Science Alliance

Experiments in Gravity & Density

with help from

Dry Ice - Frozen Carbon Dioxide

Falling Objects

We are often told that objects fall at the same rate, yet the common experiences of your students can get in the way of this notion.

What to do?

Drop stuff! Lots of stuff. I like to begin with a few demos that make use of common things like paper, pencils, textbooks, balloons, bubbles, and marbles (for dropping into water).

Most kids know that just about anything will fall faster than a sheet of paper. Yet, one should not overlook the obvious. Just do it. Some will argue that the heavier book hits first because it is heavier. This of course is wrong!

Having them (or you) drop a textbook & sheet of paper side by side works well. Dropping again with the paper under the book is the logical follow-up. They will fall together. Why? That's for your kids to figure out. Some will suggest that the book falls faster (as previously seen) and is "pushing" the paper down. Have them predict what will happen if the paper & book are dropped with the paper over the book (make sure that the paper doesn't overhang the edges of the book). Again, they will fall together. Anyone that used the "pushing" hypothesis will get caught.

The dropping of the crumpled paper vs. A sheet of paper again shows what people expect. And cries of "air resistance" or "friction" fill the room (depending on their ages).

You can use frozen CO₂ chips to fill a balloon. Make sure that all the CO₂ chips have turned to gas and then fill a different balloon with regular air (in the normal way) so that it is as close as possible to the size of the CO₂ balloon. Let everyone make predictions about dropping the two balloons side by side and then do it. The CO₂ will fall faster and hit the ground first. Why? Does "air resistance" work here? What about weight? Don't all things fall at the same rate regardless of their weight?

If you don't like balloons --make bubbles-- then do the same thing! The dry ice can also be used to make CO₂ bubbles. I used a small plastic cup, water, bubble mix, a funnel, duct tape (!), and some tubing to make my bubbles. Like the balloons, the CO₂ bubbles do fall faster than regular bubbles. They are generally cloud filled which helps to make this an interesting demonstration. (Try dropping them into a cold wet ice chest. I've used an ice chest that was filled with CO₂ gas and got some interesting results!)

So why do they fall slower? It is not because of air resistance and not really because of their weight. One can figure this problem out by analogy.

It is helpful to think about how things fall in water. Again doing the obvious is a good thing. Drop a pencil and marble into a tank of water. The marble will drop like a rock (if you are into rocks - get some pumice!) and the pencil will float. Why didn't it fall? Doesn't gravity work in water? Does it have anything to do with air resistance? (Try to find some plastics that fall in water, but much slower than a marble does. If you can find something that works - try it out).

Why don't helium balloons fall? What do they have to do with all of this? What about the floating pencil? Is there a connection between the floating things and with why the CO₂ bubbles & balloons fall faster? YES!

The key to all of this is that gravity pulls everything down at an equal rate (as demonstrated with paper & books) - but all of this stuff falls in a fluid - air. Air, just like the water is a fluid and DENSITY becomes an important consideration here. A textbook is much, much denser than air and while gravity pulls on both air and the textbook, the textbook sinks (or falls) under the air. A sheet of paper is still denser than air (although less than a textbook) it is light for its size (surface area) and runs into a lot of air on the way down. This slows it considerably. So what about the floating pencil or helium balloon? They are less dense than the medium that they are in and gravity pulls the denser medium under them - thus they float. What about the CO₂? The CO₂ is denser than air or a regular balloon and like a book sinks in air. A normal balloon (or bubble) is also denser than air - but not by much. So they fall, but slowly. They are also light so air resistance can have a big effect on them. The CO₂ balloon or bubble is much denser than regular air and falls with little (well, less) effect from air resistance.

Also

By the way, because CO₂ is heavier than regular air if you make it in a deep bowl or ice chest it will stay there. Not only will it stay there but it will push out the normal air that previously occupied that space. This means that there will not be enough oxygen to sustain a flame. This can be easily demonstrated by lowering a match or candle into a CO₂ filled bowl or placing a lit candle in a bowl and then pouring the CO₂ on to it!

Remember that the gas produced by the ol' vinegar and baking soda reaction makes the same gas - CO₂. You can use that reaction to do the balloon/bubble/flame stuff as well.

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