk; it has more mass. (I suggest you draw this explaination on the board and have the students copy it into their notebooks. The graphic notes serve retention of the concept better than linear language notes.)

16. In a different way, the air has more mass than the desk. True, the air is very thin, so thin you can see through it. But the air takes up a lot more space than the desk. The air above the plunger has more mass. Because the air and the desk have the same gravitational force acting upon them (one Earth gravity), you can weigh their masses in pounds. If they were in space, they would have no weight because there would be no gravity. But they still would have different masses; like the pudding and the chocolate milk.

17. A class discussion is called for here. Have the students think of some other analogies for:
a. similar weight, but different volumes of two objects. (Ex: Chocolate milk and chocolate pudding again, but this time the weights are the same and the chocolate milk container has a greater volume to compensate for the greater density of the chocolate pudding.)
b. similar volumes, but different weights. (Ex: A bowling ball and a soccer ball have the same volume but different masses and therefore different weights.)

Assessments

1. Award points to the students that can correctly answer the questions posed in the procedure as follows: (I recommend you have a student help by recording names and points on the board.)

-5 points: If they write their observations in their notebooks.
-5 points: If they amend their observations.
-10 points: For the first student who can correctly describe why the plunger stays on the desk. (The air pressure holds the plunger to the desk.)
-5 points: For any student who can tell how to calculate the area.

Note: Use the points as you choose.

2. Use the Post-Assessment in the associated file when you feel sure that students have grasped the concept.

Attached Files

An Assessment for students.     File Extension: pdf

Return to the Beacon Lesson Plan Library.