BK's Postings from our STEM class

Does air have weight?

The purpose of this this experiment is to test if air has weight?

There are really two questions rolled up into one.

  1. Does air have weight?
  2. Can we make a test that is able to measure the weight of air?

The second question is just as important as the first. We can try to test almost anything, but we may not be sucessful. You will see why shortly.

New Words

You may not know some of the words I use. Some of the more powerful words have been highlighted in bold. Please look them up and write down their definitions for class.

Hypothesis

Our hypothesis is: Does air have mass that we can measure as weight?
Our expected answer is: Yes it does.
Lets get to work and find out.

Materials Used

Here are the materials that we will need.

  1. Balance beam.
  2. A place to hang the balance beam
  3. String. It holds up the balance beam and the baloons
  4. Balloons, Four each of two different colors.
  5. An open space to conduct the test. A bedroom or living room will suffice.
  6. Numbers to put on the wall to make your measurements.
  7. A place to observe from.

Here are some details about those materials.

The balance beam needs to be fairly long. The length gives the balloons more leverage to move the balance beam. It must be fairly light. Expecting a couple of balloons to move a big 2x4 is unrealistic. The beam used in this test was a metal coat hanger straightened out.

The string going to the middle of the balance beam needs to be up high. You will need your parents help on this one. Mine was tied to a ceiling fan. You can use a light fixture in the center of the room The total weight of the balance and eight balloons is only a few ounces so it is no problem for the ceiling fan or the light. I started by tying the string to an air conditioning vent. But that proved to be too close to the wall.

Use the lightest string you can find. Sewing thread is strong enough, but can be difficult to manipulate and tie on to the balloons. Yarn has some drawbacks, but was very good for wrapping around the balloon necks and getting a good seal.

Scotch tape some numbers on the wall as shown in this experiment. Please DO NOT write on the wall. Use a tape measure and measure from the floor to each number to get them the same on both sides.

You should have the balloons about five or six feet from the wall. Observe the test results such that you can see the numbers behind the balloons. As you look at this web page you will understand.

The Test

The test that I discovered with an internet search is to weigh the air with a balance beam. Here is that first balance beam.

First Balance Beam

The ruler has a hole near the center and is suspended with a piece of yarn. One deflated balloon is tied on to each end with more yarn. The beam did not balance out evenly so some yarn was wrapped around the light end to make it balance. The balloon on one end was inflated and tied back on and I observed the results.

It Looked like the balloon became heavier.
But then I realized something. You can discover that problem also. Breathe slowly on a mirror and see what happens. Try that before continuing. The mirror becomes foggy, then clears up. Why?
Because every time we exhale, some moisture goes out with the air. Some of that moisture certainly condensed on the inside of the balloon. How would you compensate for that? I will leave that as a thought question for our next class.
After several more tests the results were inconclusive. Sometimes it seemed as though the inflated balloon was lighter than the deflated one. Those details and more than an hour of testing is omitted.
I decided that this test was not sensitive enough. Next is my improved balance beam.
Note: Remember to look up those bolded words.

Second Balance Beam

The balance beam is made of a coat hanger straightened out and a little loop bent into each end. It is slightly bent in the middle with the two ends hanging down some. This makes it more stable. It is more sensitive because the balance beam is longer. That makes a longer lever arm for gravity to act. We may talk about levers later in the year. Four balloons are used on each end to make the difference between uninflated and inflated larger. The numbers on the wall are 10 centimeters apart.

Wrap, don’t tie.

Before going any further, it will help to describe how I kept the balloons from deflating. If we tie the string around the balloon neck for each test, then we must carefully and tediously untie it. Wrap it instead.

For each of the eight balloons, four on each side, there is a string about 18 inches long attached to the balance. Blow up the balloon and hold on to the neck from the top. Beginning at about the halfway point of the of the string, wrap it firmly around the neck of the balloon between your fingers and the balloon to hold the air in.

To keep the string from unwinding while you look and make a measurement, wrap the last turn or two above the part of the string that goes to the balance beam. Then you can let the string go and the balloon will hold the air rather well.

Some Difficulties

When I started this test, some difficulties became apparent.

  1. When the balloon is inflated, the movement of the air causes it to develop a slight static charge. The inflated side moved towards the closet doors and stuck to them. This prevents the beam from showing the slight weight differences. When I stand close to the balloons, my presence pulls the balloons towards me.
  2. The weight of the balance beam and that of the balloons pulls on the yarn. That yarn is made of smaller twisted threads of yarn. When it is under tension it tends to unwind some. Normally that is never noticed. With these small weights, it becomes obvious.
  3. The balloons are quite light. The inflated balloons have a large surface area. That means they are susceptible to air currents, even some that you cannot feel. Gently blowing on one side will make the balance turn. Every time it is touched, it needs a minute or more to stabilize.

Now we begin to appreciate why science can be difficult. When you start a test there may be many things that disturb your results. To be successful you must be patience and must think out and work out every difficulty.

How can we resolve these difficulties?

  1. I think I resolved the static charge by moving the balance further away from the door, closer to the center of the room.
  2. Let the balance hang for several minutes so that the yarn stops twisting. It took about thirty minutes before it stopped turning. The only way to know that it has truly reached an equilibrium is when it stops rotating and begins oscillating.
  3. (Please look up the bold words.)
  4. Make sure the air conditioning, or the heater, is not blowing air and moving the balance.

Side note: Yarn is not a good material for making a balance beam. I didn’t have anything else handy so I had to be patient.

Still, the balance will not stay where I want to so there will be some movement between the images.

To help make up for this a mark was set on the door for the center of the balance. I will use this to take the pictures from the same place every time. I try to line up the cross on the door with the center of the balance beam. The view finder in the camera is above the lens so there is a slight difference between what I see and what the camera sees.

That did not work out so well. Getting lined up in the exact right position is rather difficult. I abandoned that concept.

Recording The Results

I will take two pictures for each measurement. For consistency the blue balloons will be kept on the right and the bronze on the left. I will line up the blue with the number 3 on the wall and take a picture. Then rotate the camera and take a picture of the bronze side from the same height. Our measurements will show up as differences on the bronze side.

We begin with the balloons on both sides deflated.

Begin the Test

Begin with the balloons on both sides deflated.

Blue baloons

Bronze baloons

The blue is at about 2.9. It is difficult to see the numbers through the view finder of my camera.

Before proceeding, adjust the string at the center of the balance beam so that with all balloons deflated, the bream is very close to evenly balanced. Stop here and get that good before continuing. Once you decide that center string is in the that right place do not change it for the remainder of the test. That would invalidate the entire test.

The gold side is almost right on the three while the blue side is tiny bit above the three, call it 2.9. These are pretty close. Remember about the breath condensate? We will compensate for that by blowing the balloons up on one side, measuring, then on the other side and measuring. Then we check our results and see if they are consistent. For step two, inflate the bronze balloons and make some measurements. First the blue just below. Don’t forget, I line up the blue, then from the same height, check the bronze.

Blue side

And the bronze side below.

Bronze side

This looks very good. There is a very clear difference between the two sides. The inflated bronze is about 10 centimeters below the blue. Now deflate the bronze and make an intermediate measurement. This is to make verify that the two sides are close to equal again.

Blue side

And the bronze size below.

Bronze side

Both sides are much closer to 3 than before. Some of the discrepancies are caused by movement of the balloons and not getting in the exact position to take the picture. It still takes a long time for the balloons to stabilize after each change in conditions. Now, inflate the blue balloons and make some measurements.

Blue side

And the bronze size below.

Bronze side

The bronze side, is about 10 centimeters above the blue side. The bronze side is lighter than the blue side. The difference between the two is almost identical. The inflated side is heaver. This almost proves our hypothesis.

What is something changed during the test. What could change? It might be something we are not aware of. We can verify that nothing did with another test. Deflate boths sides and see if they are even.

Blue side

And the bronze size below.

Bronze side

They are both pretty close to number 3. Now, just for fun, check with both sides inflated.

Blue side

And the bronze size below.

Bronze side

Hmmm, the blue is just above 3 and the bronze is way below the 3. Lets look at both sides, but not quite so close.

Blue side

And the bronze size below.

Bronze side

The bronze balloons are more inflated than the blue. So they are heavier. That is part of the reason to test with one side deflated and the other side inflated. Getting the balloons inflated to the same size and keeping them there long enough to make a measurement is difficult.

Conclusion:

  1. Air does have weight.
  2. Yes, We can prove that it does. We just did that.

Air is not very heavy compared to all the other things around us. There is not much difference between an inflated balloon and a deflated balloon. That makes it difficult to test. We did not measure how heavy air is. We did verify that the weight of air can be detected and measured.

Final Note

This test required about seven hours of work. Quite a while. And that does not include the time preparing this report for my web site. When you are going to do the hard stuff, be prepared to spend the time to do it right. You will be very well rewarded. This test was definitely worth the time I have spent on it.

November 2014